Excitonic Interactions in Wild-Type and Mutant PSI Reaction Centers

Abstract

Femtosecond excitation of the red edge of the chlorophyll a QY transition band in photosystem I (PSI), with light of wavelength ≥700nm, leads to wide transient (subpicosecond) absorbance changes: positive ΔA between 635 and 665nm, and four negative ΔA bands at 667, 675, 683, and 695nm. Here we compare the transient absorbance changes after excitation at 700, 705, and 710nm at 20K in several PSI preparations of Chlamydomonas reinhardtii where amino acid ligands of the primary donor, primary acceptor, or connecting chlorophylls have been mutated. Most of these mutations influence the spectrum of the absorbance changes. This supports the view that the chlorophylls of the electron transfer chain as well as the connecting chlorophylls are engaged in the observed absorbance changes. The wide absorption spectrum of the electron transfer chain revealed by the transient measurements may contribute to the high efficiency of energy trapping in photosystem 1. Exciton calculations, based on the recent PSI structure, allow an assignment of the ΔA bands to particular chlorophylls: the bands at 675 and 695nm to the dimers of primary acceptor and accessory chlorophyll and the band at 683nm to the connecting chlorophylls. The subpicosecond transient absorption bands decay may reflect rapid charge separation in the PSI reaction center.