Protein-β-ionone ring interactions enhance the light-induced dipole of the chromophore in bacteriorhodopsin

Following light absorption, the retinal chromophore of bacteriorhodospin experiences very large dipolar changes in the vertically excited state. This light-induced dipole is at least 50% larger than that of the retinal chromophore in films or in solution. We have studied the origin of the protein effect by applying second harmonic generation measurements of artificial bacteriorhodopsin pigments derived from synthetic retinal analogues, characterized by a modified polyene chain length and ring-chain conformation. The studies demonstrated a significant influence of a protein domain in the vicinity of the retinal beta-ionone. We suggest that tryptophane residues (specifically Trp 138 and 189) enhance the light-induced dipole in bacteriorhodopsin. The data also point to another protein domain (Trp 182) influencing the retinal chromophore. The effect of these tryptophane residues appears not only to stabilize the light-induced charge redistribution but also to enable the migration in the excited state of the positive charge into a region of the protein with a relatively low dielectric constant.