INFLUENCE OF DFT-CALCULATED ELECTRON CORRELATION ON ENERGIES AND GEOMETRIES OF RETINALS AND OF RETINAL DERIVATIVES RELATED TO THE BACTERIORHODOPSIN AND RHODOPSIN CHROMOPHORES

DFT-calculations were performed on retinal in the all-trans, 1, 11-cis -12-s-cis, 2, and 11-cis-12-s-trans configuration, 3, and on the corre sponding N-methyl Schiff base and protonated N-methyl Schiff base deri vatives; for the latter, the corresponding 6-s-trans conformations and the 6-s-trans-13-cis-14-s-trans isomer which play a role in the bacte riorhodopsin photocycle were also studied. All geometries were fully o ptimized using the Becke- three-parameter Lee-Yang-Parr method in conj unction with the 6-31G* basis set (E3LYP/6-31G**). The stabilities in order of increasing energy are 1, 3 and 2 regardless of the type of s ubstitution of the end group. While the energy of 3 relative to 1 is a lmost constant (5 +/- 0.2 kcal mol(-1)), the relative energy of 2 depe nds somewhat on the nature of the functional group: it is highest in t he protonated Schiff base derivative 2-SBH + with its steric congestio n along the C12-C13 bond. Comparison with results previously obtained on the basis of RHF/6-31G* ab initio calculations reveals that the B3 LYP method is more biased towards pi-electron delocalization. This is indicated by the reduced degree of double bond fixation along the chro mophore and also in the increased tendency towards planarization as ma nifest, e.g. by the change of the C5-C6-C7-C8 dihedral angle between t he cyclohexene ring and the open chain double bond system. (C) 1998 El sevier Science B.V.