RETINYLIDENE-OPSIN SCHIFF-BASE CHROMOPHORES AND THEIR ACCESSIBILITY TO WATER

We propose a conceptual model to form the basis of understanding of th e retinylidene-opsin Schiff base (SB) chromophores. The first suggesti on is that the protonated imine is H-bonded to water, and this complex is similar in H-bonding to a hydronium ion-water dimer (H5O2+). The s econd proposal is a limited accessibility of water to the SB region of the chromophore. We scrutinize data from anion-sensitive and UV-absor bing visual pigments and find consistency with expectations based on t he 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 depe ndence of protonation in SB models and in a UV-absorbing visual pigmen t. We discuss H-bonding opportunities for retinylidene-opsin SB: the s trong H-bonds to ions, the weak H-bonds of C-H groups, serving as H-bo nd donors, and of the pi-bonds, in which pi-electrons act as H-bond ac cepters.