title: 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: 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.
authors.affiliation: Department of Physics, Technical University Kaiserslautern, Kaiserslautern, Germany.
authors.forename: Miriam
authors.initials: M
authors.lastname: Colindres-Rojas
authors.forename: Matthias M N
authors.initials: MM
authors.lastname: Wolf
authors.forename: Ruth
authors.initials: R
authors.lastname: Gross
authors.forename: Sonja
authors.initials: S
authors.lastname: Seidel
authors.forename: Benjamin
authors.initials: B
authors.lastname: Dietzek
authors.forename: Michael
authors.initials: M
authors.lastname: Schmitt
authors.forename: Jürgen
authors.initials: J
authors.lastname: Popp
authors.forename: Gudrun
authors.initials: G
authors.lastname: Hermann
authors.forename: Rolf
authors.initials: R
authors.lastname: Diller
mesh_terms: D018554:Avena / Q000201:enzymology / Q000528:radiation effects; D002735:Chlorophyllides / Q000737:chemistry; D007700:Kinetics; D008027:Light; D008954:Models, Biological; D008968:Molecular Conformation / Q000528:radiation effects; D010084:Oxidation-Reduction / Q000528:radiation effects; D010088:Oxidoreductases / Q000378:metabolism; D011521:Protochlorophyllide / Q000737:chemistry*; D013055:Spectrophotometry, Infrared; D013056:Spectrophotometry, Ultraviolet; D013997:Time Factors
chemical_list: D002735:Chlorophyllides; D011521:Protochlorophyllide; D010088:Oxidoreductases