Primary processes in photosystem I reaction center at cryogenic temperatures

Abstract

Structural model at a molecular resolution has been reported for the photosystem I reaction center, where there are 6 chlorophyll molecules (Chl) in the electron transfer system. It has been, however, difficult to detect the elementary processes in the electron-transfer system. This is partly due to the large number (about 80 Chls) of antenna chlorophylls that surround the electron transfer system. We have applied a femtosecond transient absorption method to photosystem I reaction center with a reduced number of chlorophylls (about 12) per its primary donor chlorophyll (P700). The temperature of the sample was cooled to as low as 9 K, where uphill excitation transfers were suppressed. We thus aimed at revealing the primary processes in the electron transfer system when thermal activation does not work. Under the P700-preoxidized conditions, about 90 % of the initial photobleaching signal of the Chls upon a nonselective excitation at 630 nm decayed within 120 ps at 9 K. This ratio was found to be comparable to that at 290 K. This suggests that, for the most Chls, there is at least one energetically downhill and efficient excitation-transfer pathway to P700+. Under the P700-neutral conditions, the transient absorption in the 720 ¿ 780 nm region upon a preferential excitation of P700 at 9 K was measured. This has enabled us to understand the photochemistry from the excited state of P700, without interference from excited states of the other chlorophylls.