Probing the structure and Franck–Condon region of protochlorophyllide a through analysis of the Raman and resonance Raman spectra

Abstract

The structure and Franck-Condon region of protochlorophyllide a, a precursor in the biosynthesis of chlorophyll and substrate of the light-regulated enzyme protochlorophyllide oxidoreductase (POR), were investigated by Raman and resonance Raman (RR) spectroscopy. The spectroscopic results are compared to the spectra of the structurally closely related porphyrin model compound magnesium octaethylporphyrin (MgOEP), and interpreted on the basis of density functional theory (DFT) calculations. It is shown that the electronic properties of the two porphyrin macrocycles are affected by different vibrational coupling modes, resulting in a higher absorption cross section of protochlorophyllide a in the visible spectral region. Furthermore, a comparison of the Fourier transform (FT)-Raman and RR spectra of protochlorophyllide a indicates the modes that are resonantly enhanced upon excitation. Based on vibrational normal mode calculations, these modes include C-C ring-breathing and C C stretching vibrations of the porphyrin macrocycle. In particular, the strong band at 1703 cm −1 can be attributed to the C O carbonyl vibration of the cyclopentanone ring, which is attached in conjugation to the π-electron path of the porphyrin ring system. The enhancement of that mode upon electronically resonant excitation is discussed in the light of the reaction model suggested for the photoreduction of protochlorophyllide a in the POR.

Excited-State Dynamics of Protochlorophyllide Revealed by Subpicosecond Infrared Spectroscopy

“…A recent time-resolved mid-IR study on PChlide (34), which used excitation into the Soret band, reported results similar to those obtained in our previous time-resolved fluorescence and absorption studies. However, the mid-IR data do not support the branched reaction model suggested on the basis of our previous work (15)(16)(17)(18)(19)(20)(21).…”

“…Thus, only processes that are directly related to the intrinsic reaction dynamics are accessible to observation, which facilitates a more straightforward analysis of the transient IR spectra. The results not only provide insight into the structural changes associated with the initial reaction dynamics in PChlide, they also confirm the reaction model suggested for the excited-state dynamics on the basis of our previous transient absorption and fluorescence measurements (15)(16)(17)(19)(20)(21). In this respect, they reveal the partitioning of the initially excited-state population into reactive and nonreactive pathways.…”

“…We have initiated a comprehensive study on the excitedstate dynamics of PChlide alone in solution, i.e., outside the cavity of the POR apoprotein (15)(16)(17)(18)(19)(20)(21). The objective of this work is to separate the influence of the POR apoenzyme from the intrinsic excited-state relaxation dynamics of the substrate, and thus elucidate the more complex lightinduced processes in the POR enzyme.…”

Dynamics of charge separation in the excited-state chemistry of protochlorophyllide

“…4C and Table 1). As seen from the RR spectrum and the vibrational assignment by the DFT calculations (Table 1), the frequencies below 800 cm À1 correspond in general to structure specific normal modes associated with displacements of the whole porphyrin molecule rather than individual functional groups [32]. Accordingly, the 26-and 365-cm À1 modes involved in the wave packet motion are highly delocalized and arise from strongly coupled vibrations.…”

Recent advances in linear and nonlinear Raman spectroscopy. Part V

“…They noted the importance of the pump excitation wavelength in femtosecond time‐resolved experiments that probe intersection‐induced nonadiabatic dynamics. The structure and Franck–Condon region of protochlorophyllide a , a precursor in the biosynthesis of chlorophyll and substrate of the light‐regulated enzyme protochlorophyllide oxidoreductase, were investigated by Hanf and coworkers using Raman and RR spectroscopy . The spectroscopic results were compared with the spectra of the structurally closely related porphyrin model compound magnesium octaethylporphyrin and interpreted on the basis of DFT calculations.…”