Re-evaluation of rhodopsin's relaxation kinetics determined from femtosecond stimulated Raman lineshapes.

This work presents a theoretical treatment of the vibrational line shape generated in a femtosecond stimulated Raman spectroscopy (FSRS) experiment under conditions in which the probed vibration undergoes a significant frequency shift during its free induction decay. This theory is applied to simulate the FSRS lineshapes previously observed in rhodopsin (Kukura et al. Science 2005, 310, 1006). The previously determined relaxation times for formation of the trans-photoproduct of rhodopsin were calculated using an incorrect equation for the time dependence of the observed frequency shifts. Here the data are reanalyzed by calculation of the corrected frequency sweep occurring during the vibrational free induction decay. It is shown that the calculated frequency shifts and general conclusions of the original work are sound but that the coherent vibrational frequency shifts of the C(10), C(11), and C(12) hydrogen-out-of-plane vibrations occur with a 140 fs time constant rather than the previously reported 325 fs time constant. This time constant provides an important constraint for models of the dynamics of the cis to trans isomerization process.