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 (Chls) 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 Forster 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 Forster theory might be required in order to account for the severe deviation.