Spectroscopic characterization of photosystem I at the single-molecule level

Abstract

Progress in the field of single-molecule techniques has yielded enormous possibilities for studying photosynthetic pigment–protein complexes. The aim of this review is to present recent developments in spectroscopy on single photosystem I (PSI) complexes. The fluorescence emission of PSI is composed of the contributions of several chlorophyll a (Chl a) species, showing a remarkable red-shift compared with the fluorescence emission of Chl a in solution. Thus far, single complexes of PSI from Th. elongatus, Synechocystis PCC 6803, and Synechococcus PCC 7002 have been studied. Even on single-molecule level, several emitters contribute to the fluorescence emission of PSI. The spectral characteristics of these emitters found for the different organisms are compared with findings from ensemble and site-selective spectroscopy. Based on observed spectral positions and spectral diffusion properties, an assignment of different red emitters to specific positions in the X-ray structure is discussed. Favorable optical properties of PSI allow studies concerning the influence of solvent exchange on spectral diffusion. Remarkable changes in the spectral diffusion rate were observed upon H2O to D2O exchange. These results are discussed in the framework of pigment–protein coupling. Further, the fluorescence emission of different emitters within one single PSI complex shows anti-correlated intensity variations during time. This indicates changes in the excitation energy transfer pathways within a single PSI complex. Implication for excitation energy transfer within multi-chromophore systems are discussed.