Femtosecond pump-probe measurements of the excitation pathway of bovine rhodopsin to bathorhodopsin show varying transient events in the photoinduced absorption spectrum. In the spectral region where bathorhodopsin absorption dominates in the rhodopsin-bathorhodopsin difference spectrum, subpicosecond (200-600 fs) and 3 ps signals are observed. In the region where rhodopsin absorption dominates, a 3-5 ps transient is observed, which follows rhodopsin repopulation. To unravel the different models of the rhodopsin-bathorhodopsin dynamics that have been proposed to explain these transient observations, we have employed a three-beam experiment. The first pump beam launches the photochemistry, a second pump pulse interrupts the spontaneous evolution of the trajectory of excited rhodopsin at a delayed time from the first pump pulse, and a weak third beam probes changes in the amount of bathorhodopsin that is formed in response to the second pump beam. We first find that bathorhodopsin can be photoexcited, presumably back to rhodopsin, by reversing the initial photochemistry. We find that bathorhodopsin is formed very promptly (within the 300 fs resolution of our experiments). Surprisingly, the kinetics in the absorbance signal near 605 nm is found to be the superposition of two independent signals, one being characterized by events on the subpicosecond time scale and the other at 3 ps. The results suggest that the trajectory of excited rhodopsin splits into two pathways very early: one of which results in bathorhodopsin formation on the ∼200 fs time scale and another which lives in the excited state for ca. 3 ps before undergoing internal conversion to rhodopsin.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry