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RETINYLIDENE‐OPSIN SCHIFF BASE CHROMOPHORES AND THEIR ACCESSIBILITY TO WATER
Author(s) -
Haarosi F. I.,
Sándorfy C.
Publication year - 1995
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1995.tb02354.x
Subject(s) - chromophore , protonation , imine , chemistry , photochemistry , schiff base , water dimer , opsin , photochromism , rhodopsin , ion , crystallography , hydrogen bond , retinal , molecule , organic chemistry , biochemistry , catalysis
We propose a conceptual model to form the basis of understanding of the retinylidene‐opsin Schiff base (SB) chromophores. The first suggestion is that the protonated imine is H‐bonded to water, and this complex is similar in H‐bonding to a hydronium ion‐water dimer (H 5 O 5+ ). The second proposal is a limited accessibility of water to the SB region of the chromophore. We scrutinize data from anion‐sensitive and UV‐absorbing visual pigments and find consistency with expectations based on the model: (1) Hydration plays a fundamental role in the photochemistry. (2) Protonation of the imine nitrogen is not obligatory. The latter point is supported by spectroscopic evidence from UV‐absorbing visual pigments, in which unprotonated SB chromophores are inferred. Further support is derived from thermodynamic analysis of the temperature dependence of protonation in SB models and in a UV‐absorbing visual pigment. We discuss H‐bonding opportunities for retinylidene‐opsin SB: the strong H‐bonds to ions, the weak H‐bonds of C‐H groups, serving as H‐bond donors, and of the π‐bonds, in which π‐electrons act as H‐bond acceptors.