Towards a New Taxonomy of Photosynthetic Bacteria: ADMR-Monitored Triplet Difference Spectroscopy of Reaction Center Pigment–Protein Complexes

Low-temperature (1.2 K) triplet-minus-singlet absorbance difference spectra of a number of photosynthetic bacteria, including members of almost all genera with the exception of Heliobacteriaceae, have been recorded by absorbance-detected magnetic resonance (ADMR). The spectra of the purple bacteria fall into two distinct classes. It is argued that these classes reflect the extent of delocalization of the primary donor triplet state over its two bacteriochlorophylls. The spectra from the green sulfur bacteria form a third class. In some cases, the bleaching of the long-wavelength primary donor absorption is split, or shows a strong shoulder at lower energy. This phenomenon is attributed to an intrinsic heterogeneity of the reaction center with regard to the precise configuration of the primary donor dimer. It is argued that the zero-field splitting parameters and the triplet-minus-singlet difference spectra may be used as a taxonomic tool.