Interaction of the water oxidising manganese complex of photosystem II with the aqueous environment has been investigated using electron paramagnetic resonance spectroscopy and electron spin echo envelope modulation spectroscopy to detect interaction of [ 2 H]methanol with the complex in the S 2 state. The experiments show that the classical S 2 multiline signal is associated with a manganese environment which is not exposed to the aqueous medium. An electron paramagnetic resonance spectroscopy signal, also induced by 200 K illumination, showing 2 H modulation by methanol in the medium and a modified multiline electron paramagnetic resonance spectroscopy signal formed in parallel to it, are suggested to be associated with a second manganese environment exposed to the medium.z 1999 Federation of European Biochemical Societies.

To study electrogenesis the photosystem I particles from Synechococcus elongatus were incorporated into asolectin liposomes, and fast kinetics of laser flash-induced electric potential difference generation has been measured by a direct electrometric method in proteoliposomes absorbed on a phospholipid-impregnated collodion film. The photoelectric response has been found to involve three electrogenic stages associated with (i) iron-sulfur center Fx reduction by the primary electron donor P700, (ii) electron transfer between iron-sulfur centers Fx and FA/FB, and (iii) reduction of photo-oxidized P700+ by reduced cytochrome C553. The relative magnitudes of phases (ii) and (iii) comprised about 20% of phase (i).

Photovoltage responses accompanying electron transfer on the acceptor side of photosystem I (PS I) were investigated in proteoliposomes containing PS I complexes from the cyanobacterium Synechococcus sp. PCC 6301 using a direct electrometrical technique. The relative contributions of the F CF f and the F CF e electron transfer reactions to the overall electrogenicity were elucidated by comparing the sodium dithionite-induced decrease in the magnitude of the total photoelectric responses in control and in F f -less (HgCl P -treated) PS I complexes. The results obtained suggest that the electrogenesis on the acceptor side of PS I is related to electron transfers between both F and F e and F e and F f . Based on the electrogenic nature of the latter reaction in PS I complexes, we conclude that F e rather than F f is the acceptor proximal to F .z 1998 Federation of European Biochemical Societies.

An electrometrical technique was used to investigate electron transfer between the terminal iron^sulfur centers F A /F B and external electron acceptors in photosystem I (PS I) complexes from the cyanobacterium Synechococcus sp. PCC 6301 and from spinach. The increase of the relative contribution of the slow components of the membrane potential decay kinetics in the presence of both native (ferredoxin, flavodoxin) and artificial (methyl viologen) electron acceptors indicate the effective interaction between the terminal [4Fe^4S] cluster and acceptors. The finding that F A fails to donate electrons to flavodoxin in F B -less (HgCl 2 -treated) PS I complexes suggests that F B is the direct electron donor to flavodoxin. The lack of additional electrogenicity under conditions of effective electron transfer from the F B redox center to soluble acceptors indicates that this reaction is electrically silent.z 1999 Federation of European Biochemical Societies.

The kinetics of reduction of the photo-oxidized primary electron donor P700 + by redox dyes N,N,N',N'-tetr amethyl-p-phenylendiamine, 2,6-dichlorophenoMndophenol and phenazine methosulfate was studied in proteoliposomes containing Photosystem I complexes from cyanobacteria Synechocystis sp. PCC 6803 using direct electrometrical technique, in the presence of high concentrations of redox dyes, the fast generation of a membrane potential related to electron transfer between P700 and the terminal iron-sulfur clusters FA/FB was followed by a new electrogenic phase in the millisecond time domain, which contributes approximately 20% to the overall photoelectric response. This phase is ascribed to the vectorial transfer of an electron from the redox dye to the proteinembedded chlorophyll of P700 +. Since the contribution of this electrogenic phase in the presence of artificial redox dyes is approximately equal to that of the phase observed earlier in the presence of cytochrome co, it is likely that electrogenic reduction of P700 + in vivo occurs due to vectorial electron transfer within RC molecule rather than within the cytochrome c6-P700 complex.© 1997 Federation of European Biochemical Societies.