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Inorganica Chimica Acta 482 (2018) 905–913
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Biocoordination reactions in copper(II) ions and L-glutamic acid systems
including tetramines: 1,11-diamino-4,8-diazaundecane or 1,12-diamino-4,9diazadodecane
T
⁎
L. Lomozika, , R. Bregier-Jarzebowskaa, A. Gasowskaa, S.K. Hoffmannb, A. Zalewskaa
a
b
Adam Mickiewicz University, Faculty of Chemistry, 61-614 Poznan, Umultowska 89b, Poland
Institude of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, 60-179 Poznan, Poland
A R T I C LE I N FO
A B S T R A C T
Keywords:
Copper(II) complexes
Glutamic acid
3,3,3-tet
Spermine
Biocoordination reactions
Metal-free systems of glutamic acid (Glu) with 3,3,3-tet or spermine (Spm) and ternary systems of Cu(II) ions with
Glu and 3,3,3-tet or Spm were studied using the potentiometric and spectral methods: Vis, NMR and EPR. The
composition, stability and mode of interaction in the formed (Glu)Hx(PA) and Cu(Glu)Hx(PA) species were
established. Biogenic spermine interacts with Glu in a different way to its homologue 3,3,3-tet. In ternary systems
Cu(II)/Glu/Spm, metallation involves all available nitrogen atoms from Spm, while in the form Cu(Glu)(3,3,3tet) the uncoordinated –NH2 group species 3,3,3-tet can create a centre for potential interaction with other
bioligands. In glassy state at 77 K EPR shows that only stable CuL2 type complexes exist with an axial symmetry.
The geometrical structure of the complexes is discussed in the light of the results of alteration in complex
structure when dynamic equilibrium at room temperature is changed during freezing.
In this paper we have also quantitatively calculated how the change in the concentration of Cu(II) affects the
level of molecular complexes. It was found that the introduction of Cu(II) into the studied binary ligand/ligand
systems results in changes in the concentration of the molecular complexes in aqueous solution.
1. Introduction
Ternary complexes of transition metal ions, including Cu(II) with
amino acids, peptides, fragments of nucleic acids or other biomolecules
have been subjects of scientific interest for a long time [1–7]. Within
this general area we are interested in the character of interactions between the amino acids present in living organisms and other small
biomolecules, e.g. polyamines (PA), and in the influence of Cu(II) ions
on the weak interactions of the above species [8–12]. L-glutamic acid
(Glu) is the main neurotransmitter in the central nervous system
[13–19] it is involved in the work of the cellular memory mechanism, in
the mechanism of learning [20] and brain ageing [21]. Most free Lglutamic acid in the brain is derived from local synthesis from L-glutamine and Kreb’s cycle intermediates. It clearly plays an important role
in neuronal differentiation, migration and survival in the developing
brain via facilitated Ca2+ transport [22]. Moreover, it affects the neurological damage observed in people suffering from Parkinson’s disease
and AIDS [23,24], while a change in the level of free Glu in the ventricular cerebrospinal fluid allows the degree of degradation of neurons
in Alzheimer’s disease to be determined [25–27]. Similarly to amino
⁎
acids, polyamines are present in all living cells in relatively high concentrations. In physiological conditions, PAs occur in protonated form
and can react with negatively charged fragments of other bioligands,
e.g. amino acids, proteins, phospholipids, fatty acids, nucleotides or
nucleic acids. Their reactions with the latter determine the role of PAs
in genetic processes, leading e.g. to changes in the DNA structure at
several levels of organisation [28,29]. Moreover, PAs are involved in
cell migration, proliferation, differentiation, embryogenesis, tumour
promotion [30], chromatin organization, mRNA translation, ribosome
biogenesis and programmed cell death [31,32]. An understanding of
the mode of biocoordination reactions of amino acids can provide information on the nature of peptide complexes with metal ions present
in living organisms, e.g. Cu(II) ions that are necessary for the functioning of the brain as cofactors for neurotransmitter synthesis, antioxidant defence, and electron transport [33]. Disruption of Cu homeostasis has been implicated in neurodegenerative diseases such as
Alzheimer’s and Parkinson’s diseases, amyotrophic lateral sclerosis and
prion diseases [34–37]. Moreover, in the blood plasma, copper is present in ceruloplasmin and in complexes with albumin and amino acids
[38,39]. Ternary metal complexes in which one of the bioligands is an
Corresponding author.
E-mail address: lomozik@amu.edu.pl (L. Lomozik).
https://doi.org/10.1016/j.ica.2018.07.046
Received 30 April 2018; Received in revised form 25 July 2018; Accepted 26 July 2018
Available online 29 July 2018
0020-1693/ © 2018 Published by Elsevier B.V.
Inorganica Chimica Acta 482 (2018) 905–913
L. Lomozik et al.
2. Experimental
systems with metals. The modes of interactions were determined on the
basis of the spectroscopic and equilibrium investigation in the pH range
in which particular complexes dominate. Distribution of particular
species was determined by the HALTAFALL computer program [45].
2.1. Materials
2.3. Spectral measurements
The compounds 1,11-diamino-4,8-diazaundecane (3,3,3-tet) –
C9H24N4 (purity 99%) and 1,12-diamino-4,9-diazadodecane, spermine
(Spm) – C10H26N4 (purity 99%) were purchased from Sigma. Polyamine
nitrates were prepared by dissolving a proper amount of free amine in
water followed by adding an equimolar amount of HNO3. The white
precipitate obtained was recrystallised, washed out with methanol,
dried in a desiccator over P4O10 or in the air. The ligands which were
used as nitrate salts were subjected to elemental analysis whose results
(% C, % N, % H) were in agreement with the theoretically calculated
values ( ± 0.3% apparatus error). The elemental analysis was performed on an Elemental Analyzer CHN 2400, Perkin-Elmer. L-glutamic
acid (Glu) C5H9NO4 (purity 99%) used without further purification, was
bought from Sigma. Cu(NO3)2·3HNO3 from POCh–Poland was purified
by recrystallisation from water. The concentration of Cu(II) ions was
determined by the method of inductively coupled plasma optical mass
spectroscopy (ICP MS) on a Mass Spectrometer made by Varian.
Carbonate-free NaOH solution (the titrant) was prepared with Sörensen
solution and determined using the ICP MS method. D2O, NaOD and DCl
were purchased from the Institute for Nuclear Research, Swierk,
Poland.
2.3.1. NMR spectroscopy
Samples for 13C NMR and 2D 1H-15N NMR investigation of metalfree species were prepared by dissolving appropriate amounts of the
components (Glu, 3,3,3-tet and Spm) in D2O. DNO3 and NaOD were used
to adjust the pD of the solutions, correcting pH-readings (a pH meter
N517 made by Mera-Tronik) according to the formula:
pD = pHreadings + 0.40 [46]. The concentration of the ligands was
0.1–0.15 M, at a concentration rate of L:L′ = 1:1. 13C NMR spectra were
recorded on a NMR Gemini 300VT Varian spectrometer using dioxane
as an internal standard. The positions of 13C NMR signals were converted to the TMS scale. Measurements of 2D NMR were performed on a
NMR Bruker Avance III 500 MHz spectrometer using liquid ammonia as
an standard.
2.2. Potentiometric measurements
2.3.3. EPR spectroscopy
EPR spectra were recorded on an SE/X 2547 Radiopan spectrometer
at 77 K. To obtain a homogeneous glassy state and avoid water crystallization during freezing mixed water-glycerol (1:1) solutions were
used. Samples with a metal to ligand ratio taken at various pH, as
during potentiometric studies, were used with an effective copper(II)
concentration of 1 × 10−3 M. EPR spectra were simulated using the
SymFonia Bruker routine.
amino acid (or a peptide) can be considered as models to understand
metal ion-enzyme interactions, including the role of polyamines [40].
2.3.2. Vis spectroscopy
Vis spectra were taken on a UV/Vis Thermo Fisher Scientific
Evolution 300 Spectrophotometer. The samples were prepared in H2O
for the same ligand concentration as in samples for potentiometric titrations at the metal to ligand ratio of 1:1:1 or 1:2.5:2.5 using a
Plastibrand PMMA cell with 1 cm path length.
Potentiometric studies were performed on a Methrom 702 SM
Titrino with an autoburette. A glass electrode Methrom 6.0233.100 was
calibrated in terms of hydrogen ions concentration [41] with the preliminary use of phthalate (pH = 4.002) and borax (pH = 9.225) as
standard buffers. The concentration of ligands in the titrated systems
was 1 × 10−2 M in metal free-systems and 1 × 10−3 M in ternary
systems. The concentration ratio in the metal-free systems of L:L′
(where L = glutamic acid and L′ = tetramine) in the studied samples
was 1:1 and in the Cu:L:L′ systems they were at the ratios 1:1:1 and
1:2.5:2.5. Potentiometric titrations were performed at a constant ionic
strength μ = 0.1 M (KNO3), at 20 ± 1 °C under a neutral gas atmosphere (helium), using as a titrant CO2-free NaOH solution (∼0.184 M).
The addition of NaOH solution did not change the ionic strength because the measurements were performed, starting from fully protonated
polyamines, so –NHx+ cations were replaced by equivalent amounts of
Na+. The initial volume of the sample was 30 mL. No precipitate formation was observed in the entire pH range studied. The selection of
the models and determination of the stability constants of the complexes were made using HYPERQUAD computer programs [42]. Six
titrations were performed for each particular systems and 100–350
points for each titration curve were used for computer analyses. The
computer program use the nonlinear method of least squares to minimize the sum (S) of squares of residuals between the observed quantities (fobs) and those calculated on the basis of the model (fcalc) S = Σn
obs
− ficalc)2, where n is the number of measurements and wi is
i=1 wi (fi
the statistical weight. The testing usually begins with a simple hypothesis and then in the subsequent steps the models are expanded to
include progressively more species; the results are then scrutinized to
eliminate the species rejected in the refinement process.
The dissociation of protons in the reaction allowed the use of potentiometric titration to determine the composition and overall stability
constants (log β) of the adducts formed with the help of the HYPERQUAD computer program. The procedures of measurement and criteria of
model selection have been described earlier [43,44]. The procedures
were used for investigation of the binary systems amino acid/polyamine as well as for coordination compounds forming in ternary
3. Results and discussion
The ligands studied are presented in Scheme 1.
3.1. Metal-free systems
3.1.1. Potentiometric studies of Glu/3,3,3-tet and Glu/Spm systems
The results of the potentiometric as well as NMR studies have
proved the formation of molecular complexes in the investigated systems. Non-covalent interactions between Glu and tetramines 1,11-diamino-4,8-diazaundecane (3,3,3-tet) or 1,12-diamino-4,9-diazadodecane
(Spm, spermine) change the acid-base equilibrium of the system components. The formation of molecular complexes described by the
equation HxGlu + Hy(tetramine) → (Glu)H(x+y-n)(tetramine) + nH+,
was analysed on the basis of potentiometric results. The considered
equilibria of adduct formation have been selected by analyzing distribution curves presenting ranges of the occurrence of free bioligands
in relation to pH and comparing these ranges with those of the formation of molecular complexes. We assumed that these ligands are
involved in the formation of adduct which dominates at a considered
pH. In the case of ternary complexes (including metal ions), the ranges
of the occurrence of binary complexes were also analysed. The rightness
of the choice of the type of reactions was unambiguously confirmed by
the results of computer analysis. When the model choice is correct, the
convergence of the iteration process is obtained. In the case of improperly chosen reactions, the computer analysis process would be divergent. Table 1 presents the values of stability constants (log β) and
reaction equilibrium constants (logKe) for the adducts formed in the
metal-free systems studied.
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Inorganica Chimica Acta 482 (2018) 905–913
L. Lomozik et al.
+
1
H3 N
3
2
N
H2 +
4
4
5
N
H 2+
1
3
2
NH 3+
1,11-diamino-4,8-diazaundecane (3,3,3-tet)
+H N
3
1
3
2
N
H2 +
4
H2 +
N
5
5
3
4
NH 3+
2
1
1,12-diamino-4,9-diazadodecane, Spermine (Spm)
O
+H N
3
CH
2
3
C
1
O-
CH 2
4 CH 2
5C
O
OGlutamic acid (Glu)
Scheme 1. Chemical formulae of the bioligands studied.
(positive) and deprotonated carboxyl groups (–C(1)OO−) of the amino
acid (negative). In the pH range from 5 to 7 the dominant species in the
Spm system is the (Glu)H5(Spm) adduct, while above pH 7 the (Glu)
H4(Spm) species. The adduct (Glu)H5(3,3,3-tet) dominates at pH close
to 6 reaches the higher concentration and (Glu)H4(3,3,3-tet) at pH 8.
Taking into account the values of protonation constants of the bioligands involved in the complex formation, one can deduce that at low
pH, polyamines present in the studied complexes are completely protonated. The number of protons decreases with increasing pH. The
amino acid present in all the studied complexes are monoprotonated.
As follows from Table 1, the equilibrium constants of formation
(logKe), which depends on the energy of interaction of the two ligands,
are higher for the adducts of Glu with 3,3,3-tet than for the Glu adducts
with Spm. A similar relation has been observed earlier for Glu systems
with diamines: 1,4-diaminobutane (Put) and its structural homologue
1,3-diaminopropane (tn) [48] as well as with triamines: spermidine
(Spd) and its derivative 4-aza-1,7-diaminoheptane (3,3-tri) [49]. This
observation proves that in the same conditions the biogenic amines Put,
Spd and Spm form less stable species in the reaction with Glu than their
structural homologues tn, 3,3,-tri and 3,3,3-tet. Contrary to the above, in
the analogous systems with aspartic acid, i.e. shorter than glutamic acid
by one methylene group in the carbon chain, the biogenic amines form
adducts of greater stability [9–11]. This observation emphasizes the
influence of the structural matching of reagents on the character of
interactions in the aminoacid/polyamine systems.
Table 1
Overall stability constants (log β) and equilibrium constants (log Ke) of adducts
formation in Glu-3,3,3-tet (Σ = 14.21; χ2 = 16.36) and Glu-Spm (Σ = 9.06;
χ2 = 12.42) systems.
Species
Equilibrium
log β
log Ke
(Glu)H6(3,3,3-tet)
(Glu)H5(3,3,3-tet)
(Glu)H4(3,3,3-tet)
(Glu)H3(3,3,3-tet)
(Glu)H2(3,3,3-tet)
(Glu)H6(Spm)
(Glu)H5(Spm)
(Glu)H4(Spm)
(Glu)H3(Spm)
H2Glu + H43,3,3-tet (Glu)H6(3,3,3-tet)
HGlu + H43,3,3-tet (Glu)H5(3,3,3-tet)
HGlu + H33,3,3-tet (Glu)H4(3,3,3-tet)
HGlu + H23,3,3-tet (Glu)H3(3,3,3-tet)
Glu + H23,3,3-tet (Glu)H2(3,3,3-tet)
H2Glu + H4Spm (Glu)H6(Spm)
HGlu + H4Spm (Glu)H5(Spm)
HGlu + H3Spm (Glu)H4(Spm)
HGlu + H2Spm (Glu)H3(Spm)
53.09(9)
48.95(1)
41.48(1)
32.61(1)
22.89(2)
54.42(3)
50.42(2)
42.17(2)
32.36(9)
3.01
3.05
2.96
2.72
2.51
2.06
2.24
2.27
1.57
Overall protonation constants of the ligands: H4Spm, 38.67(3); H3Spm,
30.39(3); H2Spm, 21.28(2); HSpm, 10.91(2) [47]; H43,3,3-tet, 36.39(4);
H33,3,3-tet, 29.01(3); H23,3,3-tet, 20.38(3); H3,3,3-tet, 10.36(3) [47]; H3Glu,
15.96; H2Glu, 13.69; HGlu, 9.51 [48].
Because of the difference in the number of protons in particular
species (e.g. the value of β has a different dimensions (units) for (Glu)
H3(Spm) and (Glu)H4(Spm) species), the analysis of the thermodynamic
stability of complexes was conducted with the use of formation constants, without a direct comparison of logβ, as has been sometimes the
case in discussion of results. For example, the calculated equilibrium
constants for the general reaction HxGlu + Hy(PA) → (Glu)H(x+y)(PA)
are logKe = logβ(Glu)Hx+y(PA)–logβHx(Glu)–logβHy(PA), where the subsequent β values are the overall stability constant of the complex,
overall protonation constant of Glu and overall protonation constant of
PA, respectively. (An example of calculation of equilibrium constant in
Supplementary, Example 1).
As shown in Fig. 1, illustrating the distribution of species, adducts in
the systems studied: Glu/3,3,3-tet and Glu/Spm, already begin to form
below pH 3. Although pH 3 does not exist in biological systems, the
values of stability constants are presented for the adducts formed in the
whole pH range covered by our experiments.
The pH range of ligand protonation suggests that the effective interaction centres can be only –NHx+, amino groups from tetramines
3.1.2. NMR study of the metal-free Glu/3,3,3-tet and Glu/Spm systems
The centres of weak non-covalent interactions in the adducts have
been identified on the basis of 13C NMR spectra (Table 2). The positions
of signals coming from particular carbon atoms from the Glu molecules
were assigned according to ref. [50], while for the molecules of 3,3,3-tet
and Spm according to refs. [51,52].
As a result of the interactions between the biomolecules, the electron density on the carbon atoms close to the ligand interaction centres
changes, which is observed as a shift in signals in the 13C NMR spectrum. To analyze the 13C NMR results in a much clearer way, apart from
the Table 2, two examples of graphics showing the variation in chemical shifts with pH due to noncovalent interactions were added to the
907
Inorganica Chimica Acta 482 (2018) 905–913
L. Lomozik et al.
100
100
80
80
(Glu)H5(3,3,3-tet)
60
(Glu)H3(3,3,3-tet)
(Glu)H4(3,3,3-tet)
40
H43,3,3-tet
20
3,3,3-tet
H3,3,3-tet
H23,3,3-tet
0
(Glu)H4(Spm)
(Glu)H2(3,3,3-tet)
% species
% species
(Glu)H6(3,3,3-tet)
H2Glu
Glu
(Glu)H6(Spm)
(Glu)H3(Spm)
60
H4Spm
HGlu
H2Glu
(Glu)H5(Spm)
40
HSpm
Glu
HGlu
H2Spm
20
Spm
H3Spm
H33,3,3-tet
H3Glu
H3Glu
0
4
5
6
7
8
9
4
10
5
6
7
pH
pH
a)
b)
8
9
10
Fig. 1. Distribution diagrams for the Glu/tetramine systems; percentage of the species refers to total Glu; (a) Glu/3,3,3-tet: CGlu = 1.0 × 10−2 M;
C3,3,3-tet = 1.0 × 10−2 M; (b) Glu/Spm: CGlu = 1.0 × 10−2 M; CSpm = 1.0 × 10−2 M.
+
supplementary material, Fig. S1.
No significant changes have been observed in the positions of the
signals assigned to the carbon atoms located far from the reaction
centres [48,49]. In the range of (Glu)H6(3,3,3-tet) complex formation
(pH 3), the shifts of signals assigned to C(1), C(3) and C(4) from 3,3,3tet are 0.055, 0.010 and 0.016 ppm, respectively, while those of the
signals assigned to C(1), C(2) and C(5) from Glu are 0.113, 0.036 and
0.034 ppm, respectively (Table 2). This means that in the (Glu)
H6(3,3,3-tet) adduct the weak interactions between ligands involve
only a deprotonated carboxyl group –C(1)OO− from Glu (a negative
charge) and protonated terminal amine groups from PA (a positive
charge). In the supplementary material, the 13C NMR spectra were
added to view the intensity of signals assigned to −COO− group
carbon, Fig. S2. With increasing pH, the deprotonation of the second
carboxyl group from Glu is accompanied by the formation of a (Glu)
H5(3,3,3-tet) complex. The analysis of the 13C NMR spectra (pH 6) of
this adduct in comparison to free ligands (Table 2) has shown that
oxygen atoms from both carboxyl groups are involved in the interactions. The insignificant changes (0.023 and 0.003 ppm) in the positions
of signals assigned to C(3) and C(5) from tetramine indicate that the
secondary –NH2+ amino groups from 3,3,3-tet are not involved in the
interactions in this adduct, Fig. 2 (similarly as in the (Glu)H6(3,3,3-tet)
species).
The (Glu)H4(3,3,3-tet) adduct appears in the pH range in which the
process of deprotonation of 3,3,3-tet begins, (Fig. 1a), which results in a
change in the interaction mode, as also found in the (Glu)H3(3,3,3-tet)
complex. The deprotonated terminal –NH2 group from tetramine (a
partial charge of -0.033, calculated by GAUSSIAN (Ground State
method/DFT /B3LYP/LAND2DZ level, solvation model) [53] is a
Table 2
Noncovalent shifts in
Systems
Glu-3,3,3-tet
Glu-Spm
13
H 3N
COO+
+
CH
H 3N
H2 N
CH2
+
H2N
H 2C
COO+H
3N
Fig. 2. Tentative mode of interaction in the (Glu)H5(3,3,3-tet) adduct.
potential negative centre of the weak non-covalent type of interaction
with the protonated amino group –NH3+ from Glu (a positive reaction
centre, partial charge + 0.523). As follows from 13C NMR spectra
analysis (Table 2), in the (Glu)H3(3,3,3-tet) species both carboxyl
groups (negative centres) and protonated secondary –NH2+ groups
(positive centres) from 3,3,3-tet are involved in species formation. The
NMR results as well as the values of partial charges at particular groups
in the ligands, calculated by GAUSSIAN, (−0.784, −0.830, +0.523 for
–C(1)OO−, –C(5)OO−, –NH3+ groups from H(Glu), respectively, and
C NMR spectra recorded for Glu-tetramine systems in relation to the free ligands [ppm].
pH
3.0
6.0
8.3
9.5
10.5
3.0
6.0
9.0
Glutamic acid
tetramine
C(1)
C(2)
C(3)
C(4)
C(5)
C(1)
C(2)
C(3)
C(4)
C(5)
0.113
0.067
0.140
0.414
1.182
0.083
0.032
0.274
0.036
0.004
0.013
0.080
0.187
0.024
0.006
0.988
0.014
0.039
0.020
0.053
0.094
0.015
0.037
0.134
0.034
0.011
0.033
0.220
0.654
0.020
0.044
0.027
0.034
0.077
0.020
0.114
0.014
0.023
0.504
0.468
0.055
0.051
0.134
0.227
0.023
0.051
0.031
0.169
0.013
0.033
0.327
0.848
0.015
0.020
0.031
0.164
0.010
0.023
0.100
0.193
0.053
0.019
0.047
0.260
0.016
0.003
0.133
0.127
0.039
0.019
0.054
0.481
0.020
0.020
0.328
0.414
0.075
0.016
0.038
0.111
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Inorganica Chimica Acta 482 (2018) 905–913
L. Lomozik et al.
Fig. 3. 2D 1H-15N NMR spectra of Glu/3,3,3-tet system at pH = 9.5; a) Glu, b) 3,3,3-tet, c) (Glu)H3(3,3,3-tet) adduct, d) overlap spectra.
participation of secondary –NH2+ groups from Spm, and probably
terminal –NH3+ groups (Table 2). Above pH 7, deprotonation of the
first amino group from tetramine (the range according to protonation
constants), is accompanied by the formation of (Glu)H4(Spm) species.
In the 13C NMR spectrum (Table 2) the shifts of signals assigned to C(1),
C(3) and C(4) from spermine are 0.169, 0.260 and 0.481 ppm, which
suggest the involvement of the terminal groups as well as the secondary
groups from the Spm in its interactions with the amino acid. Moreover,
a significant shift by 0.988 ppm of the signal assigned to C(2) neighbouring with the still protonated –NH3+ group in the Glu (a positive
reaction centre, partial charge in this group is +0.523 [53]), indicates
its participation in the interaction with polyamine. Therefore, the deprotonation of the first terminal amino group from Spm leads to the
appearance of a negative reaction centre (–NH2, partial charge on this
group is –0.044) in the PA molecule, which can interact with the positive centre from the amino acid, i.e. the –NH3+ group. Thus, similarly
to (Glu)H3(3,3,3-tet), the inversion in the mode of interaction takes
place in the adduct (Glu)H4(Spm). Depending on pH, the amino group
from spermine can act as a positive or negative reaction centre. Unfortunately, the adduct (Glu)H3(Spm), occurs in the studied system in a
very low concentration (Fig. 1b), which did not allow the NMR technique to be used to identify the active centres involved in the noncovalent interactions on the basis of spectral studies.
−0.031, +0.318, +0.308, −0.039 for –NH2, –NH2+, –NH2+, –NH2
groups from H23,3,3-tet, respectively), suggest the occurrence of an
inversion effect in the (Glu)H3(3,3,3-tet) adduct i.e. interaction between the –NH3+ group from amino acid and the –NH2 group from
polyamine. As described earlier, in some systems, depending on pH, the
amine groups of the polyamine could act either as positive or negative
centres of interactions, (this was also found in the system with Asp
[9–12]). The proposed model of interactions in the adduct (Glu)
H3(3,3,3-tet) was verified by performing 2D 1H-15N NMR measurements. The changes in the shift of signals from nitrogen atoms in Glu
and 3,3,3-tet molecules, observed in the 1H-15N NMR spectrum of the
discussed adduct (pH = 9.5), although very small, point to the involvement of all ligand amino groups in the noncovalent interactions,
which confirms the above pattern of interactions (Fig. 3).
Moreover, the equilibrium constant of the (Glu)H3(3,3,3-tet) formation, logKe = 2.72, is slightly lower than the constant of formation of
the (Glu)H4(3,3,3-tet) adduct (logKe = 2.96, Table 1), where no inversion effect was observed (this confirms a different mode of interaction).
The analysis of signal shifts in the 13C NMR spectra of (Glu)H6(Spm)
taken at pH 3 (Table 2) indicates a similar mode of interactions as in the
corresponding adduct with 3,3,3-tet, that is with the involvement of the
–C(1)OO− group from the amino acid and terminal –NH3+ groups from
tetramine. At pH 6, the changes in the 13C NMR spectrum of (Glu)
H5(Spm) point to the involvement of the –C(5)OO− carboxyl group of
Glu in the weak intermolecular non-covalent interactions, with the
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Inorganica Chimica Acta 482 (2018) 905–913
L. Lomozik et al.
Table 3
Overall stability constants (log β), and equilibrium constants (log Ke) of complexes formation in Cu(II)-Glu-3,3,3-tet and Cu(II)-Glu-Spm systems.
log β
Species
Cu(Glu)H4(3,3,3-tet)
Cu(Glu)H3(3,3,3-tet)
Cu(Glu)H(3,3,3-tet)
Cu(Glu)(3,3,3-tet)
Cu(Glu)H3(Spm)
Cu(Glu)H2(Spm)
Cu(Glu)H(Spm)
Equilibrium of formation
2+
48.09(8)
42.70(7)
30.70(5)
20.47(8)
44.47(14)
37.06(10)
28.40(10)
Cu
Cu2+
Cu2+
Cu2+
Cu2+
Cu2+
Cu2+
+
+
+
+
+
+
+
log Ke
+
6+
4H + Glu + 3,3,3-tet Cu(Glu)H4(3,3,3-tet)
3H+ + Glu + 3,3,3-tet Cu(Glu)H3(3,3,3-tet)5+
H+ + Glu + 3,3,3-tet Cu(Glu)H(3,3,3-tet)3+
Glu + 3,3,3-tet Cu(Glu)(3,3,3-tet)2+
3H+ + Glu + Spm Cu(Glu)H3(Spm)5+
Glu + Spm Cu(Glu)H2(Spm)2+
H+ + Glu + Spm Cu(Glu)H(Spm)3+
3.18
5.17
11.82
11.95
5.75
7.26
8.97
Overall stability constants (log β) of complexes formation in binary systems:
Cu(II)-Glu: CuH(Glu), 13.03; Cu(Glu), 8.52; Cu(Glu)2, 15.01; Cu(Glu)(OH), 1.85 [48]; Cu(II)-3,3,3-tet: CuH2(3,3,3-tet), 27.49(7); Cu(3,3,3-tet), 16.36(3); Cu(H3,3,3tet)2, 42.03(7) [47]; Cu(II)-Spm: CuH2(Spm), 27.63(26); CuH(Spm)2, 29.32(14); Cu(Spm), 14.66(3) [47].
100
100
Cu(Glu)H(3,3,3-tet)
H43,3,3-tet
Cu2+
80
Glu
Cu(Glu)H4(3,3,3-tet)
Cu(Glu)H3(3,3,3-tet)
60 H2Glu
% formation relative to Cu
% formation relative to Cu
80
HGlu
Cu(H3,3,3-tet)2
40
H4Spm
Cu2+
Cu(Glu)(3,3,3-tet)
Cu(Glu)2
H33,3,3-tet
CuH(Glu)
Cu(Glu)
H3Glu
Cu(3,3,3-tet)
H3,3,3-tet
Cu(Glu)(OH)
20
H23,3,3-tet
H2Glu
Cu(Glu)H(Spm)
Glu
Cu(Glu)
60
Cu(Glu)H3(Spm)
Cu(Glu)H2(Spm)
CuH(Spm)2
H2Spm
HGlu
40 CuH(Glu)
Cu(Glu)(OH)
H3Spm
H3Glu
Cu(Glu)2
HSpm
20
3,3,3-tet
Cu(Spm)
Spm
0
0
3
4
5
6
7
8
9
3
10
4
5
6
7
pH
pH
a)
b)
8
9
10
Fig. 4. Distribution diagrams for the Cu(II)/Glu/tetramine systems, percentage of the species refers to total Cu(II); (a) Cu(II)/Glu/3,3,3-tet: CCu2+ = 4 × 10−4 M;
CGlu = 1.0 × 10−3 M; C3,3,3-tet = 1.0 × 10−3 M; (b) Cu(II)-Glu-Spm: CCu2+ = 4 × 10−4 M; CGlu = 1.0 × 10−3 M; CSpm = 1.0 × 10−3 M.
4. Ternary Cu(II)/L-Glutamic acid/tetramine systems
This proves that in the complex Cu(Glu)H4(3,3,3-tet), the metal ions are
coordinated only by the donor atoms from glutamic acid, while tetramine is outside the inner coordination sphere.
As shown in Fig. 4a, in the pH range from 4 to 7, the complex Cu
(Glu)H3(3,3,3-tet) forms. The value of logKe = 5.17 of this species (Ke
corresponds to the energy of ligand bonding with metal ions) is higher
than logKe = 3.18 of Cu(Glu)H4(3,3,3-tet), which suggests a different
coordination mode. For the Cu(Glu)H3(3,3,3-tet) species, the position of
d-d transition, λmax = 645 nm, corresponds to the chromophore
{2 N,Ox} [11,12]. A new centre in the Cu(II) ion coordination sphere
appears, this is the deprotonated terminal amino group of tetramine.
This mode of coordination is also supported by the fact that
logKe = 5.17 of the complex Cu(Glu)H3(3,3,3-tet) is similar to
logKe = 5.0 of the binary complex CuH(Put) [54] and to logKe = 5.06
of the species CuH(tn) [47], in which one nitrogen atom from PA is also
involved in the metallation. Unexpectedly, no presence of the diprotonated** complex Cu(Glu)H2(3,3,3-tet), at least in detectable concentration, was found. Another species forming in the system is Cu(Glu)
H(3,3,3-tet) which reaches its maximum concentration at a pH close to
7 and definitely dominates in physiological pH. The equilibrium constant of this species formation is 11.82, which is similar to
logKe = 11.70 of the binary complex Cu(Spd) [47] with three amino
groups from spermidine involved in the coordination This was also
confirmed by the fact that the value of logKe = 11.82 is higher than
logKe = 7.11 of the binary complex CuH2(3,3,3-tet), chromophore
{2 N} and lower than logKe = 16.36 of Cu(3,3,3-tet) chromophore
{4 N} [47]. Starting from a pH close to 9, the presence of a heteroligand
complex Cu(Glu)(3,3,3-tet) is formed. The value of logKe of this species
4.1. Potentiometric studies of the Cu(II)/Glu/3,3,3-tet system
On the basis of potentiometric titration data, overall stability constants logβ of the complexes, forming in the ternary systems were determined (HYPERQUAD program [42]), (Table 3). Fig. 4a presents the
distribution of species forming in the analysed system.
The Cu(Glu)H4(3,3,3-tet) species is present in the system at a pH
range from 3 to 6, where 3,3,3-tet is fully protonated. As reported in [9],
there is a relationship between the number and type of donor atoms in
the inner coordination sphere of Cu(II) and the energy of d-d transition.
The value of this transition for the Cu(Glu)H4(3,3,3-tet) complex,
λmax = 750 nm, taken at pH 4.5, indicates the involvement of only one
nitrogen atom and oxygen atoms ({N,O} chromophore) [10,11]. The
equilibrium constant of this species formation is 3.18 (similarly to logKe
values for Cu(Asp)H4(3,3,3-tet) or Cu(Asp)H4(Spm) complexes [11]),
which suggests that it is a molecular complex of the ML∙∙∙L’ type, in
which polyamine is outside the coordination sphere of Cu(II) ions and
non-covalently interacts with the anchoring binary species of Cu(II) ion
with glutamic acid. The mode of interactions concluded for the complex
Cu(Glu)H4(3,3,3-tet) is supported by analysis of Vis spectra in an additional experiment, Fig. 5.
The introduction of increasing amounts of tetramine to the binary
system Cu(II)/Glu, at pH = 5, does not result in significant changes in
the position of the maximum absorption (Fig. 5a), but introduction of
increasing amounts of Glu to the system Cu(II)/3,3,3-tet, leads to shifts
in the position of the d-d bands towards higher frequencies (Fig. 5b).
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Inorganica Chimica Acta 482 (2018) 905–913
L. Lomozik et al.
4x10-2
5x10-2
1:1:1
1:1:2
1:1:5
1:5:1
4x10-2
3x10-2
1:1:3
1:3:1
1:2:1
3x10-2
1:1:1
A
A
1:1
2x10-2
2x10-2
1:1
10-2
10-2
0
0
500
600
700
800
900
500
600
700
max
max
a)
b)
800
900
Fig. 5. Vis spectra of Cu(II)/Glu/3,3,3-tet system (pH = 5); CCu2+ = 1 × 10−3 M: (a). excess 3,3,3-tet; (b). excess Glu.
by the EPR studies presented in the next section.
Calculations have also been made to show how the presence of
metal ions quantitatively changes the concentration of the molecular
complexes formed between glutamic acid and biogenic amine, spermine
and 3,3,3-tet at physiological pH, (Fig. 6). A change in the level of
molecular complexes is observed even from the very low concentration
of copper(II) of 0.05 mM. When the concentration of Cu(II) ions in solution reaches approximately 1 mM, the molecular complexes formed in
the physiological condition disappear.
is 11.95, which is close to the logKe = 11.82 value of the monoprotonated species (Table 3). This indicates that in both complexes: Cu
(Glu)H(3,3,3-tet) and Cu(Glu)(3,3,3-tet), tetramine is coordinated with
Cu(II) ions in an analogous mode, i.e. with the participation of three
nitrogen atoms. This is clearly confirmed by similar positions of d-d
transitions; λmax = 598 for Cu(Glu)H(3,3,3-tet) and λmax = 594 for Cu
(Glu)(3,3,3-tet), taken at pH 7 and pH 10.5, respectively (chromophore
{3 N,Ox}).
4.2. Potentiometric studies of the Cu(II)/Glu/Spm system
4.3. EPR studies of the binary and ternary Cu(II)/L-Glutamic acid/
tetramine systems
In the ternary system of copper(II) ions, glutamic acid and spermine,
protonated complexes Cu(Glu)Hx(Spm), where x = 3–1, are formed
(Fig. 4b). In the pH range from 4.5 to 8.5, the complex Cu(Glu)H3(Spm)
is present in the system, whose equilibrium constant of formation, logKe
is 5.75. A comparison of the above value with logKe = 5.08 of the
binary species CuH(tn) [47] and logKe = 5.0 of the complex CuH(Put)
[54] (chromophores {N}), indicates a coordination involving one nitrogen atom from the tetramine ligand in this triprotonated mixed
species, as confirmed by the value of λmax = 646 nm, measured at pH 6.
The involvement of the second amino group from Spm in metallation in
the Cu(Glu)H2(Spm) complex is proved by the increase in logKe = 7.26
of this species with respect to logKe = 5.75 of the triprotonated species,
as well as by the location of d-d transition, λmax = 591 nm (pH 8).
Moreover, the equilibrium constant of diprotonated species formation is
in good agreement with logKe = 7.11 of the binary complex
CuH2(3,3,3-tet), in which two nitrogen atoms from tetramine are involved in Cu(II) coordination [47]. Above pH 7.5 the Cu(Glu)H(Spm)
complex starts forming and its highest concentration is reached at pH
close to 9.5. The value logKe = 8.97 points to the metallation with the
involvement of 3 nitrogen atoms from PA, similarly as in the case of the
binary Cu(HSpm) species. In the case of the CuH2(Spm) binary species
logKe = 6.35 (chromophore with 2 nitrogen atoms) and in that of the
Cu(Spm) binary complex logKe = 11.70 (chromophore with 4 nitrogen
atoms). The position of λmax = 563 nm indicates the {4 N,Ox} type of
coordination [47] with the involvement of one nitrogen donor atom
from Glu.
A comparison of the distribution diagrams for binary (Fig. 1) and
ternary systems (Fig. 4) indicate that Cu(II) strongly modifies intermolecular interactions as a result of coordination by polyamine and
glutamic acid molecules. All Cu(II) ternary complexes assigned in potentiometric studies are asymmetric units suggesting low effective
complex symmetry. Information on the coordination mode in frozen
solution, where only high symmetry species were dentified, is provided
EPR spectra of Cu(II) complexes were measured at glassy state at
77 K at pH 6. EPR spectra of solutions at room temperature were not
recorded since they were non-informative, being composed of broad
overlapping lines. The frozen solution spectra were recorded both for
binary systems, treated as a reference sources, and for ternary systems.
The spectra are shown in Fig. 7 with simulated spectra represented by
dashed lines.
They are characteristic of a complex geometry, having axial
20
(Glu)H5(3,3,3-tet)
(Glu)H5(Spm)
%
15
10
5
0
0.0
0.02
0.05
0.1
0.5
1.00
5.00
Cu2+ (mM)
Fig. 6. Influence of copper(II) ions on the formation of the glutamic acid/
polyamine molecular complexes, concentration of glutamic acid and polyamine
1 × 10−3 M. Percentage of the complexes in relation to total aminoacid.
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Inorganica Chimica Acta 482 (2018) 905–913
L. Lomozik et al.
Fig. 8. Correlation diagram of A|| vs. g|| for the studied binary and ternary
systems. The straight line is the theoretical plot of Eq. (1) with the delocalization parameter α2 = 0.8 for the d x 2 − y2 ground state.
AII = P [(κ−4/7] α 2 + (gII −2.0023) + 3/7(g⊥−2.0023)
where P = 0.036 cm−1 is the radial d orbital extension, κ ≈ 0.43 is
the isotropic contribution to the hyperfine interaction reduced by the
unpaired electron density delocalization on ligands α2. For the typical
value g⊥ = 2.05 a diagram is shown in Fig. 7, where the solid line
represents correlation for α2 = 0.8. The position of a Cu(II) complex in
the diagram is characteristic of various Cu(II) coordinations and a delocalization parameter value, as suggested in the diagram of Fig. 8.
Cu(II)/polyamine complexes are characterized by relatively low gIIfactors and relatively high hyperfine splitting AII. This is typical of the
pseudotetrahedral or distorted square-planar ligand configuration expected for fully deprotonated amines. We propose a model of amine
coordination by two terminal nitrogen atoms in trans configuration for
Cu(polyamine)2 complexes [55]. The Cu(II)/polyamine binary complexes are located in the top left corner of the diagram (numbers
2,3,4,5) in close positions showing that the complex structures are
nearly identical.
The Cu(II)-glutamic acid EPR spectrum (spectrum No.1 in the correlation diagram) has a slightly lower g-factor compared to Cu(II)/
polyamines. Amino acids are well coordinated bidentate molecules
depending on the deprotonation of the amine group. At neutral and
basic pH they form axial symmetry Cu(Glu)2 complexes in cis and trans
configuration in water solutions [56]. EPR parameters of these two
configurations have been added to Table 4 and to the correlation diagram (Fig. 8).
It seems that the Cu(II)/Glu complex in our investigation is an
averaged form of the above two configurations with effective planar
geometry. Complexation of Cu(II) by poly(L-glutamic acid) has been
studied in detail by UV–Vis and circular dichroism (CD) in water solutions [57]. It was found that visible absorption and CD intensities
depend on the pH-value due to an interplay between two dynamical
configurations of the Glu polymeric chain. A broad structure less absorption band from d-d- transitions was observed at around 700 nm
(14300 cm−1) whereas the λmax = 750 nm (13300 cm−1) in our
system.
In EPR spectra of the ternary Cu(II)/Glu/PA complexes the competition in coordination between Glu and polyamines is well reflected.
Glu and polyamines are comparable in coordination strength and give
separate EPR lines. This means that there are no mixed complexes
where Glu and 3,3,3-tet or Spm coordinate simultaneously, although
such coordination is suggested by the potentiometric results (Fig. 4). In
Cu(II)/Glu/3,3,3-tet the spectra of Cu(II)/Glu and Cu(II)/3,3,3-tet coexists with an intensity ratio of about of 3:1. In Cu(II)/Glu/Spm at
Fig. 7. EPR spectra of binary and ternary systems in glassy state at 77 K.
Simulated spectra are shown by dashed lines.
Table 4
EPR parameters of Cu(II) in binary and ternary systems.
Systems
1
2
3
4
5
6
7
8
9
10
11
12
Cu/Glu
Cu/3,3,3-tet
Cu/Spm
Cu/Spd
Cu/3,3-tri
Cu/Glu/3,3,3-tet:
1 Cu/Glu
2 Cu/3,3,3-tet
Cu/Glu/Spm:
1 Cu/Glu
3 Cu/Spm
1 Cu/Glu
Cu/Glu/3,3-tri
Cu/Glu/Spd
Cu/Glu/tn
Cu(L-Glu)2 trans
Cu(L-Glu)2 cis
pH
7.0
7.0
6.5
>7
>6
7
6.5
8.0
8.0
8.6
8.0
Solid state
EPR parameters
A-values in 10−4 cm−1
Refs
g||
g⊥
A||
A⊥
2.261
2.225
2.225
2.226
2.227
2.059
2.049
2.049
2.045
2.045
182
183
183
194
194
16
25
25
19
19
2.261
2.225
2.321
2.261
2.225
2.261
2.232
2.234
2.246
2.255
2.255
2.059
2.049
2.050
2.059
2.049
2.059
2.035
2.040
–
2.122
2.122
182
183
166
182
183
182
179
184
177
173
192
16
25
22
16
25
16
24
20
–
13
19
(1)
This paper
This paper
This paper
[55]
[55]
This paper
This paper
[49]
[49]
[48]
[56]
Glu = glutamic acid; Polyamines: Spd = 1,8-diamino-4-azaoctane (spermidine);
Spm = 1,12-diamino-4,9-diazadodecane (spermine); 3,3,3-tet = 1,11-diamino4,8-diazaundecane; tn = 1,3-diaminopropane. Errors: g⊥ ± 0.002; g⊥ ± 0.004;
A⊥ ± 3; A⊥ ± 5.
symmetry with orbital ground state d x 2 − y 2 or dxy which exist in distorted
octahedral D4h or pseudotetrahedral C2v symmetry. The g-factors and
hyperfine splitting obtained from a computer simulation of the spectra
are summarized in Table 4.
EPR spectra of Cu(II) coordinated by Spm and 3,3,3-tet are identical,
indicating the identical coordination of these polyamines at solid state.
This is also true of the polyamines Spd and 3,3-tri which we have recently studied [55] at pH > 7. Their EPR parameters are added to the
Table 4. A relationship between EPR parameters and complex stereochemistry can be easily recognized from the A|| vs. g|| correlation diagram. For orbital ground state d x 2 − y 2 the correlation diagram can be
plotted according to the equation
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Inorganica Chimica Acta 482 (2018) 905–913
L. Lomozik et al.
pH = 6 two spectra coexist with nearly equal intensity: Spectrum Cu
(II)/Glu (No.1) and a new spectrum (No.7) having a clearly larger gfactor. At pH > 8 the Cu(II)/Glu spectrum (No.1) dominates with a
trace of Cu(II)/Spm spectrum (No.3). The position of the new spectrum
7 in the correlation diagram suggests that there is a square-planar
complex CuO4 with oxygens of glutamic acid at low pH where Spm
molecules do not coordinate Cu(II) ions. The above results indicate that
in solid state the strongest coordinating ligand is Glu, while 3,3,3-tet is
stronger than Spm.
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5. Conclusion
As a result of noncovalent dipole–dipole interactions in the Glu/PA
systems molecular complexes are formed. In the system Glu/3,3,3-tet,
up to pH 7, only the terminal –NH3+ amino groups from polyamine
(positive reaction centre) are involved in the interaction with the carboxyl groups from amino acid (negative centre). However in the adduct
(Glu)H3(3,3,3-tet), the terminal deprotonated amino groups from this
tetramine (negative centres) interact with the amino group from Glu (a
positive centre), so the inversion effect occurs (above pH about 8.5). In
the (Glu)H4(Spm) species the inversion effect also takes place (above
pH about 7). EPR results at glassy state (77 K) show that there is a weak
correlation between potentiometric results at room temperature and
EPR results at low temperature glassy state, both at qualitative and
quantitative level. It seems that recording of a stationary absorption
between spin levels in EPR or between orbital levels in UV–Vis spectroscopy is not possible if Cu(II) complexes exists in rapid dynamical
equilibrium. The greatest discrepancy between low-temperature EPR
and room-temperature potentiometric results is that in the liquid state
the single ligand asymmetric units CuLHx type dominate, whereas in
glassy state only axially symmetric CuL2 complexes exist. This can be
associated with a transition from dynamical equilibrium in solution to
static equilibrium during freezing with significant change in the concentration distribution of various species. Such behaviour of Cu(II)
complexes under freezing has already been reported [58]. It is known
that the pH-value of a solution decreases during freezing and enthalpy
favours complexes with a high coordination number. Thermochemistry
of metal-polyamine complexes giving enthalpies and entropies of
complex formation correlate well with EPR and UV–Vis results [59,60].
The calculations have shown that the introduction of metal ions to
the studied binary ligand/ligand systems results in a change in the
concentration of molecular complexes. The change in the concentration
of the complexes begins from the copper(II) level of 0.05 mM Although
the level of Cu(II) in cells is usually low, the presence of the metal
should be taken in account because increased concentration of copper
in biological fluid has been noted in some diseases, e.g. Alzheimer.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
online version, at https://doi.org/10.1016/j.ica.2018.07.046.
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