THE BACTERIORHODOPSIN CHROMOPHORE RETINAL AND DERIVATIVES - AN EXPERIMENTAL AND THEORETICAL INVESTIGATION OF THE 2ND-ORDER OPTICAL-PROPERTIES

The first hyperpolarizabilities, beta, of retinal and related derivati ves were measured experimentally by using the hyper-Rayleigh scatterin g (HRS) technique and compared to the calculated values obtained with the semiempirical intermediate neglect of differential overlap/configu ration interaction/sum-over-states (INDO/CI/SOS) method. The experimen tal and theoretical results are in excellent mutual agreement. By subs titution of the trans-retinal aldehyde group for stronger or weaker el ectron-withdrawing groups, the positions of the absorption band maxima could be systematically varied, Theoretical and experimental data con firm the expected linear dependence between beta and the inverse of th e square of the first transition energy as well as an exponential incr ease of beta with the number of double bonds in the polyene segment. I t was found that, upon trans to 13-cis or 9-cis isomerization of a ret inal double bond, a constant fraction of the beta value is lost, regar dless of the nature of the electron-withdrawing group or the solvent. We find an increase in the beta value by about 1 order of magnitude in passing from the neutral form of the retinal Schiff base to the proto nated form which is present in the bacteriorhodopsin protein. The obse rved changes in the depolarization ratios are in accordance with the c hanges in molecular structure.