Four positional isomers of trans-retinal (1) differing in the location of the side-chain methyl groups have been prepared by a combination of Wittig and highly stereocontrolled Suzuki coupling reactions. The incubation of 9-demethyl-10-methylretinal (5) with bacterioopsin yielded an artificial pigment with an opsin shift of 4630 cm(-1) The other three analogs, namely 13-demethyl-14-methylretinal (3), 13-demethyl-12-methylretinal (4), and 9-demethyl-8-methylretinal (6) did not bind to the apoprotein. In order to rationally address the intrinsic structural differences among analogs which could be relevant to the discrimination exhibited by the protein binding site, ab initio calculations with complete optimization at the 3-21G level were performed on model N-methylretinal iminium salts derived from aldehydes 1 and 3-6. The validity of the approach was inferred from the remarkable coincidence between the minimized structure of N-methylretinal Schiff base (PSB-1) and the 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 structural effects, added to the dominant steric and electronic restrictions of the binding pocket, would explain the observed discrimination among the analogs 3-6, with minor structural changes, and perhaps among other retinals reported in the literature. Additionally, the theoretical and experimental results obtained with 9-demethyl-8-methylretinal (6) provide further indirect evidence of the importance of the 6-s-trans conformation for the native chromophore in bacteriorhodopsin.