Examination of the photophysical processes of chlorophyll d leading to a clarification of proposed uphill energy transfer processes in cells of Acaryochloris marina

摘要

A comprehensive study or the photophysical properties of chlorophyll (Chl) d in 1:40 acetonitrile-methanol solution is performed over the temperature range 170-295 K. From comparison of absorption and emission spectra, time-dependent density-functional calculations and homologies with those of Chi a. we assign the key features of the absorption and fluorescence spectra. Possible photophysical energy relaxation mechanisms are summarized, and thermal equilibration processes are studied in detail by monitoring the observed emission profiles and quantum yields as a function of excitation energy. In particular, we concentrate on emission subsequent to excitation in the extreme far-red tail of the Q(y) absorption spectrum, with this emission partitioned into contributions from hot-band absorptions as well as uphill energy transfer processes that occur subsequent to absorption. No unusual photophysical processes are detected for Chi d; it appears that all intramolecular relaxation processes reach thermal equilibration on shorter timescales than the fluorescence lifetime even at 170 K. The results from these studies are used to reinterpret a previous study of photochemical processes observed in intact cells and their acetone extracts of the photo-synthetic system of Acaryochloris marina. In the study of Mimuro et al., light absorbed by Chi d at 736 nm is found to give rise to emission by another species, believed to also be Chi d, at 703 mu, this uphill energy transfer process is easily rationalized in terms of the thermal equilibration processes that we deduced for Chi d. However, no evidence is found in the experimental results of Mumuro et al. to support claims that (nonequilibrium) uphill energy transfer is additionally observed to Chi a species that emit at 670-680 nm. This finding is relevant to broader issues concerning the nature of the special pair in Photosystem H of A marina because suggestions that it is comprised of Chi a can only he correct if nonthermal uphill energy transfer processes from Chi d are. operative. [References: 41]