INTERMOLECULAR INTERACTIONS IN VISUAL PIGMENTS. THE HYDROGEN BOND IN VISLON

Thermodynamic and UV Studies of Association Reaction between Phenols and Benzil Mono Benzylidene Aniline

The study includes two complementary topics. Firstly, the influence of various concentrations of phenols on the UV absorption spectrum of benzil mono benzylidene aniline (BMBA) in different solvents. Experiments show a bathochromic shift in the UV spectrum of BMBA obtained after addition of phenol. When phenol is replaced by 2,4-dimethylphenol, rather similar results are observed. The results are interpreted interms of hydrogen bond formation between BMBA and phenol. Secondly, it is decided to confirm the hydrogen-bond formation stated above by thermodynamic method. Accordingly, the effect of temperature on the equilibrium constant value of the interaction between BMBA and phenol is considered. Evaluated thermodynamic parameters Δ

Chromophore-anion interactions in halorhodopsin from Natronobacterium pharaonis probed by time-resolved resonance Raman spectroscopy

Halorhodopsin of Natronobacterium pharaonis which acts as a light-driven chloride pump is studied by time-resolved resonance Raman spectroscopy. In single-beam experiments, resonance Raman spectra were obtained of the parent state HR578 and the first thermal intermediate HR520. The parent state is structural heterogeneous including ca. 80% all-trans and 20% 13-cis isomers. The resonance Raman spectra indicate that the all-trans conformer exhibits essentially the same chromophoric structure as in the parent states of bacteriorhodopsin or halorhodopsin from Halobacterium salinarium. Special emphasis of the resonance Raman spectroscopic analysis was laid on the C=C and C=N stretching region in order to probe the interactions between the protonated Schiff base and various bound anions (chloride, bromide, iodide). These investigations were paralleled by spectroscopic studies of retinal Schiff base model complexes in different solvents in an attempt to determine the various parameters which control the C=C and C=N stretching frequencies. From these data, it was concluded that in the parent state the anion is not involved in hydrogen bonding interactions with the Schiff base proton but is presumably bound to a nearby (positively charged) amino acid residue. On the other hand, the anion still exerts an appreciable effect on the chromophore structure which is, for instance, reflected by the variation of the isomer composition in the presence of different anions and in the anion-depleted form. In contrast to the parent state, the intermediate HR520 reveals frequency shifts of the C=N stretching in the presence of different anions. These findings indicate a closer proximity of the bound anion to the Schiff base proton which is sufficient for hydrogen bonding interactions. These changes of the anion-chromophore interaction upon transition from HR578 to HR520 may be related to the coupling of the chromophore movement with the anion translocation.

Relative ground and excited state energies of CH3(CH = CH)5CH = NC4H9, its hydrogen-bonded and proton-transferred species, and charge partitioning and distribution in the protonated Schiff base of retinal

CH3(CH = CH)5CH = NC4H9 (compound 1) is structurally related to the Schiff base of retinal, the prosthetic group in visual pigments. Dilute solutions of a weak acid (phenol) and 1 in a hydrocarbon solvent, when subjected to decreasing temperature, show striking changes in electronic absorption spectra. Initially only the spectrum of compound 1 is present, but as the temperature is lowered, the absorbance of 1 decreases, and the spectrum of the H-bonded form of 1 appears and increases. Continued temperature lowering then causes a decrease in absorption of the H-bonded form and an appearance and rise in absorption of the proton-transferred form of 1. Concentrations of the various species are measured as a function of temperature, and by standard procedures, the thermodynamic constants for both reaction steps are computed. Values of delta H0 are taken as relative energies among the three ground states, and the lambda max value of each species yields relative energies among excited states. By employing data from electronic absorption spectroscopy, nuclear magnetic resonance (NMR) and theoretical calculations for retinal Schiff base, charge partitioning between nitrogen and the polyene chain and charge distribution among the carbon atoms of the polyene chain are calculated.

The protonation of a retinyl Schiff base: a study by FTIR spectroscopy at low temperature in solution

The hydrogen bonding-protonation equilibrium for retinyl Schiff base/propionic acid or 3-chloropropionic acid systems was examined by Fourier transform infrared spectroscopy in non polar solutions at temperatures ranging from 25 degrees C to about -150 degrees C. The spectra give evidence for the gradual increase in the degree of protonation as temperature is lowered. The bearing of this on applying low temperature spectroscopic results to physiological conditions in rhodopsin research is discussed.

Fourier-transform infrared spectroscopy applied to rhodopsin. The problem of the protonation state of the retinylidene Schiff base re-investigated

By measuring the rhodopsin--bathorhodopsin, isorhodopsin--bathorhodopsin, rhodopsin--isorhodopsin and rhodopsin--meta-II difference spectra with the method of Fourier-transform infrared spectroscopy we have identified the C = N stretching vibration of the protonated retinylidene Schiff base of rhodopsin, isorhodopsin and bathorhodopsin. In contrast to resonance Raman spectroscopy additional strong bands were observed between 1700 cm-1 and 1620 cm-1. Most of them depend on the isomeric state of the chromophore. The origin of these bands will be discussed. In the fingerprint region isorhodopsin and bathorhodopsin are quite similar but no similarities with infrared spectra of model compounds of any isomeric composition are observed. Therefore, no conclusions on the isomeric state of the retinal in bathorhodopsin can be drawn. We provide evidence for the modification of one or two carboxylic group(s) during the rhodopsin--bathorhodopsin and isorhodopsin--bathorhodopsin transition.

Investigation of the primary photochemistry of bacteriorhodopsin by low-temperature Fourier-transform infrared spectroscopy

The method of Fourier-transform infrared difference spectroscopy was applied to investigate the transition at 77K of bacteriorhodopsin in its light-adapted form to K6(10), the first intermediate which is stable at low temperature. In addition to unmodified bacteriorhodopsin, bacteriorhodopsin in 2H2O and bacteriorhodopsin containing [15-2H]retinal was used. The results show that major rearrangements occur in the Schiff base in this transition. It is not possible to identify a C = N stretching vibration of the Schiff base in K6(10). The identification of an N-H bending vibration in K6(10) shows that the nitrogen of the previous Schiff base still has a proton attached. The fingerprint region exhibits very unusual features for K6(10) and bears no similarity to protonated retinylidene Schiff base model compounds of any isomeric composition. Therefore, no conclusions on the isomeric state of the retinal in K6(10) can be drawn. The spectra show that the terminal part of the retinal is predominantly reflected in the difference spectra. This indicates that the most polar part of the retinal is located near the Schiff base. We have evidence for protein molecular changes occurring in this transition at 77K.

On the rise time of the R1-component of the "early receptor potential": evidence for a fast light-induced charge separation in rhodopsin

The rising phase of the R1-component of the early receptor potential from isolated cattle retinas was measured with high time resolution. When the measuring capacitance was 133 pF, a latency of about 200 ns was observed. A rise time of about 0.8 mus at 0 degrees C and 1.6 mus at 37 degrees C (extrapolated to ideal measuring conditions) was found. The negative temperature dependence indicates that the rise is not directly related to the production and decay of photolysis products of rhodopsin since the latter have positive temperature coefficients. An increase of the external measuring capacitance caused a slower rise time. The analysis of this effect allowed the determination of the source impedance of the R1-component. The experimental results can be described with a model in which it is assumed that a fast charge separation (ns or ps) takes place in the outer segment of a photoreceptor cell, and spreads passively to the inner segment via the resistance of the interconnecting cilium. The "inner" relaxation could be circumvented by using isolated rod outer segments which lack the passive inner segments, i.e., a rise time of 90 ns could be measured when isolated rod outer segments were attached to Millipore filters. The results suggest that the molecular event leading to the R1-component is an early charge separation which may be as fast as the cis-trans isomerization of the retinal chromophore.

Investigations of the rhodopsin/Meta I and rhodopsin/Meta II transitions of bovine rod outer segments by means of kinetic infrared spectroscopy

We have applied our recently developed technique of flash induced kinetic infrared spectroscopy to the rhodopsin/Meta I and rhodopsin/Meta II transitions. Features of the infrared spectrum reflecting the C = C-vibration and the isomeric form of the chromophore are in agreement with resonant Raman experiments. Different results are obtained for the C = N-vibration of the Schiff base retinal opsin link. They are interpreted in terms of a Schiff base protonated via an hydrogen bond. A proton transfer in the excited state is suggested to explain the deviating results. In addition we have obtained spectral changes which cannot be attributed to molecular changes in the chromophore. We assume that these spectral features reflect molecular events in the protein part of rhodopsin.