Spectroscopic and chemical properties of isomeric retinals and visual pigment analogs

Abstract

Complementing the recent interest in utilizing vibrational spectroscopies to probe for structural information of protein bound retinyl chromophores, we have recorded FT-IR spectra of all sixteen isomers of retinal. Characteristic trends of the poly-cis isomers including C=C double bond stretching modes, C-C single bond stretching modes and hydrogen out of plane bends (HOOP) have been discussed and compared with those of the mono-cis and all-trans isomers. The normal modes of C-C stretching character of two sterically hindered 7-cis and 7,9-dicis isomers and their Schiff bases (SB) and protonated Schiff bases (PSB) have been assigned by using their isotopically labeled analogs. These assignments are further substantiated through normal mode calculations. These vibrational, data will provide a probe for studying the specific changes in the chromophore-opsin interactions during the photobleaching processes of synthetic isomeric rhodopsins. Temperature-dependent isomerization of retinal isomers have been examined, including the photoisomerization of mono-cis (7-cis, 9-cis, l l-cis) and 7,9-dicis-retinal isomers and the thermal isomerization of. four unstable retinal isomers (11, 13-dicis, 7, 11, 13-tricis, 9, 11-13-tricis and all-cis). The initial product distributions of the photoisomerizations have been determined as a function of temperature. Relative quantum yields for photoisomerization of 7-cis and 7,9-dicis-retinal have also been determined at various temperatures. The results show a general trend of higher torsional energy barrier values for those double bonds near the electron withdrawing carbonyl group. The experimental trend is consisted with the bond orders of the conjugated polyene chain. The four unstable isomers were known to undergo facile stereospecific rearrangements to their corresponding 11-transisomers (13-cis, 7, 13-dicis, 9, 13-dicis and 7,9,13-tricis). The enthalpy and entropy values of activation of these thermal rearrangement reactions, which were monitored by UV/Vis absorption spectroscopy, have been determined. The data are in agreement with the postulated Kluge-Lillya mechanism of isomerization, involving consecutive 6e-electrocyclization reactions for the stereospecific reactions. Two dicis-retinal isomers, 7, 9-dicis-retinal and 9, l l-dicis-l z-fluoro-retinal, have been incorporated into cattle opsin to yield stable pigments. Therefore, they are available to study the mechanism of photobleaching of dicis isomeric rhodopsin. The photochemical reactions of these two pigments have been investigated by low temperature UV/Vis spectroscopy and HPLC analysis of extracted chromophores. Low temperature spectroscopies revealed that the batho-intermediates from dicis-rhodopsin analogs are blue-shifted than that from 11-cis-rhodopsin. HPLC extraction analysis results revealed that two consecutive steps of one-photon-one-bond isomerization from 9, ll-dicis-12fluororhodopsin to 12-fluoro-batho rhodopsin (all-trans) via its 9-cis-intermediate, while 7,9-dicis-rhodopsin demonstrated exclusive two-bond-isomerization to the all-trans isomer. Possible steric interactions between the isomeric chromophores and the hydrophobic protein pocket are discussed.