Mutations of ligands to connecting chlorophylls perturbs excitation dynamics in the core antenna of PSI from Chlamydomonas reinhardtii

Abstract

The connecting chlorophyll molecules cC and cC¿ are proposed to functionally bridge excitation energy transfer between the PS I bulk antenna and the core electron transfer cofactors (Schubert et al. 1997). The connecting chlorophylls are coordinated by His-714 of PsaB and His-730 of PsaA. Double mutants were construct that changed both His residues to Leu with the aim producing mutants in which the cC's are perturbed or removed.. Both transient absorption and fluorescence measurements show clearly a very large increase in the contribution of long living components (>>100 ps) compared to WT control. This indicates a significant decrease in the coupling between the antenna and electron transfer core. In the mutant PSI the difference spectra of the uncoupled antenna is excitation wavelength dependent (four wavelengths were applied: 670 nm, 680 nm, 695 nm and 700 nm). Also, in contrast to WT, a peak position of a 20 ps trapping component in the decay associated spectra at RT depends on excitation wavelength. This reveals poor connection between different spectral pools of chlorophyll molecules in the mutant. Selective excitation of electron transfer chlorophylls using spectrally narrow pulses (FWHM = 5 nm) centered at 695 nm and 700 nm leads to the appearance of a ~2 ps phase in the transient absorption dynamics both at RT and 10 K. This phase is absent in WT and may be interpreted as a decay of the exciton due to rapid charge separation in the uncoupled electron transfer core.