Factors affecting the C:N stretching in protonated retinal Schiff base: a model study for bacteriorhodopsin and visual pigments
- 2 June 1987
- journal article
- research article
- Published by American Chemical Society (ACS) in Biochemistry
- Vol. 26 (11), 3210-3217
- https://doi.org/10.1021/bi00385a041
Abstract
Factors affecting the C.dbd.N stretching frequency of protonated retinal Schiff base (RSBH+) were studied with a series of synthetic chromophores and measured under different conditions. Interaction of RSBH+ with nonconjugated positive charges in the vicinity of the ring moiety or a planar polyene conformation (in contrast to the twisted retinal conformation in solution) shifted the absorption maxima but did not affect the C.dbd.N stretching frequency. The latter, however, was affected by environmental perturbations in the vicinity of the Schiff base linkage. Diminished ion pairing (i.e., of the positively charged nitrogen to its anion) achieved either by substituting a more bulky counteranion or by designing models with a homoconjugation effect lowered the C.dbd.N stretch energy. Decreasing solvation of the positively charged nitrogen leads to a similar trend. These effects in the vicinity of the Schiff base linkage also perturb the deuterium isotope effect observed upon deuteriation of the Schiff base. The results are interpreted by considering the mixing of the C.dbd.N stretching and C.dbd.N.sbd.H bending vibration. The C.dbd.N mode is shifted due to electrostatic interaction with nonconjugated positive charges in the vicinity of the Schiff base linkage, an interaction that does not influence the isotope effect. Weak hydrogen bonding between the Schiff base linkage in bacteriorhodopsin (bR) and its counteranion or, alternatively, poor solvation of the positively charged Schiff base nitrogen can account for the C.dbd.N stretching frequency of 1640 cm-1 and the deuterium isotope effect of 17 cm-1 observed in this pigment. Conversion of bR to the photochemically induced intermediate K610 involves environmental perturbation in the vicinity of the C.dbd.N linkage, lowering the C.dbd.N stretch energy. The C.dbd.N stretching frequency (1600 cm-1) observed for rhodopsin indicates very effective hydrogen bonding with the Schiff base counteranion and/or effective solvation by protein dipoles or residual water.This publication has 27 references indexed in Scilit:
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