Excited-state dynamics of protochlorophyllide revealed by subpicosecond infrared spectroscopy

Abstract

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 = 3.6±0.2, t 2 = 38±2, and t3 = 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, 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 (»00 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. © 2011 by the Biophysical Society.