V. M. Ramesh

Excitonic interactions in wild-type and mutant PSI reaction centers

Femtosecond excitation of the red edge of the chlorophyll a Q(Y) transition band in photosystem I (PSI), with light of wavelength > or = 700 nm, leads to wide transient (subpicosecond) absorbance changes: positive DeltaA between 635 and 665 nm, and four negative DeltaA bands at 667, 675, 683, and 695 nm. Here we compare the transient absorbance changes after excitation at 700, 705, and 710 nm at 20 K 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 DeltaA bands to particular chlorophylls: the bands at 675 and 695 nm to the dimers of primary acceptor and accessory chlorophyll and the band at 683 nm to the connecting chlorophylls. The subpicosecond transient absorption bands decay may reflect rapid charge separation in the PSI reaction center.

A0 → A1 Electron Transfer in Chlamydomonas reinhardtii PS I with Replaced A0 Axial Ligand

Replacement of methionine, the natural axial ligand to the primary electron acceptor (A0) in Photosystem I, with a series of different amino acids results in dramatic increase of the A0 − lifetime from 20 ps in wild type to a few nanoseconds in the mutants in the case of Chlamydomonas reinhardtii (Ramesh et al. 2004, 2007). This effect is similar independently if the mutation affects A-side or B-side A0. This observation confirms an existence of two equivalent primary electron acceptors in both symmetric branches of Photosystem I in Chlamydomonas reinhardtii, which makes this photosystem unusual among other photosystems (from purple bacteria, PS II), which are essentially unidirectional. However, it is still not clear if the bidirectionality of electron transfer in Photosystem I is complete, i.e. if the electron from A0 − reaches A1 in both branches or takes another route in the “non-active” branch. In order to solve this issue, in this contribution we attempted to compare kinetics of A0 − reoxidation to the kinetics of A1 − formation in the case of B-side A0 mutant with methionine replaced by serine.