AB-INITIO STUDY OF C-13 NMR CHEMICAL-SHIFTS FOR THE CHROMOPHORES OF RHODOPSIN AND BACTERIORHODOPSIN .2. COMPREHENSIVE ANALYSIS OF THE C-13 CHEMICAL-SHIFTS OF PROTONATED ALL-TRANS-RETINYLIDENE SCHIFF-BASE

Theoretical analysis was performed for the C-13 chemical shifts of the retinal chromophore in bacteriorhodopsin (bR) by means of ab initio N MR shielding calculation, based on the localized orbital/local origin method. In order to comprehensively investigate the correlation betwee n the C-13 chemical shieldings of the unsaturated carbons and physicoc hemical perturbations relating to the spectral tuning of bacteriorhodo psin, the following three factors are taken into account in the presen t calculation: (1) change in strength of the hydrogen bonding between protonated retinylidene Schiff base and its counterion, (2) conformati onal changes about single bonds of the conjugated chain, and (3) elect rostatic interactions between the Schiff base and electric dipoles. On the basis of these calculations, we successfully find a molecular mod el for which the shielding calculation almost completely reproduces th e observed chemical shift data for the chromophore of bR.