Antenna size dependence offluorescence decayinthecoreantenna ofphotosystem I:Estimates ofcharge separation andenergy transfer rates

Wehaveexamined thephotophysics ofener- gymigration andtrapping inphotosystem Ibyinvestigating the spectral andtemporal properties ofthefluorescence fromthe coreantenna chlorophylls asafunction oftheantenna size. Time-correlated single photon counting wasusedtodetermine thefluorescence lifetimes intheisolated P700chlorophyll a-protein complex andina mutantofChlamydomonas reinhardtii that lacks thephotosystem IIreaction center com- plex. Thefluorescence decayinbothtypes ofsample is dominated byafast (1545psec) component that isattributed tothelifetime ofexcitations inthephotosystem Icore antenna. These excitations decay primarily byanefficient photochemical quenching onP700.Themeasured lifetimes showalinear relationship tothecore antenna size. Alinear dependence ofthe excitation lifetime onantenna size waspredicted previously in alattice model forexcitation migration andtrapping inarrays ofphotosynthetic pigments (Pearlstein, R.M.(1982) Photo- chem. Photobiol. 35,83544M). Based onthis model, ourdata predict atime constant forphotochemical charge separation in thephotosystem Ireaction center of2.8±0.7or3.4±0.7psec, assuming monomeric ordimeric P700, respectively. Thepre- dicted average single-step transfer timeforexcitation transfer between coreantenna pigments is0.21± 0.04psec. Under these conditions, excitation migration inphotosystem Iisnear thediffusion limit, witheachexcitation making anaverage of 2.4visits tothereaction center before photoconversion. Theprimary steps inphotosynthesis areabsorption oflight andcreation ofasinglet excitation, transfer oftheexcitation between pigment molecules, andphotochemical charge sep- aration inthereaction center. Therelative kinetics ofthe excitation transfer andtrapping reactions determine critical aspects oftheoverall light-harvesting process. Excitation transfer isbelieved tooccurbyanincoherent hopping mechanism (1), withtheexcitation following arandom walk through theantenna pigments tothereaction center (2). The mechanism ofcharge separation depends ontheunique redox properties ofthereaction center pigments intheexcited state andappears tobesimilar inbacteria andplants. Thereaction center complex ofpurple photosynthetic bacteria contains only sixpigment molecules (3), atleast four ofwhich aredirectly involved intheprimary photochemical