Jerzy Olejnik

A proton pathway with large proton polarizability and the proton pumping mechanism in bacteriorhodopsin — Fourier transform difference spectra of photoproducts of bacteriorhodopsin and of its pentademethyl analogue

Abstract The first part of the photocycle of bacteriorhodopsin (BR) (BR 570 , K 630 , L 550 and M 412 ) is studied by difference Fourier transform-infrared (FT-IR) spectroscopy. These results are compared with the respective difference spectra of the pentademethyl analogue (BR a ). With the BR intermediates, the same bands are observed as in previous studies. This is especially true in the carbonyl region. With BR a , however, the behaviour of the intermediates is different from the respective intermediates of BR. The photocycle is interrupted before the L state and returns to BR a . In the case of BR, in the K intermediate, a very broad band is observed in the region 2800-2200 cm −1 . In the L intermediate, a continuous absorption is observed beginning at 2800 cm −1 and extending toward smaller wavenumbers over the whole region studied. This continuum vanishes with the formation of the M 412 intermediate and, instead, two very broad bands are found in the region 2700-2200 cm −1 . The broad band in the K state indicates that at least one strong hydrogen bond is formed in which the proton is not well localized. From the continuum observed in the L intermediate, it is concluded that a hydrogen-bonded chain is formed showing large proton polarizability caused by collective proton motion. This chain is discussed on the basis of a structural model based on literature data. The Schiff base—Asp 85 and Tyr 185—Asp 212 bonds show proton polarizability. With the transition to the intermediate M 412 , within both hydrogen bonds, the protons are shifted within these hydrogen bonds to Asp 85 and Asp 212, due to changes of the local electrical fields and to specific interactions arising due to conformational changes of the protein. In this way the negative charges in the neighbourhood of Arg 82 are neutralized and hence the proton potential at Arg 82 raised. Thus, the positive charge is shifted to the outside of the proton channel and released to the bulk water phase. The continuum is no longer observed in M 412 since the proton potentials in the chain are now asymmetrical. Hence, in the M 412 intermediate only broad bands are found, indicating strong but asymmetrical hydrogen bonds present in the active centre.

On hydrogen and deuterium bonds as well as on Li+, Na+ and Be2+ bonds: IR continua and cation polarizabilities

Abstract Experimentally observed and calculated infrared (IR) continua of the homoconjugated hydrogen bonds in H 5 O + 2 or deuterium bonds in D 5 O + 2 , both of which show large polarizability, are compared. With N + H⋯N⇌N⋯H + N bonds, for instance, the far IR continua give information on how strongly these bonds are polarized to one or the other side. In the case of heteroconjugated AH⋯B ⇌ A − ⋯H + B bonds it is shown how the reaction field creates two minima within these bonds. The wavenumber-dependent intensity distribution of the IR continua is discussed as a function of the length and strength of the hydrogen bonds. Li + , Na + and Be 2+ bonds also show cation polarizabilities, due to shifts of these cations within the bonds formed by them. The wavenumber-dependent far IR intensity distribution of the continua caused by homoconjugated Li + bonds is discussed as a function of the length and strength of these bonds. Furthermore, it is shown that with O − Li + ⋯ON ⇌ O − ⋯Li + ON bonds the equilibria can be shifted as a function of the electron density at the acceptor or donor atoms, respectively. These shifts are analogous with such shifts in hydrogen bonds as a function of Δp K a . The wavenumber-dependent far IR intensity distribution with homoconjugated polarizable O − Na + ⋯ − O ⇌O − ⋯Na +− O bonds is also discussed. In the case of heteroconjugated Na + bonds it is shown that the fluctuation frequency may be so small that the coalescence point is in the far IR region. It is demonstrated that large cation polarizabilities due to collective cation motion are not specific for chains of hydrogen bonds, but occur also in chains of two Li + or Na + bonds. H + ,D + ,Li + and Na + bonds with large cation polarizabilities are compared in terms of the spectral regions in which the continua of these bonds occur and in terms of the size of their cation polarizabilities. Finally, the Be 2+ polarizability due to the motion of Be 2+ between four acceptors is discussed with three types of system. It is shown that the broad absorption of the Be 2+ bonds is shifted toward higher wavenumbers. Because of the high affinity of Be 2+ ions for the acceptor groups, the potential has a steep slope at the acceptors and is relatively narrow.

1H NMR studies of gossypol and its complexes with some organic compounds

Abstract The 1H NMR spectra of gossypol and its complexes with some organic compounds in various solvents have been analyzed. In CDCl3 solution gossypol exists as the (±) aldehyde tautomer. In other solvents, aldehyde and lactol tautomers are in equilibrium, depending on the nucleophilicity of solvent (B). In the case of complexes of gossypol with organic compounds, the equilibrium is shifted to the aldehyde form, and the (±) diastereoisomers were observed in the 1H NMR spectrum due to asymmetric nonspecific interactions. Two types of tetrabutylammonium complexes of gossypol, 2:1 and 4:1 have been prepared. The spectrum of the 2:1 salt points to a very complex equilibrium in solution without a ketol tautomer. In this case a new type of intramolecular hydrogen bond is formed. Ketol is the main tautomeric form of the 4:1 salt, which shows a very strong intermolecular OHO− type hydrogen bond.

Fourier transform infrared study on the identification of gossypol tautomers

Abstract Gossypol and its adducts with some organic compounds, as well as its two tetrabutylammonium salts (2:1 and 4:1), were studied in various solutions by Fourier transform IR (FTIR) spectroscopy. In CH 2 Cl 2 solution gossypol exists as the aldehyde tautomer. In the DMSO-d 6 solution an equilibrium between the aldehyde and lactol tautomers was observed in which the predominant form is the latter. The IR spectrum of the 2:1 tetrabutylammonium salt indicates that a very complex equilibrium occurs in CD 3 CN solution. In the case of the 4:1 salt the ketol tautomeric form is associated via very strong intermolecular (O⋯⋯H⋯⋯O) − hydrogen bonds with large proton polarizability.

An intramolecular hydrogen-bonded system with large proton polarizability — a model with regard to the proton pathway in bacteriorhodopsin and other systems with collective proton motion

Abstract 3-Diethylaminomethyl-2,2′-biphenol was synthesized and studied by FT-IR and 1 H NMR spectroscopy. The compound forms a system with two hydrogen bonds which shows large proton polarizability due to collective proton motion. This result supports our earlier suggestion that the first part of the proton pathway in bacteriorhodopsin conducting protons is a hydrogen-bonded chain with large proton polarizability built up by arginine and tyrosine residues. Furthermore, we show that in the monotetrachloroaurate of 3,3′-bis(diethylaminomethyl)-2,2′-biphenol and in the tritetrachloroaurates of 3,3′,5,5′-tetrakis(diethylaminomethyl)-2,2′-biphenol there is proton polarizability due to collective proton motion.

Discussion of the proton potential with proton-transfer equilibria: thermodynamic data and infrared continua as a function of temperature

The infrared continua caused with intramolecular hydrogen bonds of the type AH⋯B ⇌ A–⋯H+B have been studied as a function of the ΔpKa of the hydrogen-bond donor and acceptor as well as a function of the temperature. From the behaviour of these continua it is shown that the shape of the proton potential may change with decreasing temperature from a double minimum with the deeper well at the donor to proton potential with the deeper well at the acceptor. The In (KPT) over 1/T plots result, however, always in negative apparent ‘ΔH° values’. Hence with all such proton-transfer hydrogen bonds the deeper well of the double-minimum proton potentials should always be at the acceptor group.Using the reaction-field theory, it is shown that for proton-transfer equilibria the slope of these In KPT vs. 1/T plots remains linear since ∂P/∂T, i.e. the derivative of the Onsager parameter P against the temperature T is constant if it is considered with not too large a temperature range, and since ΔH° changes with temperature by a term which is linear in T. Therefore, from the In (KPT)vs. 1/T plots apparent ‘ΔH° values’ are obtained from which the true values can be calculated if ∂P/∂T is known.

Intramolecular O-Li+...ON⇄O-...Li+ON bonds with large Li+ polarizability. A FTIR study

Abstract Five Li+ salts of (4-R-2-pyridyl-N-oxide) acetic acid were studied by FTIR spectroscopy. Intramolecular O−Li…ON ⇌O−…Li+ ON bonds with large so-called Li+ polarizability are formed. The double minimum cation potential has changed in the −OC2H5 substituted compound to a broad flat single minimum, i.e. the nature of the bond to the O−…Li+…ON type.

Intramolecular Na+ bonds with double minimum potential wells

Abstract Four Na+ salts of (4-R-6-tert-butyl-pyridyl-N-oxide) acetic acids are studied by FTIR spectroscopy. An intramolecular (I) COO−Na+...ON⇌COO−...Na+ON (II) bond is present. Within the series of compounds R = NO2, Br, H, CH3 the νCO character of the carbonyl vibration in structure I decreases and the equilibrium shifts more and more to the right.

Phenol—retinal schiff base hydrogen bonds—influence of steric hindrance and phenol acidity on the thermodynamic data of formation and proton transfer

Abstract CD2Cl2 solutions of four trans-retinal Schiff base (containing methylamine, n-butylamine, t-butylamine and 5-butyl-nonylamine), their perchlorates and their 1:1 complexes with 3,4-disubstituted and 4-monosubstituted phenols were studied as a function of temperature using Fourier transform infrared (FTIR) spectroscopy. The thermodynamic quantities ΔH0F and ΔS0F of hydrogen-bond formation and ΔH0PT and ΔS0PT of the proton transfer process across the hydrogen bonds were determined. All (I) OH⋯N⇌O−⋯H+N (II) bonds show large proton polarizability as indicated by continua in the IR spectra. In all the systems studied, the ΔH0PT and ΔS0PT values of the (I) OH⋯N⇌O−⋯H+N (II) equilibrium were negative. This is a result of the highly ordered arrangement of the solvent molecules around these complexes. ΔH0F and ΔS0F of formation of the OH⋯N⇌O−⋯H+N bond and ΔH0PT and ΔS0PT decrease significantly with decreasing acidity of the phenol. Furthermore, ΔH0PT and ΔS0PT decrease significantly with increasing screening of the hydrogen bonds from the solvent environment by bulky groups. This indicates that the ΔH0PT and ΔS0PT values are determined by the interaction of the hydrogen bonds with their environment.

Change of the proton potential from a single to a double minimum in intramolecular hydrogen bonds with increasing pKa of the phenolic group in 4-R-Mannich base N-oxides

Abstract Nine 4-R-Mannich base N -oxides have been studied by FTIR spectroscopy. The intramolecular hydrogen bonds formed between the phenolic and the N -oxide group show proton polarizability. If the phenolic group is relatively acidic a broad flat single minimum proton potential with the well at the NO group is found, whereas if the phenolic group is more basic a double minimum proton potential is observed. An analogous change of the shape of the proton potential was observed earlier in the case of carboxylic acid/trimethylammonium N -oxide systems if the acidity of the carboxylic acid increases. The reasons for this change of the proton potential are explained.