EXPERIMENTAL AND THEORETICAL-ANALYSIS OF THE STERIC TOLERANCE OF THE BINDING-SITE OF BACTERIOOPSIN WITH THE USE OF SIDE-CHAIN METHYL-SHIFTED RETINAL ANALOGS

Four positional isomers of trans-retinal (1) differing in the location of the side-chain methyl groups have been prepared by a combination o f Wittig and highly stereocontrolled Suzuki coupling reactions. The in cubation of 9-demethyl-10-methylretinal (5) with bacterioopsin yielded an artificial pigment with an opsin shift of 4630 cm(-1) The other th ree analogs, namely 13-demethyl-14-methylretinal (3), 13-demethyl-12-m ethylretinal (4), and 9-demethyl-8-methylretinal (6) did not bind to t he apoprotein. In order to rationally address the intrinsic structural differences among analogs which could be relevant to the discriminati on exhibited by the protein binding site, ab initio calculations with complete optimization at the 3-21G level were performed on model N-met hylretinal iminium salts derived from aldehydes 1 and 3-6. The validit y of the approach was inferred from the remarkable coincidence between the minimized structure of N-methylretinal Schiff base (PSB-1) and th e structural parameters displayed by N-methyl-N-phenylretinal iminium perchlorate (38b). Computations clearly show that the location of the methyl groups on the polyene side chain is of the utmost importance in determining the overall shape of the retinal ligands. Those structura l effects, added to the dominant steric and electronic restrictions of the binding pocket, would explain the observed discrimination among t he analogs 3-6, with minor structural changes, and perhaps among other retinals reported in the literature. Additionally, the theoretical an d experimental results obtained with 9-demethyl-8-methylretinal (6) pr ovide further indirect evidence of the importance of the 6-s-trans con formation for the native chromophore in bacteriorhodopsin.