Excitonic Interactions in Wild-Type and Mutant PSI Reaction Centers

Abstract

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.

Excitation dynamics in Photosystem I from Chlamydomonas reinhardtii. Comparative studies of isolated complexes and whole cells

“…1, 2), however it is more pronounced in the case of PSI-LHCI complexes. In the case of PSI core, the long-wavelength emission may come from the long-wavelength chlorophylls located in the RC [19,51,52] or in the core antenna [23,49,53]; neither possibility can be definitively excluded based on the present data.…”

Spectral Resolution of the Primary Electron Acceptor A0 in Photosystem I

“…The measured (A 0 − − A 0 ) spectrum is very similar in shape to the (Chl a − − Chl a) spectrum in solution (Figure 7b), which indicates that excitonic coupling of A 0 with the neighboring pigments are not significant in our experimental conditions. Indeed, excitonic coupling between A 0 and its closest neighboran accessory pigmentwas estimated to be between 35 and 181 cm −1 , 45,46 which is lower than thermal energy kT ∼ 200 cm −1 at room temperature. The excitonic effects (several bands in the ΔA spectrum) have been observed earlier in pump−probe ΔA signals for PS I, but at a much lower temperature of 20 K (kT = 14 cm −1 ) and only under conditions when (possibly several) excitonic states were excited at ∼700 nm.…”

Structure of cyanobacterial Photosystem I

“…Recent mutagenesis studies on the ligands of these connecting chlorophylls (Gibasiewicz et al 2003) show minor alterations in the trapping of the excitation energy, but the question as to whether the connecting chlorophylls play a crucial role in energy transfer is still not resolved.…”