Femtosecond Spectroscopy of a 13-Demethylrhodopsin Visual Pigment Analogue: The Role of Nonbonded Interactions in the Isomerization Process

The photoisomerization reaction of the visual pigment analogue 11- cis -13-demethylrhodopsin is investigated using femtosecond pump−probe techniques. Following excitation with a 40-fs pump pulse at 500 nm, differential transient absorption spectra are measured from 470 to 560 nm using a 10-fs probe pulse centered at 500 nm and from 560 to 650 nm using a 10-fs probe pulse centered at 620 nm. The persistence of the excited-state absorption, the recovery kinetics of the ground-state bleach, and the formation time of the photoproduct absorption all indicate that this photoisomerization reaction is complete on the 400-fs time scale. Comparison of the reaction dynamics of 11- cis -rhodopsin with 11- cis -13-demethylrhodopsin suggests that the removal of the nonbonded steric interaction between the C 13 -methyl group and the C 10 -hydrogen atom slows down the initial torsional dynamics and that this in turn results in a lower isomerization quantum yield and a slower overall reaction time. Our results support the hypothesis that the quantum yield and the speed of the isomerization reaction are coupled according to a dynamic potential-surface crossing mechanism.