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: t1 ¼ 3.650.2, t2 ¼ 3852, and t3 ¼ 21558 ps. As indicated by the C130¼O 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, t2 describes the formation of a state with a strong change in electronic structure, and t3 represents the partial recovery of the PChlide electronic ground state.Furthermore, t1 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 ~25 ps, its decay into the PChlide ground state with 200 ps, and a parallel reaction path to the long-lived PChlide triplet state.