Optical Microscopy and Spectroscopy on Single Photosystem I

Abstract

Single-molecule spectroscopy dealing with single photosynthetic aggregate provides a possibility to resolve the optical transition of individual pigment, even under non-selective excitation of antenna chlorophylls. We present the first results of single-molecule spectroscopy on reaction-center containing photosynthetic aggregate Photosystem I of Synechococcus elongatus. Spectroscopic properties of low-energy antenna pigments were probed in individual phtotosynthetic complexes. Temperature activated fluorescence quenching unravels energy transfer pathways within the red-most pigment pool and to the reaction center on individual complex level. Low-temperature spectroscopy indicates that two subgroup of pigments which are present in the red antenna pool differs not only by their spectral position but also by the strength of electron-phonon coupling. Possible dimeric nature of the strongly phonon coupled red- most state is discussed in context of hole-burning data (Rätsep at al., J. Phys. Chem. B 2000, 104: 836). Room-temperature bleaching dynamics shows that light energy absorbed by antenna chlorophylls is transferred to red-most pigments before being capturing in the reaction center. The present work demonstrates that fluorescence of single PS1 complexes can be detected at physiological temperatur.