Spectroscopic and MO Study of Peptide Models for the M412 Intermediate of Bacteriorhodopsin. Nε-Retinylidenelysyl Peptides Containing Aromatic Amino Acids

Abstract N ε-Retinylidenelysyl peptides containing Tyr and Phe were prepared in order to investigate the effect of the surrounding microenvironment on the absorption maximum of the chromophore of bacteriorhodopsin. The absorption maximum of the retinylidene Schiff base of the lysyl peptide, Boc–Lys–Phe(or –Gly)–Tyr–OMe, was shifted to the red relative to the peptides lacking Tyr, Boc–Lys–Phe–Phe–OMe and Boc–Lys–Phe–OMe. UV and 13C NMR studies on the Tyr-containing lysyl-peptide have confirmed an intramolecular interaction between Tyr and the retinylidene portion in low dielectric media. A hydrogen-bond formation of the Schiff base with a Tyr side chain is supported by a theoretical estimation of the transition energy and the shielding constants for some models of Tyr-Schiff base complexes. The side-chain conformation of the Tyr-contaning lysyl-peptide is discussed in terms of the 1H NMR data. The hydrogen-bonding shift induced by Tyr was estimated as being no less than 10 nm.