Access to the Antenna System of Photosystem I via Single-Molecule Excitation-Emission Spectroscopy

Abstract

Photosystem I (PSI) is one of the two photosystems embedded in the thylakoid membrane in oxygenic photosynthetic organisms. It plays an important role in electron transfer in the photosynthesis reaction. The PSIs of many organisms contain a few red-shifted chlorophylls (Chls) with much lower excitation energies than the ordinary antenna Chls. The fluorescence emission spectrum originates primarily from the red-shifted Chls, whereas the excitation spectrum is sensitive to the antenna Chls that are upstream of red-shifted Chls. Using single-molecule excitation-emission spectroscopy (SMEES), we obtained the inclining 2D excitation-emission matrix (2D-EEM) of PSI particles isolated from a cyanobacterium, Thermosynechococcus vestitus (equivalent to elongatus), at 80 K. Interestingly, by decomposing the inclining 2D-EEMs, we found prominent variations in the excitation spectra of the red-shifted Chl pools with different emission wavelengths, strongly indicating the variable excitation energy transfer (EET) pathway from the antenna to the terminal emitting pools. SMEES helps us to directly gain information about the antenna system, which is fundamental to depicting the EET within pigment-protein complexes.