Ultrafast energy transfer in the core light harvesting complex of photosynthetic bacterium Rhodobacter sphaeroides observed by fluorescence upconversion.

Abstract

Energy transfer in the core light harvesting complex (LHI) of the purple photosynthetic bacterium Rb. sphaeroides has been measured by the fluorescence upconversion technique1. LHI is thought to be constructed from aggregates of small building blocks containing transmembrane polypeptides and associated carotenoid and bacteriochlorophyll a (BChla) pigment moieties.2 There is evidence for a high degree of ordering of the pigments in the basic "minimal unit". The mechanism of energy transfer from carotenoid to BChla and between BChla pigments is not well understood. Although the two energy transfer processes proceed via different mechanisms, they are both highly efficient, and are both expected to occur on sub-picosecond timescales. LHI makes an excellent system for study of ultrafast energy transfer in a biological system because of the existence of only a single BChl spectral pool and one type of carotenoid. We have investigated energy transfer between B870 BChla pigments in the complex as well as between carotenoid and BChla pigments. A sub-picosecond decay observed in the fluorescence depolarization of LHI excited at 870 nm is assigned to energy transfer between BChla's in the minimal unit of the complex. In preliminary measurements where the carotenoid band is excited at 505 nm, a rise in the isotropic emission from BChla is characterized by two time constants.