RETINOIDS AND RELATED-COMPOUNDS .17. CONFORMATIONAL STUDY OF RETINOCHROME CHROMOPHORE - SYNTHESIS OF 8,18-ETHANORETINAL AND A NEW RETINOCHROME ANALOG

In order to investigate the chromophore conformation around the trimet hylcyclohexene ring in retinochrome, (all-E)-8,18-ethanoretinal (2), i n which C-8 and C-18 positions of retinal 1 was connected by an ethyle ne group, was synthesized via the Dieckmann condensation of diester 6 from 2,2-dimethylcyclohexanone 7. This analog 2 was incorporated into aporetinochrome, forming a pigment with lambda(max) at 498 nm. Its ops in shift (2200 cm(-1)) is of the same order of magnitude as that of na tive retinochrome (2400 cm(-1)), suggesting that the conformations of both chromophores are fairly similar in the protein. In addition, MMX calculations indicate that the torsional angle around the 6-7 single b ond in retinochrome might correspond to the allowable torsional angle in 8,18-ethanoretinal (2).