Evidence for light-induced 13-cis , 14-s-cis isomerization in bacteriorhodopsin obtained by FTIR difference spectroscopy using isotopically labelled retinals

Abstract

We have obtained by Fourier transformed infra‐red (FTIR)‐spectroscopy BR‐K, BR‐L and BR‐M difference spectra of bacteriorhodopsin regenerated with isotopically labelled retinals. Thereby, we are able to assign reliably the C₁₄–C₁₅ and C=N stretching vibrations of the various intermediates. The lower C₁₄–C₁₅ stretching vibration frequency in L as compared with 13‐cis protonated Schiff base model compounds indicates a 13‐cis, 14‐s‐cis configuration of the retinal in this species. The unusually low C=N stretching vibration in K at 1615 cm⁻¹ indicates less stabilization of the positive charge at the Schiff base by the protein environment. Based on these results, a mechanism is suggested by which the stored light energy is transformed into proton transfers.