Triplet states in reaction center, light-harvesting complex B875 and its spectral form B840 from Rubrivivax gelatinosus investigated by absorbance-detected electron spin resonance in zero magnetic field (ADMR)

Absorbance-detected magnetic resonance (ADMR) spectroscopy was used for characterization of the triplet states in several pigment-protein complexes from Rubrivivax gelatinosus, namely the isolated reaction center, the core light-harvesting complex B875, and the B875 subunit form that absorbs in the infrared at 820 nm (B820), and, in the presence of glycerol, at about 840 nm (B840). The zero-field splitting parameters of the triplet states were determined and T - S spectra were recorded at 1.2 K. The absorbance and T - S spectra of the reaction center resemble closely those of Rhodobacter sphaeroides, except for slight shifts in the band positions. For the isolated B875 complex, only the triplet state of carotenoid was observed, indicating that triplet-triplet transfer from bacteriochlorophyll a to the carotenoid proceeds as in the native system. The near-infrared absorption spectrum of B840 at 1.2 K revealed several overlapping bands, which were deconvoluted in five Gaussian components corresponding to free BChl a (795 nm), B820 (826 nm), small population of B875 (888 nm) and two new spectral forms B846 and B868, The origin of these two latter spectral forms is proposed to reflect inaccurate reassociation of B820. Probably, oligomers of various sizes are formed, with different strengths of pigment-pigment and pigment-protein interactions. The T - S spectrum of B840 is consistent with that of a BChl a dimer containing a localized triplet state.