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Influence of External Negative Charges on the Absorption Maxima of Symmetrical Cyanines. A Study with Model Compounds and Artificial Bacteriorhodopsin Pigments

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for l a - l b + e 3 is 18.6 eV."'] It already follows from the
presence of the narrow signal at m/z 21 in Figure IB that
charge stripping of 2a also yields a stable dication and
preliminary measurements of the ionization energies for
2a --r 2b e0 give IE = 18- 19 eV. Ab initio MO calculations
(MP2/6-31G*//4-31G) yield a value of 18.6eV for the
adiabatic ionization of 2a ;"'I the recently computed
(MP3/6-31G*//6-31G*)1'51 value for the vertical transition
to the triplet ground state of 2b is 18.5 eV.
Conclusion: The existence of keto/enol tautomeric pairs
of neutral and singly or doubly charged species is now well
established. The present work provides the first experimental evidence that the ynol of the most simple ketene/
ynol tautomeric pair, hydroxyacetylene, exists as a stable
neutral molecule 2, radical cation 2a, and dication 2b, as
predicted by theory.
Received: November 8. 1985:
revised: December 19, 1985 12 1526 I€]
German version: Angew Chem. 98 (1986) 275
[I] a ) W. J. Bouma, R. H. Nobes, L. Radom, C. E. Woodward, J . Org.
Chem. 47(1982) 1869; b) W. J. Bouma, P. M W. Gill, L. Radom, Org.
Mass Spertrom. 1Y (1984) 610; c) W. Koch, F. Maquin, D. Stahl, H.
Schwarz, J. Chem Soc. Chem. Commun. 1984. 1679.
121 G. Maier, H. P. Reisenauer, T. Sayrac, Chem. Eer. l l S (1982) 2192.
[3] F. W. McLafferty (Ed.): Tandem Mass Spectrometry. Wiley-lnterscience,
New York 1983.
[4] J. L. Holmes, Org. Mass Spectrom. 20 (1985) 169.
[S] Review. W. Koch, F. Maquin, D. Stahl, H. Schwarz, Chimia 3Y (1985)
161 Selected references: a ) P. 0. Danis, C. Wesdemiotis, F. W. MCLdfferty,
J Am. Chem. Soc. 105 (1983) 7454; b) P. C. Burgers, J. L. Holmes, A. A.
Mommers, J. K. Terlouw, Chem. Phy.7. Letr. 102 (1983) I : c) P. C. Burgers, J . L. Holmes, A. A Mommers, J . €. Szulejko, 5. K. Terlouw, Org.
Mass Spectrom. 19 (1984) 442; d) J. L. Holmes, A. A. Mommers, J. K.
Terlouw, C. E. A. A. Hop, Int. J. Mass Spectrom. Ion Proc.. in press.
[7] Review: K. Levsen, H. Schwarz, Mass Specrrom. Rei!. 2 (1983) 17.
181 P. C. Burgers, J. L. Holmes, Org. Mass Spectrum. 17 (1982) 123.
[9[ H M. Rosenstock, K. Draxl, R. W. Steiner, J. T. Herron, J . Phy.7. Chem.
Re( Data Suppl. I . 6 (1977).
[lo] J . L. Holmes, private communication.
[ I 11 Note that the relative stability of hydroxyacetylene versus ketene decreases in going from the neutral molecules 211 to the radical cations
2a/la, whereas the reverse invariably obtains with enol/keto tautomeric
pairs, where the order of stability may even change. Review: H.
Schwarz, Proc. 10. Intern. Moss Specrrom. Conf. Swansea (1983, in
[I21 R. Stockbauer, lnt J. Mass Specrrom. /an Phys. 25 (1977) 401: N. J.
Hijazi, J. L. Holmes, .I. K . Terlouw. Org. Mass Specrrom. 14 (1979)
(131 J C. Lorquet, Int. J . Mass Spectrom. Ion Phys 38 (1981) 343.
[I41 The relatively intense peak at m/z 28 in the NRMS of la relates to the
large cross section [6d] for collisional re-ionization of CO generated by
dissociation o f la in the first CA cell.
(151 W. Koch, unpublished results.
Influence of External Negative Charges on the
Absorption Maxima of Symmetrical Cyanines.
A Study with Model Compounds and Artificial
Bacteriorhodopsin Pigments**
By Mordechai Sheves* and Noga Friedman
Symmetrical cyanine dyes show narrow, red-shifted absorption maxima (relative to the corresponding protonated
Schiff base) as a result of their highly delocalized n electrons. Perturbation that reduces the symmetry of a symmeI*]
Dr. M. Sheves, Dr. N. Friedman
Department of Organic Chemistry, The Weizrnann Institute of Science
Rehovot 76 100 (Israel)
M S , is an incumbent of the Morris and Ida Wolf Career Development
Chair. This work was supported by the Frances and Lillian Schermer
Trust. We thank Prof. K . Nakanishi for stimulating discussions.
0 VCH VedagsgeseIl.whaft mbH. 0-6940 Wernherm, 1986
trical cyanine should result in blue shifts and broadening
of the electronic spectrum.
Bacteriorhodopsin (bR) is the protein pigment of the
purple membrane of the haiophilic microorganism Halobacterium halobium."] Its role is to convert light energy
into a proton gradient across the membrane, which is subsequently used to synthesize ATP via a chemiosmotic
mechanism. bR consists of all-rrans-retinal linked to the F amino group of a lysine residue to form a protonated
Schiff base.[*]
The absorption maximum of bR
= 570 nm) is redshifted relative to that of the protonated retinal Schiff base
in methanol solution (L,,,,=440 nm). At present, it is assumed that this red shift is a consequence of several factors: (1) weak hydrogen bonding to the positively charged
Schiff base nitrogen;I3l (2) a planar s-trans ring-chain conformation of the retinal chromophore in the binding site;[41
(3) an interaction between an ion-pair in the protein near
the cyclohexene moiety and the retinal c h r ~ m o p h o r e . e.41
Recently, an artificial bR pigment containing a cyanine
dye chromophore, which exhibited a narrow, red-shifted
absorption maximum (relative to bacteriorhodopsin), was
prepared.l5I Calculations'61predicted that this characteristic
absorption should originate from interaction of the polyene with negative charges symmetrically distributed along
the chromophore.
In this study, we have examined the effect of a nonsymmetrical distribution of negative charges along the polyene
on the absorption maximum of symmetrical cyanines, using compounds 1-3, which bear symmetrically ( 1 and 3)
or nonsymmetrically ( 2 ) distributed negative charges.
2 , R ' -COO',RZ=H
3,R' = R' =COOo
a,n=l, b , n = 2 , c,n=3, d,n=L
Cyanines 1 were prepared as described earlier."] Compounds 2 (mixtures of syn-anri isomers) were prepared by
condensation of the corresponding merocyanines (obtained by basic hydrolysis of I , cf. 5 and 6 ) with proline.
Compounds 3 (likewise mixtures of svn-anti isomers) were
synthesized by condensation of 4 with proline followed by
addition of Et-,N. Compounds 1 , 2, and 3 exhibited very
similar absorption maxima in both ethahol and methylene
chloride. The introduction of the second negative charge
(see 3) only slightly changed the absorption maxima and
the bandwidth (as measured at the half-height) (Table 1).
None of the recorded absorption maxima was shifted following dilution up to l o p 5 M, which was necessary to minimize the possibility of aggregation. Thus, the absorption
maxima Of
are not affected significantly by the location Of their counterion in organic Solvents.
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Angew. Chem. lnt. Ed. Engl 25 (1986) No. 3
Table I . Absorption maxima,
symmetrical cyanines 1-3.
52 I
63 I
and bandwidths at half-height, G,I
43 I
The symmetry of chromophores la-d and high degree
of charge delocalization in the ground state are reflected in
their 'H- and I3C-NMR spectra (Table 2).
Table 2. ' H - a n d "C-NMR data of la-d and l a , b, Za, b. For numbering of
the atoms see 5 and 6 . J in Hz.
' H - N M R (CD,OD): l a : 6 = 5.52 (t. 1, J = 12, 7-H), 7.92 (d, 2, J = 12, 6(8)-H);
6(10)-H); lC:6=5.74(t,2,J=11.8,7(II)-H),6.26(t,
J=11.8, 2, 8(10)-H), 7.70 (d, 2, J=11.8, 6(12)-H); Id: 6=5.95 (t. 2, J = 1 2 ,
7(13)-H),6.21 (t,2,J=12,9(11)-H),7.17(t,1,J~12,10-H),7.25(t,2,J=12,
8(12)-H), 7.45 (d, 2, J = 12, 6(14)-H)
"C-NMR (CD,OD): l a : 6=92.90 (C-7), 159.66 ( C - 6 4 ; 2a [a]: 6=93.49,
93.12 (C-7), 159.65, 159.74, 159.83, 160.88 (C-6/8); l b : 6 = 105.16 (C-7/9),
158.76 (C-6/10), 163.57 (C-8); Zb [a]: 6=105.44, 105.55, 105.59, 106.31 ( C 7/9), 158.50, 159.09, 160.12 (C-6/10), 163.45, 163.72 (C-8)
[a] syn-anti mixture
The I3C-NMR spectra indicate that 2a, b d o not differ
significantly from l a , b with respect to charge delocalization and the high degree of symmetry. The corresponding
carbon atoms in l a and 2a have virtually identical chemical shifts. This is also true for l b and 2b. This insensitivity
to location of the counterion (symmetrical in 1 and unsymmetrically fixed in 2) is reflected in the absorption maxima
of these cyanines (Table 1).
To gain further information o n the influence of external
charges on the electronic spectra of symmetrical cyanines,
we incubated merocyanine 5 (obtained by basic hydrolysis
of l c ) with bacterioopsin"] for 48 h (pH = 6.5, 20 mM
HEPES buffer). This resulted in formation of the artificial
Fig. I . Incubation of merocyanine 5 with bleached purple membrane at
25°C. pH =6.5; formation of the artificial bacteriorhodopsin pigment V
(---) Absorption after 1 min; (. - ..) after 12 h; (-)
after 48 h.
Angew. Chem. I n [ . Ed. Engl. 25 (1986) No. 3
pigment V , which has an absorption maximum at
;1=530 nm (Fig. 1) with a narrow bandwidth (1350cm-').
Competition experiments with all-trans-retinal showed
that the chromophore occupied the binding site. Similarly,
incubation of merocyanine 6 with bacterioopsin ( I h) afforded pigment VI, which absorbed at ;1=640 nm with a
bandwidth of 1100 cm-'. Both artificial pigments exhibit
similar absorption maxima and bandwidths to those of the
corresponding cyanines l c and 2c or Id and 2d, respectively, in EtOH and CH& (Table 1).
Artificial b R pigments based on dihydroretinals exhibit
opsin shifts that vary, depending on the length of the chromophore. (The opsin shift is defined as the difference between the absorption of the pigment and that of a protonated Schiff base of the corresponding retinal chromophore
in MeOH.)L91For example, it was suggested that the pigments derived from 5,6- and 7,8-dihydroretinals differ
quite significantly from each other on account of different
charge distribution introduced by the protein around these
two c h r o m o p h ~ r e s . [ Pigments
~ ~ ~ ~ ] V and VI are comparable to 7,8- and 5,6-dihydro pigments, respectively, in their
chromophore length. However, the absence of significant
opsin shifts points to the insensitivity of symmetrical aliphatic cyanines to charge distribution around their polyene skeleton. This result is consistent with our experimental observation in organic solvents that the absorption
maxima of symmetrical cyanines are not affected significantly by interaction with external negative charges.
The red-shifted absorption maxima and the narrow
bandwidth of symmetrical cyanines reflect the high degree
of charge delocalization in both the ground and excited
states ; the charge in these cyanines is distributed similarly,
in each case, along the polyene chain.['01This phenomenon
is not perturbed by interaction with nonsymmetrically distributed negative charges in the vicinity of the nitrogen
atoms. Similar arguments hold for the insensitivity of the
absorption maxima to solvent polarity. This behavior is
completely different from that found in protonated retinal
Schiff bases, which are markedly influenced by nonconjugated charge^,'^'] because the positive charge is distributed
differently in the ground and excited states.
Received: August 30, 1985;
revised: December 30, 1986 [Z 1444 IE]
German version: Angew. Chem. 98 (1986) 260
CAS Registry numbers:
la, 56337-40-7; lb, 100683-37-2; lc, 100683-39-4; Id, 2 1044-72-4; Za, 10068340-7; Zb, 100683-41-8; Zc, 100683-42-9; Zd, 100683-43-0; 3a, 100701-08-4; 3b,
100683-45-2; 3e, 100683-47-4; 3d, 100683-49-6; 5, 100683-50-9.
[I] D. Oesterhelt, W. Stoeckenius, Nature (London) New Biol. 233 (1971)
[Z] Reviews: a) W. Stoeckenius, R. Lozier, R. Bogomolni, Biochim. Biophys
Acra 505 (1979) 215; b) M. Ottolenghi, Adu. Phorochem. 12 (1980) 97: c)
R. Birge, Annu. Rev. Biophys. Bioeng. I0 (1981) 315.
131 a) P. Blatz, J. Mohler, Biochemistry 14 (1975) 2304; b) M. Sheves, N.
Friedman, A. Albeck, M. Ottolenghi, ibrd. 24 (1985) 1260; c) M. Muradin-Szweykowska, J. A. Pardoen, D. Dobbelstein, L. J. P. Van Amsterdam, J. Lugtenburg, Eur. J . Biochem. 140 (1984) 173; d) J. L. Spudich,
D. A. McCain, K. Nakanishi, M. Okabe, N. Shimizu, H. Rodman, B.
Honig, Biophys. J.. in press; e) T. Baasov, M. Sheves, J. A m . Chem. Soc.
107 (1985) 7524; f ) G . Harbison, J. Herzfeld, R. Griffin, Biochemistry 22
(1983) I.
[4] a) G. Harbison, J . Herzfeld, S . 0. Smith, R. A. Mathies, J. Pardoen, J.
Lugtenburg, R. G. Griffin, Biophys. J. 47 (1985) 92a; b) G, Harbison, S .
0. Smith, J. Pardoen, J. M. L. Courtin, J. Lugtenburg, J. Herzfeld, R. A.
Mathies, R. G. Griffin, Biochemistry 24 (1985) 6955; c) T. Schreckenbach, 8. Walckhoff, D. Oesterhelt, ibid. 17(1978) 5353.
[5] F. Derguini, C . G. Caldwell, M. G. Motto, V. Balogh-Nair, K. Nakanishi, J. A m . Chem. Soc. I05 (1983) 646.
0 VCH Verlagsgesellschaf! mbH. D-6940 Weinheim. 1986
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[6] T. Kakitani, H. Kakitani, B. Honig, K. Nakanishi, J. Am. Chem. Soc. 105
(1983) 648.
[7] S. S . Malhotra, M. C. Whiting, J . Chem. Soc. 1960. 3812.
18) F. Tokunaga, T. Ebrey, Biochemistry 1 7 (1978) 1915.
[U] K. Nakanishi, V. Balogh-Nair, M. Arnaboldi, K. Tsujimoto, B. Honig, J.
Am. Chem. Soc. 102 (1980) 7945.
[lo] M. Klessinger, 77ieor. Chim. Acta 5 (1966) 251.
(b); formulated for octaethylporphyrin, Eqn. (b) then describes a redox system analogous to the “special pair”.
Indeed, the oxidation is realized on a preparative scale by
reaction of the Ce” double-decker Ce(TTP), with the hexachloroantimonate of the phenoxathiin radical cation[’] in
1,2-dichloroethane (DCE) according to Eqn. (c).
Remarkable Ease of Ring Oxidation in Cerium(iv)
Bisporphyrinates with Double-Decker Structure**
By Johann W. BuchIer,* Kyra Elsasser,
Martina Kihn- Botulinski, and Bernd Scharbert
Dedicated to Professor Hans Herloff Inhoffen on the
occasion of his 80th birthday
+ BPhe’
The ionized “special pair”, (BChl)?, is recognized by a
characteristic absorption band at about 1300nm in the
near infrared (NIR).[’]
We recently described sandwich-like lanthanoid bisporphyrinates, e.g., the cerium(1v) derivative of 5,10,15,20tetra@-tolyl)porphyrin, Ce(TTP),,[31 and the derivatives
of 2,3,7,8,12,13,17,18-octaethylporphyrin, Ce(OEP)2,’4.51
Pr(OEP)2,[41and Eu(OEP),,[~] which contain CerV,Pr”’,
and Eu”’, respectively, as central ions; in the case of the
trivalent lanthanoid ions, the formula Ln(OEP), can only
be explained in terms of one porphyrin ligand being present in the complexes as dianion (0EP)’O and the other as
radical monoanion OEPoe. The combination of two porphyrin systems in different oxidation states is recognized
in Pr(OEP)* and EU(OEP)~,inter aha, by characteristic absorption bands in the NIR at 1400 and 1280nm, respectively, which can be tentatively assigned to an internal
charge transfer (CTI).l6I
The finding that the cation (BChl)? in Eqn. (a) likewise
absorbs in the N I R at 1300 nm, suggests that these absorptions could be a common characteristic of such mixed valence tetrapyrrole dimers. In the meantime, the doubledeckers Ln(OEP), with all lanthanoids excepting Pm have
been prepared. With exception of the C e i Vderivative all
complexes show a NIR band whose wave number monotonously decreases with increasing ionic radius of the rareearth metal.@’ These observations finally prompted attempts to convert the neutral Ce” double-deckers likewise
into mixed valence cations by oxidation according to Eqn.
Prof. Dr. J. W. Buchler, DipLlng. K. Elsasser,
DipLlng. M. Kihn-Botulinski, DipLChem. B. Scharbert
lnstitut fur Anorganische Chemie der Technischen Hochschule
Hochschulstrasse 4, D-6100 Darmstadt (FRG)
Metal Complexes with Tetrapyrrole Ligands, Part 41. This work was
supported by the Deutsche Forschungsgemeinschaft, the Fonds der
Chemischen Industrie, and the Vereinigung van Freunden der Technischen Hochschule Darmstadt.-Pafi 40: [51.
0 VCH Verlagsgesellschaft mbH. 0-6940 Weinheim, 1986
After evaporation to dryness and redissolution in DCE/
toluene, blue-violet crystals of l a . 2 DCE separate. 2 and
3, the derivative of 5,10,15,20-tetraphenylporphyrin,are
obtained likewise. The salts are characterized by elemental
analyses and conductivity measurements, and by their
UV/VIS, IR, and NMR spectra.[*] As expected, NIR bands
are observed at 1340 nm ( l a ) , 1270 nm (2), and 1350 nm
(3) showing halfwidths of about 300 nm.
The redox potentials for the reversible first ring oxidation to l a or 2 are taken from the cyclic voltammograms
(Fig. 1). The waves appearing in the more positive potential region correspond to an oxidation to the dication; the
quasi-reversible waves lying in the negative region presumably indicate the reduction to Cell‘. Electrolysis of
Ce(TTP), in CH2C12/NBu4PF6at a controlled potential of
about 0.7 V affords 1bIB1after removal of the solvent and
recrystallization from acetone.
Very informative is a comparison of the redox potentials
(calomel electrode, CH,CI2/NBu,PF6) for the first ring oxidation. On comparison with the documented monoporphyrins the following series emerges: l T P complexes:
Zn(TTP): 0.81 V, Ce(lTP),: 0.62 V; TPP complexes:
Zn(TPP): 0.71 V,[lol Ce(TTP),: 0.68 V, Mg(TPP): 0.54V;[Io1
O E P complexes: Zn(0EP): 0.69 V, Mg(0EP): 0.53 V,
Ce(OEP),: 0.17 V. In general, these potentials decrease in
going from the zinc to the magnesium complexes;[’*.“]
Mg(0EP) belongs to the metal porphyrins that are particularly readily oxidized at the ring. In the case of the tetraarylporphyrins the value for the cerium double-decker lies
between those of the magnesium complex and the zinc
+ phenoxathiin
The structure of the reaction center of bacterial photosynthesis in Rhodopseudomonas viridis was recently elucidated by Huber et al.[’I This extensive membrane protein
complex proves to contain the long postulated “special
pair”, a bacteriochlorophyll b dimer (BChl)2, in which the
tetrapyrrole ligands cohere about 3
apart from each
other via 7c-n interactions of the two pyrrole rings I. According to present knowledge,“’ this “special pair” donates an electron to a neighboring bacteriopheophytin
monomer (BPhe) according to Eqn. (a) under the action of
light quanta.
(BChl)2+ BPhe + hv
+ [pheno~athiin]~%bCI~
+l 5
-1 0
Fig. I . Cyclic voltammograms of the cerium(1v) bi5porphyrinates C‘e(lTP)2
mol/L, and Ce(OEP)> (-),
1.2 x l o d 4mol/L; scan rate
(----), 1.0 x
100 mV/s, solvent CH2C12, supporting electrolyte NBu,PF,, Pt-disk electrode, calomel reference electrode.
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Angew. Chem Int. Ed. Engl.
2s (1986) No 3
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maxim, mode, compounds, symmetrically, external, influence, artificial, pigment, cyanine, stud, negativa, absorption, bacteriorhodopsin, charge
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