Key Chlorophyll Molecules in the Uphill Energy Transfer from Chlorophyll to P700 in Far-Red Light-Adapted Photosystem I.

Multiple far-red light-adapted photosystem I (FR-PSI) reaction centers are recently found to work in oxygenic photosynthesis. They contain a small amount of a new type pigment chlorophyll (Chl ) in addition to the major pigment chlorophyll (Chl ). FR-PSI differs from the conventional PSIs in plants and cyanobacteria, which use only visible light absorbed by Chl , although the mechanism of FR-PSI is not fully clear yet. We theoretically studied the light-harvesting mechanism of FR-PSI of PCC 7521, in which a small amount of Chl transfers the excitation energy of FR-light uphill to Chl . We constructed two types of exciton models for FR-PSI using pigment arrangements based on the structural information. A model that assumes the same site energy value for all of the antenna Chl molecules reproduced most of the experimentally obtained properties. The transient absorption spectra, excitation energy relaxation, and mean first passage time (MFPT) of the excitation energy transfer from Chls and to the special pair P700 (a pair of Chl /Chl ) were numerically calculated. The model, however, could not reproduce the low but distinct absorption intensity between the Chl - and Chl -bands and predicted a rather slow energy transfer from Chl to P700. Advanced "modified models" further tested the effect of modification of the site energy values at individual antenna Chl molecules. The optical properties and MFPTs of FR-PSI were calculated for each model with modified site energy values to evaluate the uphill light-harvesting process. The analysis showed that Chl -1131 and -1222 play key roles in the light-harvesting process from Chl molecules to P700, regardless of the excitation wavelength. The locations and site energy values of these Chl molecules were found to be essential to reproduce the unique uphill energy transfer function of FR-PSI.