FLUORESCENCE-DETECTED MAGNETIC-RESONANCE OF MONOMERS AND AGGREGATES OF BACTERIOCHLOROPHYLLS OF GREEN SULFUR BACTERIA CHLOROBIUM SP

Fluorescence detected magnetic resonance (FDMR) was used to study the lowest triplet state of bacteriochlorophylls (BChls) c and d in Chloro bium (Chl.) tepidum and Chl. vibrioforme, respectively. These pigments were studied both in the oligomeric form (in whole cells) and in the monomeric form (after conversion using a 1% 1-hexanol treatment). Fluo rescence spectra show the presence of lower-state aggregates, apart fr om monomers, in samples treated with 1-hexanol. Values of the zero fie ld splitting (ZFS) parameter D, obtained from FDMR spectra, were found to decrease with an increasing aggregate size. The observed ZFS trend s are explained by a delocalization of the triplet spins, including a charge resonance (CR) contribution, over the aggregate. A simple model is presented relating the changes of D and E as a result of monomer a ggregation to the aggregate geometry. Application of this model to BCh ls c and d indicates approximately diagonal stacking of the monomers i n the dimer. Results for oligomeric BChl c and d were compared with th ose previously obtained for oligomeric BChl e. FDMR transitions of BCh ls c, d and e differ both in frequencies and in signs. The D and E val ues of Car's and BChl a (in whole cells) agree well with those reporte d for Chl. phaeobacteroides and Chl. limicola.