RESOLUTION OF LOW-ENERGY CHLOROPHYLLS IN PHOTOSYSTEM-I OF SYNECHOCYSTIS SP PCC-6803 AT 77-K AND 295-K THROUGH FLUORESCENCE EXCITATION ANISOTROPY

Fluorescence excitation spectra of highly anisotropic emission from Ph otosystem I (PS I) were measured at 295 and 77 K on a PS II-less mutan t of the cyanobacterium Synechocystis sp. PCC 6803 (S. 6803). When PS I was excited with light at wavelengths greater than 715 nm, fluoresce nce observed at 745 nm was highly polarized with anisotropies of 0.32 and 0.20 at 77 and 295 K, respectively. Upon excitation at shorter wav elengths, the 745-nm fluorescence had low anisotropy. The highly aniso tropic emission observed at both 77 and 295 K is interpreted as eviden ce for low-energy chlorophylls (Chls) in cyanobacteria at room tempera ture. This indicates that low-energy Chls, defined as Chls with first excited singlet-state energy levels below or near that of the reaction center, P700, are not artifacts of low-temperature measurements. If t he low-energy Chls are a distinct subset of Chls and a simple two-pool model describes the excitation transfer network adequately, one can t ake advantage of the low-energy Chls' high anisotropy to approximate t heir fluorescence excitation spectra. Maxima at 703 and 708 nm were ca lculated from 295 and 77 K data, respectively. Upper limits for the nu mber of low-energy Chls per P700 in PS I from S. 6803 were calculated to be 8 (295 K) and 11 (77 K).