The photo-induced electron spin resonance signal of the primary donor P840+ in unit membrane fractions of the title photosynthetic bacterium was measured. Bacteria containing natural isotopic abundances and 82% 13C-enrichment were examined. Second moment analysis of the lineshape of the 13C-enriched P840+ signal shows that P840+ consists of two bacteriochlorophyll a-type macrocycles. The data thus obtained are independent of difficulties in interpreting either peak-to-peak linewidths or ENDOR data with respect to intermolecular spin delocalization.
