J. Phys. Chem. 1987, 91, 3077-3080
3077
Proton Polarizability Caused by Collective Proton Motion in Intramolecular Chains Formed by Two and Three Hydrogen Bonds. Implications for Charge Conduction in Bacteriorhodopsin Bogoumil Brzezinski, Department of Chemistry, A. Mickiewicz University, 60- 780 Poznari, Poland
Georg Zundel,* and Rainer Kramer Institute of Physical Chemistry, University of Munich, 0-8000 Munich 2, FRG (Received: December 23, 1986)
The tetrabutylammonium monosalts of the following compounds were studied by IR and NMR spectroscopy in acetonitrile-d3 solutions: 2,6-di(hydroxymethyl)phenol, 1; 2-hydroxyisophtalicacid, 2; 11,12-dihydroxy-1,10-naphthacenedicarboxylicacid, 3. In all compounds chains of intramolecular hydrogen bonds are formed. In the chain of 1 the protons are localized at the alcoholic groups. Compounds 2 and 3 must be represented by three proton-limiting structures. In these compounds triple minimum proton potentials are present for the proton motion in the hydrogen-bonded chains, whereby for 3 the central minimum is a broad flat well. In 2 the central minimum is deeper but nevertheless the hydrogen-bonded system shows considerable proton polarizability. In 3 due to the presence of an additional phenolic OH group the chain is extended by one hydrogen bond. A three-minimum proton potential occurs whereby the middle minimum is very broad and flat. The collective fluctuation of the three protons is very fast and the hydrogen-bonded chain shows large proton polarizability. From these results it is presumed that the much longer hydrogen-bondedchain postulated for charge conduction in bacteriorhodopsin should show a very large proton polarizability.
Introduction In a previous study of the tetrabutylammonium monosalt of 1,2,3-benzenetricarboxylicacid' it was shown that an intramolecular hydrogen-bonded system formed by one carboxylate and by two carboxylic acid groups shows a large proton polarizability2" as indicated by the intense continua in the IR spectra. This study confirmed a self-consistent field treatment that hydrogen-bonded chains may show particularly large proton polarizabilities caused by collective proton m ~ t i o n . Furthermore, ~ it was shown in ref 1 that this carboxylic acid-carboxylic acid-carboxylate system is represented by three proton-limiting structures, whereby the weights of these structures are a function of the temperature.' Other systems with hydrogen-bonded chains showing particularly large proton polarizabilities are formed by side chains of proteins with phosphate group^.^,^ Such hydrogen-bonded chains are of particular interest for charge conduction in biological membranes. It was postulated that in the case of bacteriorhodopsin the positive charge is conducted from the active side to the outside of the membranes by a hydrogen-bonded chain formed by one aspartic acid, six tyrosines, and one glutamate In the following, two compounds with two intramolecular hydrogen bonds are studied. Furthermore, the system with two intramolecular bonds is compared with a similar system with a chain of three hydrogen bonds. Experimental Section Compounds 1-3 were synthesized by following the procedures given in ref 8-10, The tetrabutylammonium monosalts of all (1) Brzezinski, B; Zundel, G.; Krlmer, R. Chem. Phys. Lett. 1986, 124,
395. (2) Weidemann, E. G.; Zundel, G. Z . Naturforsch. A 1970, ZSa, 395. (3) Janoschek, R.; Weidemann, E. G.; Pfeiffer, H.; Zundel, G.J . Am. Chem. SOC.1972, 94, 2387. (4) Fritsch, J.; Zundel, G.; Hayd, A.; Maurer, M. Chem. Phys. Lett. 1984, 107, 65. ( 5 ) Zundel, G.; Merz, H.; Burget, U. In: ATPase Structure, Function, Biogenesis, the F$, Complex of Coupling Membranes, Papa, S . , Altendorf, K.,Ernster, L., Packer, L., Eds.; Adriatica Editrice: Bari, 1984. (6) Zundel, G. In Methods in Enzymology, Vol. 127, Part 0, Packer, E., Ed.; Academic: New York, 1986. (7) Mcrz, H.; Zundel, G. Biochem. Biophys. Res. Commun. 1981, 101, 540. ( 8 ) Cartwright, N. J.; Jones, J. I.; Marmion, D. J. Chem. Soc. 1952, 3499.
0022-3654/87/2091-3077$01 .50/0
TABLE I: Chemical Shifts (ppm) of OH and the Ring hotons of the Tetrabutylammonium Monosalt of 1-3 OH ~
compd
0.05
0.1
M
M
0.2 M
0.3 M
ring protons 4-H
5-H
6-H
1 2
11.02 11.02 11.02 11.02 7.24 17.07 17.07 17.07 17.07 8.87
7.24 8.16
7.24 8.87
3
17.47 17.47
2 - and 9-H 8.16
3- to 8-H 7.67
compounds were prepared by addition of an equimolar amount of a 1 M methanol solution of tetrabutylammonium hydroxide to a solution of the free acid in 100% ethanol. The solvents were removed under reduced pressure and the residue crystallized from acetonitrile plus ethyl ether. All manipulations were carried out in a carefully dried glovebox under a nitrogen atmosphere. The IR and N M R spectra were recorded of solutions in CD3CN which were stored over a 3-A molecular sieve. IR spectra were taken by using a cell with Si windows (sample thickness 0.125 or 0.240 mm). The cells were described in ref 1 1. The IR spectra were taken with a PerkinElmer 325 spectrophotometer. The N M R spectra were plotted with a Varian HA-100 spectrometer and calibrated against Me4Si as internal standard.
Results and Discussion The tetrabutylammonium monosalts of the following compounds 1-3 were studied in acetonitrile-d3 solutions using N M R and IR spectroscopy: 2,6-di(hydroxymethyl)phenol, 1; 2-hydroxyisophtalic acid, 2; 11112-dihydroxy-1,lO-naphthacenedicarboxylicacid, 3. In Table I the N M R data of the tetrabutylammonium monosalt solutions are given. In Figures 1 and 2 the IR spectra of the monosalt solutions are compared with the respective acids. 2,6-Di(hydroxymethyl)phenoZ, I . In the spectrum of the phenol (Figure 1, broken line) in the region 3600-3200 cm-' the uOH vibrations of the alcoholic and phenolic groups are observed; the maximum of the free OH groups is found at 3500 cm-' and that (9) Todd, D.; Mortell, A. E.; Mcknsick, B. C.; Andereades, S. Org. - Syn. .
1960,40, 48.
(10) Brzezinski, B.; Jedruszek, T. Pol. J . Chem., in press. (1 1) Kramer, R.; Zundel, G. Z . Phys. Chem. (Frankfurt om Main) 1985, 144, 265.
0 1987 American Chemical Society
3078 The Journal of Physical Chemistry, Vol. 91, No. 11, 1987
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WAVENUMBER [l/CM ] Figure 1. IR spectra of 0.20 M acetonitrile-d, solutions (sample thickness 0.125 mm) of 1 (---)
and its tetrabutylammonium monosalt
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WAVENUMBER [1/CM] Figure 2. IR spectra of acetonitrile-d3solutions of (a, top) 2 (---) and its tetrabutylammonium monosalt (-) (sample thickness 0.125 mm, concentration 0.2 M) and (b, bottom) 3 (---) and its tetrabutylammonium monosalt (-) (sample thickness 0.240 mm, concentration 0.06 M).
of the hydrogen-bonded groups is at 3300 cm-'. This band complex vanishes completely with monosalt formation (Figure 1, solid line) although two hydrogen-bonded OH groups are still present. The NMR signal of these protons is shifted to 11.02ppm, Le., toward lower field, and the position of this signal is concentration independent (Table I). These facts show that both OH groups form hydrogen bonds, Le., systems of two hydrogen bonds are present in the molecules. The stretching vibration of the two hydrogen-bonded OH groups is observed as a very broad absorption in the region 3200-1750 cm-' with a maximum near 2300 cm-I. Intense phenolate bands occur in the spectrum of the monosalt, for instance, at 1305,1275,
and 1250 cm-'. All these results taken together show that the protons are localized at the alcoholic groups and the system is described by only one proton-limiting structure:
The fact that the protons are largely localized at the alcoholic groups is confirmed by the NMR data. The NMR signal of the CHI protons remains a doublet at 3.17 ppm. If the proton would
The Journal of Physical Chemistry, Vol, 91, No. 11, 1987 3079
Proton Polarizability
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WAVENUMBER [ l/CM] Figure 3. IR spectra of a 0.20 M acetonitrile-d3solution of tetrabutylammonium salt of 2 at 60 (-) and -30 OC (---). fluctuate only one singlet for the CH2protons would be obser~ed.'~ Thus, since no unusual charge fluctuation in the system occurs no IR continuum is found (Figure 1) and hence the proton polarizability of the hydrogen-bonded system is very small. The behavior of this hydrogen-bonded system is similar to that of the system in 2,6-pyridinedicarboxylic acid N-oxide.' 2-Hydroxyisophtalic Acid, 2. Figure 2a shows the IR spectrum of acetonitrile-d, solutions of the tetrabutylammonium monosalt of 2. For comparison the respective spectrum of a solution of the acid is given. Figure 3 shows the carbonyl region as a function of temperature. The N M R signal of the hydrogen-bonded protons is found at 17.07 ppm and is concentration-independent (Table I). This result shows that the OH groups form a system of two intramolecular hydrogen bonds. A comparison of the IR spectra of this monosalt with that of the acid in Figure 2a shows that the broad band complex in the region 3500-2500 cm-', caused by the VOH vibrations of the acid, vanishes if the hydrogen-bonded chain in the monosalt is formed. A continuous absorption occurs which begins at about 3000 cm-' and extends over the whole region toward smaller wavenumbers. It shows two less pronounced bandlike structures with maxima near 2500 and 1850 cm-l. This continuum demonstrates that the hydrogen-bonded chain shows proton polarizability due to collective proton motion. Figures 2a and 3 show the following: the carboxylic groups of the hydrogen-bonded chain cause an intense vcpo vibration at 1680 cm-', a v, vibration of -COT groups at 1650 cm-', and, in vibration at 1725 cm-*. addition, a weak shoulder of a second -v The intensity of the latter two vibrations increase with decreasing temperature. Phenolate bands arise with salt formation, for instance, a t 1312 and 1298 cm-'. All these results taken together show that the hydrogen-bonded system must be represented by the following three proton-limiting structures:
I
I1
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The intense v M vibration at 1680 cm-l as well as the phenolate bands show that proton-limiting structure I1 has a relatively high ~
~~
~
(12) Brzezinski, B.; Zundel, G. Can. J. Chem. 1981, 95, 458. J . Chem. SOC.,Faraday Trans. 2 1981, 77, 1101. Chem. Phys. Lett. 1983, 95, 458.
weight. Nevertheless, the weight of structures I and I11 is, especially at lower temperatures (Figure 3), also considerable as shown by v,,,co2- at 1650 cm-' and the additional v M vibration a t 1725 cm-'. The 4-H and 6-H ring protons cause only one doublet signal at 8.87 ppm whereas the 5-H proton causes a triplet at 8.16 ppm (Table I). The low-field shift shows the deshielding of these protons by the anisotropic effect.of the carbonyl groups, and the fact that only one doublet of 4-H and 6-H is observed confirms the fast charge fluctuation.'* The proton potential of the hydrogen-bonded chain is a triple-minimum potential with a deeper well corresponding to proton-limiting structure I1 and two higher wells corresponding to structures I and 111. Caused by the charge fluctuation in this hydrogen-bonded chain it shows considerable proton polarizability. 11,I 2-Dihydroxy-1 ,IO-naphthacenedicarboxylic Acid, 3. Figure 2b shows the I R spectrum of acetonitrile-d3 solutions of the tetrabutylammonium monosalt of 3. For comparison the spectrum of the respective acid is given. Parts a and b of Figure 4 show two regions of the spectra of the monosalt at two different temperatures. The N M R signal of the hydrogen-bonded protons of the monosalt is found at 17.47 ppm and is concentration-independent (Table I). This result shows that all OH groups form systems of three intramolecular hydrogen bonds. The intense band of the vOH vibrations observed in the region 3600-2800 cm-' in the spectrum of the acid vanishes completely with monosalt formation (Figure 2b). Instead of this band a continuum is found which extends from about 2500 to 400 cm-'. This continuum demonstrates that the charge fluctuates rapidly in the hydrogen-bonded chain, and thus, this chain shows large proton polarizability. A vc4 vibration is found at 1700 cm-I, a vibration at 1652 cm-I, and the respective v, vibration at 1380 cm-I. In addition, a vcpo vibration is observed at 1730 cm-I. vC4 of phenolate groups is found a t 1345 cm-' and a ring vibration at 1605 cm-I. Particularly important is, however, the fact that phenol bands are no longer observed in the region 1600-1000 cm-' with the exception of a weak shoulder at about 1400 cm-' (Figure 4). All these results taken together show that this hydrogen-bonded system must be represented by the following three proton-limiting structures:
I
I11
Herewith, proton-limiting structures I and I11 cause the v c ~ vibration at 1700 cm-' and v,, and vs at 1652 and 1380 cm-', respectively. Furthermore, the weak shoulder at 1400 cm-l may be assigned to a phenol ring vibration in these structures. The other phenol bands have probably vanished because of the very fast charge fluctuation within the hydrogen-bonded chain. The ring vibration at 1605 cm-' and the phenolate vibration near 1345 cm-' are caused by proton-limiting structure 11. The additional of this proton-limiting vibration at 1730 cm-'is caused by the vstructure since its intensity decreases also with decreasing temperature. The changes of the intensities of all these bands with temperature demonstrate (Figure 4) that with decreasing temperature
3080 The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 ?I
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Figure 4. IR spectra of a 0.06 M acetonitrile-d3 solution of tetrabutylammonium salt of 3 in the region (a, left) 1750-1550 cm-' and (b, right) 1420-1300 cm-'; 65 OC (-), -30 OC (---).
the weight of limiting structures I and 111 increases whereas that of limiting structure I1 decreases. With regard to proton-limiting structure I1 it is supposed that the central proton of the chain moves in a broad single minimum potential. If this proton would fluctuate in a double minimum instead of structure 11, two proton-limiting structures would be realized. The assumption of a single minimum is, however, supported by the following facts: (1) only one ring vibration is found at 1605 cm-'; (2) in phenol-phenolate systems one broad flat single-minimum proton potential is p r e ~ e n t 'and ~ ? ~with ~ these systems also IR continua are observed which are particularly intense in the region below 1600 cm-', similar to that observed in the spectrum (solid line in Figure 2b). Thus, the potential of the proton motion in the chain is probably a triple minimum whereby the central minimum would be very broad and flat. With regard to the weights of the three protonlimiting structures the well representing structure I1 should be energetically slightly higher than the other two. In such a system the charge can easily fluctuate because of the coupling of the motion of the three protons via proton dispersion force^,'^,^^ explaining the large proton polarizability of this hydrogen-bonded chain. Comparison of Systems 2 and 3. In both monosalts the charge can fluctuate in the hydrogen-bonded chain. In system 2 the central minimum is lower than the other two minima. Nevertheless, the charge can fluctuate and the hydrogen-bonded chain shows considerable proton polarizability due to collective proton motion. For compound 3 a third hydrogen bond is present in the chain. Hence, in this case, the motion of three protons is coupled. The motion of the central proton proceeds probably in a broad flat potential well and therefore also in this system only a triple minimum proton potential is present. The central well is slightly higher compared with the two other wells. But caused by the coupling of the motion of three protons via proton dispersion forces instead of only two protons, the fluctuation of the charge in this chain is faster and this hydrogen-bonded chain shows a large proton polarizability caused by the collective proton motion. The fact that the proton polarizability of such a hydrogen-bonded chain increases strongly with increasing chain length was recently (1 3) Pawlak, Z.; Sobczyk, L. In Advances in Relaxation Processes, Vol. 6; Elsevier: Amsterdam, 1973. (14)Zundel, G.In The Hydrogen Bond-Recent Developments in Theory and Experiments, Vol. 2, Schuster, P.;Zundel, G.; Sandorfy, C., Ed.; North-Holland: Amsterdam, 1976. (15) Weidemann, E. G.; Zundel, G. 2.Phys. 1967,288. (16) Weidemann, E.G. In The Hydrogen Bond-Recent Development in Theory and Experiments, Vol. 1, Schuster, P., Zundel, G . , Sandorfy, C., Ed.; North Holland: Amsterdam, 1976.
confirmed by calculations with model p0tentia1s.l~ In both compounds with decreasing temperature the protontransfer equilibria are shifted in favor of the proton-limiting structures in which a carboxylate group is present. Thus, the standard enthalpy AHo of the transfer equilibria phenolatecarboxylate structure must be negative.17~'* In the classical approximation AHo is the difference between the minima of the proton potentials. Thus, this result confirms that the minima of the proton potential corresponding to proton-limiting structures in which carboxylate groups are present are lower.
Conclusions In 1 the two protons are localized at the alcoholic groups and as with the earlier studied hydrogen-bonded chain in 2,6-dicarboxylic acid N-oxide] no fluctuation of charge in the intramolecular chain is possible. Thus, it shows no noticeable proton polarizability. It is shown that 2 and 3 must be described by three protonlimiting structures. Three-minimum proton potentials are present within these systems. In 2 the central well is deeper, but nevertheless due to charge fluctuation within the hydrogen-bonded chain it shows considerable proton polarizability. In 3 an additional hydrogen bond is present. In this system the proton potential shows also only three minima since the proton potential in proton-limiting structure I1 shows a broad flat well. In the triple minimum of the proton potential the minimum of the well corresponding to proton-limiting structure I1 is energetically slightly higher than those corresponding to structures I and 111. The charge fluctuation in the hydrogen-bonded chain is very fast and the chain shows a large proton polarizability. Thus, one can presume that if that chain would be enlarged by additional hydrogen-bonding phenolic groups the proton polarizability of such chains would further increase. The latter result was recently confirmed by calculations using model p0tentia1s.I~ Hence, the postulated hydrogen-bonded chain in bacterior h ~ d o p s i n ,built ~ . ~ from one aspartic acid group, six tyrosins, and one glutamate, should have a very large proton polarizability. Acknowledgment. Our thanks are due to the Deutsche Forschungsgemeinschaft and to the Fonds der Chemischen Industrie for providing the facilities for this work. Registry NO. l.NBud+, 107453-83-8;2*NBu4+,107453-85-03*NBu4+, 107453-87-2;H+, 12408-02-5. (17) Zundel, G.;Fritsch, J. J . Phys. Chem. 1984,88, 6295. (1 8) Zundel, G.;Fritsch, J. In Chemical Physics of Soluation, Vol. 2, Dogonadze, R.R.;KBlmBn, E.; Kornyshev, A. S.; Ulstrup, J., Ed.; Elsevier: Amsterdam, 1986. (19) Eckert, M.;Zundel, G., manuscript in preparation.
