The Bacteriorhodopsin Chromophore Retinal and Derivatives: An Experimental and Theoretical Investigation of the Second-Order Optical Properties

The first hyperpolarizabilities, β, of retinal and related derivatives were measured experimentally by using the hyper-Rayleigh scattering (HRS) technique and compared to the calculated values obtained with the semiempirical intermediate neglect of differential overlap/configuration interaction/sum-over-states (INDO/CI/SOS) method. The experimental and theoretical results are in excellent mutual agreement. By substitution of the trans-retinal aldehyde group for stronger or weaker electron-withdrawing groups, the positions of the absorption band maxima could be systematically varied. Theoretical and experimental data confirm the expected linear dependence between β and the inverse of the square of the first transition energy as well as an exponential increase of β with the number of double bonds in the polyene segment. It was found that, upon trans to 13-cis or 9-cis isomerization of a retinal double bond, a constant fraction of the β value is lost, regardless of the nature of the electron-withdrawing group or the solvent. We find an increase in the β value by about 1 order of magnitude in passing from the neutral form of the retinal Schiff base to the protonated form which is present in the bacteriorhodopsin protein. The observed changes in the depolarization ratios are in accordance with the changes in molecular structure.