AB-INITIO STUDY OF C-13 NMR CHEMICAL-SHIFTS FOR THE CHROMOPHORES OF RHODOPSIN AND BACTERIORHODOPSIN .1. THEORETICAL ESTIMATION OF THEIR RING-CHAIN CONFORMATIONS

The ring-chain conformations of retinals within rhodopsin (Rh) and bac teriorhodopsin (bR) are estimated by means of ab initio NMR-shielding calculations, based on the localized orbital/local origin method. Firs t, we calculated the C-13 shieldings for the unsaturated carbons of al l-trans-retinal in double-xi level and confirmed that there isa good a greement between the, calculated and experimental data. Subsequently, the shielding calculations were carried out in the same. level for a r etinal analogue to investigate the correlation between the C-13 chemic al shieldings and-the C5=C6-C7=C8 dihedral angle (phi(6-7)). Character istic magnetic properties are found in the C5 and C8 shieldings, which behave in periodic manners depending on the value of phi(6-7). The ma jor source of the periodicity is attributed to the recovering and brea king of the conjugated system rather than to steric interaction becaus e the sigma(22). and the sigma(33)-shielding tensor elements in the co njugated plane depend strongly on the phi(6-7) value, whereas the phi( 6-7) dependence of the sigma(11) element perpendicular to the plane is less affected. We also propose that the C6-C7 conformation can be est imated through an observation of the chemical shift difference between C5 and C8 (Delta sigma(C8-C5)) without requiring the corresponding da ta for a reference compound. We conclude that only for the in-planar 6 -s-trans form the calculated value of Delta sigma(C8-C5) has negative sign as observed experimentally and that the chromophore of bR takes t he planar 6-s-trans form (phi(6-7) = similar to 180(degrees)). However , information only on the C5 and C8 shieldings is insufficient to dete rmine whether the chromophore of Rh takes a skewed 6-s-cis or 6-s-tran s form.