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: τ₁ = 3.6 ± 0.2, τ₂ = 38 ± 2, and τ₃ = 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, τ₂ describes the formation of a state with a strong change in electronic structure, and τ₃ represents the partial recovery of the PChlide electronic ground state. Furthermore, τ₁ 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.