Mechanism of Absorption Wavelength Shift of Bacteriorhodopsin During Photocycle.

Bacteriorhodopsin, a light-driven proton pump, alters the absorption wavelengths in the range of 410-617 nm during the photocycle. Here, we report the absorption wavelengths, calculated using 12 bacteriorhodopsin crystal structures (including the BR, BR 13- cis , J, K 0 , K E , K L , L, M, N, and O state structures) and a combined quantum mechanical/molecular mechanical/polarizable continuum model (QM/MM/PCM) approach. The QM/MM/PCM calculations reproduced the experimentally measured absorption wavelengths with a standard deviation of 4 nm. The shifts in the absorption wavelengths can be explained mainly by the following four factors: (i) retinal Schiff base deformation/twist induced by the protein environment, leading to a decrease in the electrostatic interaction between the protein environment and the retinal Schiff base; (ii) changes in the protonation state of the protein environment, directly altering the electrostatic interaction between the protein environment and the retinal Schiff base; (iii) changes in the protonation state; or (iv) isomerization of the retinal Schiff base, where the absorption wavelengths of the isomers originally differ.