Peptide models for the bacteriorhodopsin chromophore. Retinylidene-lysine systems containing aspartic acid and serine residues

Abstract

The retinylidene Schiff base derivative of seven lysine containing peptides have been prepared in order to investigate solvent and neighboring group effects, on the absorption maximum of the protonated Schiff base chromophore. The peptides studied are Boc-Aib-Lys-Aib-OMe (1), Boc-Ala-Aib-Lys-OMe (2), Boc-Ala-Aib-Lys-Aib-OMe (3), Boc-Aib-Asp-Aib-Aib-Lys-Aib-OMe (4), Boc-Aib-Asp-Aib-Ala-Aib-Lys-Aib-OMe (5), Boc-Lys-Val-Gly-Phe-OMe (6) and Boc-Ser-Ala-Lys-Val-Gly-Phe-OMe (7). In all cases protonation shifts the absorption maxima to the red by 3150-8450 cm^-1. For peptides 1-3 the protonation shifts are significantly larger in nonhydrogen bonding solvents like CHCl₃ or CH₂Cl₂ as compared to hydrogen bonding solvents like CH₃OH. The presence of a proximal Asp residue in 4 and 5 results in pronounced blue shift of the absorption maximum of the protonated Schiff base in CHCl₃, relative to peptides lacking this residue. Peptides 6 and 7 represent small segments of the bacteriorhodopsin sequence in the vicinity of Lys-216. The presence of Ser reduces the magnitude of the protonation shift.