Excitation Wavelength Dependence of the Excitation Transfer in Photosystem I Reaction Center with Reduced Number of Antenna Chlorophylls

The most recent X-ray analysis of the crystal structure of Photosystem I (PS I) has shown six chlorophyll a molecules (Chl) in the electron transfer system [1]. There seem to be two pathways of the electron transfer: P700→A→A0 and P700→A’→A0’. It is not yet clarified which pathway is used nor how A and/or A’ are involved in the electron transfer. This situation is analogous to those of purple bacterial reaction centers (RC). One unique feature of PS I RC is that there are two “connecting” Chls which structurally link core antenna Chls to the electron transfer system [1]. Main excitation transfer pathway might be like: core antenna→ “connecting” Chl→A0→A→P700 and core antenna→ “connecting” Chl’→A0’→A’→P700. It is thus important to characterize the spectral properties and the excitation transfer processes associated with the “connecting” Chls, A, A’, A0 and A0’. This is, however, an extremely difficult task, because there are ≈ 100 core antenna Chls which mask the excitation transfer steps.