Spectroscopic studies on light-harvesting complexes of green plants and purple bacteria

Abstract

Light-harvesting properties of light-harvesting complex 2 (LH2) from purple bacterium Rhodospirillium (Rs.) molischianum and antenna complexes of green plant photosystem I (PSI) were characterized by spectroscopic methods. LH2 is a peripheral antenna protein complex that absorbs light, i.e. collects excitation energy, for chemical reactions of purple bacteria. The collection of sunlight is pe1fonned by bacteriochlorophylls organized as two separate rings, named as B800 and B850 according to their absorption maxima. Steady-state absorption properties of isolated LH2 complexes were characterized and the rate of excitation energy transfer between the B800 and B850 rings was determined by transient absorption technique. Green plant PSI has two antenna complexes, light-harvesting complex I (LHCI) and PSI-core antenna, which are bound together forming a complex called PSI-200. The isolated LHCI proteins appear in dimeric forms and these dimers were characterized by means of (polarized) steady-state absorption and fluorescence spectroscopy at low temperatures. The preparation was shown to contain a 1nixture of two spectroscopically different dimers evident in two distinct fluorescence e1nission maxima. Both LHCI and PSI-core antenna have a number of strongly coupled pigments with lower excitation energy than that of the primary electron donor, P700, of PSI complex. These low energy bands were further characterized, both in separated (LHCI or PSI-core antenna) and in whole (PSI-200) form, in te1111s of phonon coupling, phonon mean frequencies, and inhomogenous broadening by using site-selective fluorescence and absorption (hole-burning) techniques. The green plant PSI-core complex consists of at least 14 different protein subunits. The pigment organization of small subunits (PSI-G, PSI-K, PSI-L, and PSI-N) of PSI-core antenna were determined and the influence of the subunits on the energy transfer properties of PSI-200 complex was investigated. Time-resolved fluorescence technique revealed the excitation energy transfer times between various pigment pools and the excitation energy trapping times in the PSI-200 complex.