Kinetically distinct three red chlorophylls in photosystem i of thermosynechococcus elongatus revealed by femtosecond time-resolved fluorescence spectroscopy at 15 K

Time-resolved fluorescence spectra of photosystem I (PS-I) trimeric complex isolated from a thermophilic cyanobacterium, Thermosynechococcus (T.) elongatus, were observed at 15 K over the time range from 100 fs to a few nanoseconds under P700-oxidized condition and 10 ps to a few nanoseconds under P700-reduced condition. Global-fitting analysis of the data of P700-oxidized condition revealed the existence of three kinetically different red chlorophylls (ChIs) having the energy-transfer times to P700⁺ of 6.1 ps (C _{6.1 ps}), 140 ps (C_{140 ps}), and 360 ps (C _{360 ps})- According to the spectral shape of DAS, C_{6.1 ps}, C_{140 ps}, and C_{360 ps} were assigned to the previously reported red Chls with the absorption maxima at 715 nm (C715), 710 nm (C710), and 719 nm (C719), respectively. In PS-I containing P700⁺, ca. 60 Chls funnel the excitation energy into C_6.1 ps in a subpicosecond time region at 15 K. The analysis of the present data together with the conclusions of the previous reports revealed that in PS-I containing a neutral P700 the direct energy transfer from the bulk Chls to P700 seems to dominate the energy-flow process. Simulation of the energy-transfer time to P700⁺ based on Förster theory suggested the dimeric Chls A32-B7 and A33-A34 as the most probable candidates for C_{140 ps} (C710) and C _{360 ps} (C719), respectively. C_{6.1 ps} (C715) was tentatively assigned to the dimeric Chl B24-B25 or A26-A27, for which the fastest energy transfer to P700⁺ was predicted from the simulation. However, the estimated energy-transfer times to P700⁺ for these dimeric Chls were 44-46 ps, which were still much slower than the observed value of 6.1 ps. A theoretical framework beyond the standard Förster theory might be required in order to account for the severe deviation.