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Molecular Recognition of Anions by Zwitterionic Host Molecules in Water.

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COMMUNICATIONS
(C;imhridge Skieoscm 250 M3). The density of the synthesized material
({I = 2.47 gcni I ) was determined with the suspension method using butyl acetate
and broinoforni as suspension liquids N M R experiments were carried out on a
Bruker ASX 400 spectrometer at 79.495 MHz for Si.
Received: July 2. 1994 [Z7092IE]
German version: Arrgm.. C,'iwi. 1995. 107. 12.5
-
Keywords: silicon compounds structure elucidation zeolites
zinc compounds
bonds results in the specific and enthalpicallq strong binding of
the anionic guest ion.[5.61
In order to mimic this natural binding principle we designed
the artificial hosts 1 and 2. By virtue of the electrostatic repulsion of the inner cationic centers these molecules are inclined to
adopt inflated conformations with cavities which may encapsu-
-
111 J. V Smith. ~ ' / 7 ~ 1 ? 7Reij.
.
1988. 8. 149.
(21 (3 0. Brunner. W. M. Meier. Nurure 1989. 337, 146.
[3] S . Merlino. t'ur. J Minrrul. 1990. 2,809.
[4] M. J. Anncn. M . E. Davis. J. B. Higgins. J. L. Schlenker. J. C/irm. so^^. Clwin.
Coiiiiiiuii 1991, 1115.
[S] M. J. Anncn. M. E. Davis. J. B. Higgins. J. L. Schlenker. Mur. Res. So<.S w ~ p .
P w c . 1991. .?.?.?.245.
[h] M. .I. Annen, M. E. Davis. in Molcwrrlur Sr~i~c~.s
(Eds.: M. L. Occelli. H. Robson). v i m Nohtrand Reinhold 1992. p. 34Y.
[7] C. ROhrig. H. Gies. B. Marler. Zedirrs 1994, 14. 498.
[ X I C' Baerlocher. A. Hepp. W. Meier. Distance Least Squares Refinement Prosram DLS-76. Institut fur Kristallographie&Petrographie. ETH Zurich. 1977.
191 C. Baerloctier. X-ray Rietveld System XRS-82. Institut fur Kristallographie &
Petrographic. ETH Zurich, 1991.
[lo] X-i-ay structural refinement of RUB-I7 K,Na,,[Si,,Zn,O,,].l8H*O:
Cm.
( I = 7.332((1). h = 40.56(1). c = 7.308(1),&. fi = 91.8(1)". Z = 1. Siemens
D.5000 poudei- diffractometer with Ge(l11) monochromatized Cu,,, radiation
in Dehye Scherrer geometry, 0.3 mm glass capillary. room temperature. scan
region' 8 < 7 0 5 8 7 . number of data points: 7.516. number of observed reflections: 803. number of relined structure parameters: 107, isotropic temperature
factors taken from known zincosilicates and not refined, number of profile
parameters 4. number of geometrical restrictions: 109, Rietveld program
XRS-X2. R, = 0.238. R,, = 0.i64, R, = 0 090. Further details of the crystal
structure investigation may be obtained from the Fachinformationszentrum
K;irlsruhe. D-76344 Eggenstein-Leopoldshafen (Germany), on quoting the depositoi-y nmnher CSD-58472.
3
late the guest anion. Rigid substituents extend radially to the
exterior carrying a negative charge at their distal ends. Internal
collapse by ion pairing is avoided owing to the high connectivity
of the molecular skeleton. Preliminary molecular modeling
studies['' in vacuum, however, showed that these hosts acquire
an almost spherical shape due to the carboxylates approaching
the neighboring quaternary ammonium centers. The arene rings
then cover the tetrahedral faces. In molecular dynamics simulations a bromide ion on the outside thus has no chance to make
its way into the cavity even when pushed by high kinetic energy
(1 000 K). Nevertheless molecular mechanics calculations find
the inclusion complex more stable by about 23 kcalmol-' than
the next best host-guest complex with the guest anion associated from the outside. The results of our calculations support the
Molecular Recognition of Anions by
basic concept underlying the host design, because solvation of
Zwitterionic Host Molecules in Water**
the ionic groups most likely will disrupt their mutual interaction
and will restore the accessibility of the molecular cage.
Karin Worm and Franz P. Schmidtchen*
Starting from the macrotricyclic tertiary amines['] the target
compounds 1 and 2 were obtained by Menshutkin alkylation
Cationic artificial molecular hosts are of proven utility in
using a-bromotoluic ester, followed by hydrolysis and desalting
molecular recognition of anions.['' However, they must inby reverse-phase chromatography, in greater than 85 % yield.
evitably be accompanied by counterions creating competition
The zwitterionic betaines are only sparingly soluble in water at
which disfavors complexation of the desired anionic guest. Elecambient temperature, but dissolve much better on addition of
troneutral hosts would avoid this and successful examples employing polytopic Lewis acids"] or ion-dipole i n t e r a c t i o n ~41~ ~ , salt. This correlates with a progressive shift of the ' H N M R
signals of the endo- and exocyclic N-CH, groups to lower and
are known. In general these host -guest interactions are quite
higher fields, respectively. The picture is compatible with the
weak and cannot overcome the competition from protic solhosts taking part in chemical exchange processes which are
vents.
rapid with respect to the N M R time scale. Titrations were perIn these protic solvents the natural hosts for anions (e.g. the
formed at 54 "C in homogeneous solution, and the results could
majority of enzymes) bind the negatively charged guests by dibe easily fitted to a 1 : l host-guest binding model.['] The
rect contact to positively charged anchor groups. An approxiderived association constants K,,, and maximum chemical shift
mately equivalent number of anionic functions are placed somedifferences AS,,, are given in Table 1 . l l o l With Na,HPO,, Na,S,
what remote from this binding site, yielding a structural pattern
;I-aminobutyric acid, and trimethylazoniaacetate (betaine) no
with a small overall charge. This zwitterionic character of bioshifting of N M R signals was observed, which makes inclusion
genic hosts combined with a well-defined network of hydrogen
complexation of these substrates highly unlikely. K,,, decreases
with increasing ionic strength (i.e. 0.2 M N a F ) suggesting that
[*I Prof. Dr. F. P. Schmidtchen, Dr. K . Worm
Institut fur Organische Chemie und Biochemie
electrostatic interactions dominate complexation. The maxider Techiiirche Uiiiversitit Munchen
mum shift difference Ah,,, roughly correlates with the size of the
Lichtenbergstrasse 4, D-85747 Garching (Germany)
anion. Considering that the 1 : 1 stoichiometry appears to reTelefm: [nt. code + (8913209.3345
main fixed even if a high excess of guest is applied, one is forced
[**I This uork wris supported by the Deutsche Forschungsgemeinschdft and the
to assume the penetration of the guest into the molecular cavity
Fonds der chemischen Industrie.
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Table 1. Association constants K,,, of various anions uith host 1 in D,O at 327 K.
Ion radius
[pi111 [ I 61
AGK
CI -
181
BrI
1- [bl
CN
196
220
220
-81 2
-75.2
-65.7
Anion
[a] 360 M H r
'HNMR
'1 [I61
- 70.5
191
~
[kcal mol-
K." W'I
A&,,,,
[HI] [a]
270
2150
6480
1650
950
6.3
27.0
54.5
55.3
6.9
determination. [b] In the presence of 0 . 2 NaF.
~
of the tetrahedral host and consequently the formation of a
well-structured complex. Another argument in favor of this
view may be drawn from a competition experiment taking chloride as a probe monitoring the 3sCl NMR resonance (Fig. 1 ) . In
water the quadrupolar species 35Cl- yields a sharp sin1
glet. due to the rapid interd)
change of water and the
resulting time-averaged high
symmetry of the hydration
sphere of the CI- ion. On
c)
addition of host I increasing
amounts of complex are obtained which has a distinctly
lower symmetry. As a corolb)
lary the quadrupole relaxation is augmented and leads
to severe line broadening or
even complete disappeara)
ance of the signal.[12JIf now
50
0
-50
iodide, which forms a more
-6
stable comdex with 1. is
A
I
gressive sharpening of the
3'CI peak. Evidently, iodide
displaces chloride from the
complex. Considering the relationship between the size of host and guests the only plausible
location for this process to happen is the central cavity of the host.
In the complexation of bromide and iodide by host 1 the
' H NMR peak shift sufficed to determine the enthalpy and entropy of association from the dependence of K,,, on temperature
(Table 2). For both anions host-guest binding is driven by en-
of 5 lo-' M N ~ toI solution b): d ) addition of 4 x
M NaI to solution h).
Tahle 2. Thermodynamic parameters of host - guest binding of hromtde and iodide
by host 1.
312K
AG" [kcal mol327K
337K
-5.08
-6.17
-4.98
-5.70
Anion
BrI-
-4.91
-5.38
'1
AH"
AS'
347K
[kculmol-I]
[calmol-'K-']
-4.84
-5.07
-7.6
-16.7
- 8.0
-33.9
thalpy, with iodide exhibiting the distinctly better interaction. In
addition negative entropies of association are found which apparently are modulated by enthalpy -entropy compensatory effects.[' 3 1 These results may be rationalized qualitatively, if one
assumes that the guests have to strip off their solvation shells in
the course of complexation. In relation to iodide, bromide requires the higher enthalpic expenditure for this desolvation and
consequently shows a smaller overall gain in enthalpy. On the
other hand. the release of tightly bound water molecules en-
hances entropy and thereby counteracts the inherently negative
entropy of association. Although this reasoning undoubtedly
covers only part of the total interaction, it provides another
(though indirect) argument in favor of guest encapsulation,
since only this process can serve to explain the loss of the hydration shell. One can extrapolate from the thermodynamic data
that host 1 discriminates between the heavy halide ions at 0 "C
by a factor of 40, whereas at 85 "C no selectivity is to be seen.
The bigger tetrahedral host 2, on titration even with the
biggest halide iodide, displays only rather meager shifts of the
'H NMR signals, which did not allow the extraction of meaningful association constants. However, the host-guest complexation could be determined from the fluorescence intensity
enhancement of the fluorophoric probe TNS 5 following the
Benesi- Hildebrandt protoc01.I'~~Complex stability (K,,, =
1.46 x lo3M - I ) , which may be attributable to hydrophobic interactions, is similar to that of the
complex of 5 with 8-cyclodextrin
(K,,, =1.5x 1 0 3 ~ - t ) , r ' s 1but much
H,
weaker than with certain cyclophane~."~~
The hosts 1 and 2 show that binding
of heavily solvated anions is possible,
indeed, using abiotic receptors of zero net charge to form welldefined host-guest complexes with respectable affinity. The
structurally fixed spatial separation of positively and negatively
charged substructures appears to be a mandatory requirement,
which in much the same fashion is also realized in the natural
receptors.
Dw
,
46
Received: August 6. 1994 (27214IEl
German version: Ang(,w. Cliem. 1995, 107, 71
-
Keywords: anion complexation host-guest chemistry * molecular recognition
[I] a ) F. P. Schmidtchen, Nudir. Chrrn. G Y ~Luh.
. 1988. 36. 8-17: b) B. Dietrich,
Pure .4pp/. Cheiii. 1993. 65, 1457-1464.
121 a ) H . E. K a t ~ J.
, Org. U i m . 1989, 54, 2179-2183: h) M. T. Reetz, 8. M.
Johnson. K . Harms. Terrohl~rfrorrLrtr. 1994. 35. 2525-2528; c) M. F.
Hawthorne, X. Yang. Z. Zeng, Purr Appl. Cliiw., 1994, 66. 245-254; d) S.
Aoyagi. K. Tanaka. Y Takeuchi. J. Chrni. S o t . Perkin Truns. 2. 1994. 15491553; e) S. Jacobson, R. Pizer, J. Ani. C'heni. Soc. 1993, 115, 11216-11221.
[ 3 ] K . Worm, F. P. Schmidtchen. A. Schier, A. Schifer. M. Hesse, An,syw. Chem.
1994. /Oh. 360-362: Aiigcw. Cheni. hi[. Ed. EngI. 1994, 33, 327-329.
[4] P. B. Savage. S. K. Holmgren. S. H. Gellman. J Ant. Cham. So<. 1994. 116,
4069-4070.
[5] S. Mangani. M. Ferraroni. P. Orioli, Inorg. Clicwi. 1994. 33. 3421-3423.
161 One of the rare examples of proteins binding anions without the assistance
from cationic functional groups is provided by sulfate-binding-protein (SBP):
F. A. Qumcho. H. Luecke. J. Aqviqt, A. Warshel, Proc. N u t / . A d . Sci. U S A
1991. 88, 1026.2030.
[7] Molecular inodeling software Insight I1 /j Discover 3 7 (Biosym). CVFF force
field.
[8] F. P. Schmidtchen. C'hrwt. Err. 1980. 113, 864-874.
191 Regression software Enzfiric,r (Biosoft, Cambridge, UK.)
7H3
[lo] The K,.. v'ilues communicated here are nominally higher than reported for the tetracationic host
4 [ l I]. which. however. was studied at 25 'C and
@N//x/x+ x\@,CH,
0.1 M ionic strength.
[ I 11 F. P. Schmidtchen C/iwii. Bcr. 1981. 114. 5971 -x-i-'
ji\
CH3
607.
@ 'CH3
[I21 The complexation of chloride ion by host 4 was
estimated by 3sCl NMR from signal hroarden4, X = ( CHZ 16
ing: K. Ichikawa. A. Yarnamoto. M. A. Hossain. C/illiw.Leti. 1993, 2175-2178; cf.: M. W. Hosseini, J. P. Kintzinger, J.-M.
Lehn. A. Zahidi. / f t h Chini. A c i a 1989. 72. 1078-1083.
(131 A comprehensive discussion of this phenomenon appeared in the literature
recently: a) D. H. Williams. M. S. Searle. J. P. Mackay. U . Gerhard, R. A.
Mqdestone, Proi. :Vud Acud. Scr. USA 1993. 90. 1172-1 178: b) Y Inoue, T.
Hakushi. Y. Lui. L.-H. Tong. B.-J. Shen. D.-S.J1n.J Am. C h m . Soc.. 1993,115.
47.5-481
[I41 E Diederlch. K. Dick, J A m ('hrm. Soc. 19x4. 106, 8024.8036,
[I51 G Catena. F. V. Bright. A n d . Chwii. 1989, 61. 905-909.
[I61 Y. Markus. J. Chcni. Snc. F h h r Trans. 1991, 87. 2995-2999.
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