Excited-State Dynamics of Protochlorophyllide Revealed by Subpicosecond Infrared Spectroscopy

To gain a better understanding of the light-induced reduction of protochlorophyllide (PChlide) to chlorophyllide as a key regulatory step in chlorophyll synthesis, we performed transient infrared absorption measurements on PChlide in d4-methanol. Excitation in the Q-band at 630 nm initiates dynamics characterized by three time constants: tau(1) = 3.6 +/- 0.2, tau(2) = 38 +/- 2, and tau(3) = 215 +/- 8 ps. As indicated by the C13'=O carbonyl stretching mode in the electronic ground state at 1686 cm(-1), showing partial ground-state recovery, and in the excited electronic state at 1625 cm(-1), showing excited-state decay, tau(2) describes the formation of a state with a strong change in electronic structure, and tau(3) represents the partial recovery of the PChlide electronic ground state. Furthermore, tau(1) corresponds with vibrational energy relaxation. The observed kinetics strongly suggest a branched reaction scheme with a branching ratio of 0.5 for the path leading to the PChlide ground state on the 200 ps timescale and the path leading to a long-lived state (>> 700 ps). The results clearly support a branched reaction scheme, as proposed previously, featuring the formation of an intramolecular charge transfer state with similar to 25 ps, its decay into the PChlide ground state with 200 Ps, and a parallel reaction path to the long-lived PChlide triplet state.