An Ab initio analysis of charge redistribution upon isomerization of retinal in rhodopsin and bacteriorhodopsin

In the framework of quantum-chemical methods (RHF, MP2, and DFT), we study the reaction of photoinduced isomerization of biological molecules-retinal in rhodopsin (Rh) and bacteriorhodopsin (BRh)-the key intermediates in the vision process. The charge redistribution in the retinal chromophore of these biological molecules as the mechanism of the photoinduced isomerization is investigated. The results of the Mulliken, Lowdin, Natural Bond, and ESP population analyses are compared and validated. We demonstrate that the isomerization reaction is accompanied by the substantial charge relocation within the isomerization region with the electron localization on C11 for the retinal in Rh and C14 for the retinal in BRh.