New Isomers of Bovine Porphyropsin: A Nondestructive Method for Determining Chromophore Configuration during Formation of Visual Pigment Analogs

Abstract

Binding interactions of six isomers (five previously unknown) of 3-dehydroretinal (3-DHR) with bovine opsin have been investigated. Of these only 7-cisand 7,9-di-cisisomers retained the original polyene configuration, giving the corresponding isomeric porphyropsin pigment analogs. The 7,13-di-cis, 7,9,11-tri-cis, and 7,9,13-tri-cisisomers of 3-DHR were found to lose readily the 13-cisor 11-cisgeometry, yielding instead the 7-cisor 7,9-di-cispigments. The 7,11-di-cisisomer yielded a pigment, believed to be the 7,11-di-cis, which subsequently isomerized to the stable 7-cispigment. While the general behavior of the 3-DHR isomers is similar to that of the retinals, current results on the two tri-cisisomers were not available for the corresponding retinal isomers. Instead of using the established chromophore extraction procedure, we demonstrated that curve resolution of the spectra obtained during binding interactions, a procedure requiring a much smaller sample, could yield equally reliable information on stereochemical properties of the chromophore.