SINGLET-TRIPLET SPECTROSCOPY OF THE LIGHT-HARVESTING BCHL A COMPLEX OF PROSTHECOCHLORIS-AESTUARII - THE NATURE OF THE LOW-ENERGY 825 NM TRANSITION

The process of energy transfer in the trimeric water-solule BChl a complex of Prosthecochloris aestuarii was studied (as a function of temperature) using singlet-triplet annihilation. At 77 K the presence of one tripler state per trimer (21 BChl a molecules) is sufficient to quench the fluorescence of the trimer completely. Therefore, it was concluded that the aggregation state of the complex in solution is trimeric. At 4 K the efficiency of quenching by triplets is reduced; a mechanism for energy transfer between subunits is proposed to explain this observation. Laser-flash-induced (polarized) triplet-singlet difference spectra show that all Qy absorption bands of the complex are coupled by (strong) exciton interactions. The polarized T-S difference spectrum indicates that the 825 nm band of the timer is split into three transitions, two degenerate transitions, which are oriented in the plane of the trimer, and one slightly red-shifted transition, which is oriented parallel to the trimer C-3 axes. The nondegenerate transition was found to have a negligibly small contribution to the ground-state absorption spectrum. A model for the organization of the 825 nm transition dipoles within the trimer is presented.