The photo-excited triplet state of chlorophyll d in methyl-tetrahydrofuran studied by optically detected magnetic resonance and time-resolved EPR

Chlorophyll d (Chl d) is the major pigment in the antenna proteins of both photosystems (PSI and II) of the oxyphotobacterium Acaryochloris marina. This fact suggests that photosynthesis based upon Chl d rather than Chl a may be an interesting alternative in oxygenic photosynthesis. While a great deal of spectroscopic information relative to Chl a is available, both in vivo and in vitro, the literature on Chl d is scarce. In particular, the triplet state of Chl d has not been studied in vitro to date. Although triplet states do not represent the main excitation path in the photosynthetic process, they are involved in light stress events both in the antenna complexes and in the reaction centers and may also be used as endogenous paramagnetic probes of the molecular environment. In this paper we make use of both time-resolved EPR and ODMR to characterize, for the first time, the Chl d triplet state in the polar solvent methyl-tetrahydrofuran. Comparison with the spectra of Chl a obtained under the same experimental conditions is also discussed.