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When the photosystem II quinone acceptor complex has been singly reduced to the state Q AQ −B, there is a 22 s half-time back-reaction of Q −Bwith an oxidized photosystem II donor (S 2), directly measured here for the first time. From the back-reaction kinetics with and without inhibitors, kinetic and equilibrium parameters have been estimated. We suggest that the state Q AQ −Bof the complex is formed by a second-order reaction of vacant reaction centers in the state Q −Awith plastoquinone from the pool, and discuss the physico-chemical parameters involved.

The two hemes of cytochrome b6in the cytochrome b6fcomplex isolated from chloroplasts exhibit slight spectral differences in the α-band at room temperature, sufficient to deconvolute redox kinetics of cytochrome b6into the contributions of the two components. The deconvoluted changes observed can be explained by fast electron transfer from low- to high-potential cytochrome b6.

We discuss energy-dependent fluorescence lowering (q E-quenching), and suggest a model to explain the experimental data currently available. The main elements of the model are: (a) the q E-quenching reflects a mechanism associated with a component of the light-harvesting antenna rather than the reaction center of photosystem (PS) II — we suggest that it occurs through formation of an efficient quencher in one of the minor chlorophyll protein (CP) complexes; (b) the minor CPs have glutamate residues instead of glutamines at positions shown in light-harvesting complex II (LHCII) to be ligands to chlorophylls near the lumenal interface. We suggest that the quenching reflects a change in ligation of chlorophyll on protonation of these glutamate residues leading to formation of an exciton coupled dimer with a neighboring pigment, in which additional energy levels allow vibrational relaxation of the excited singlet. The model accounts for the dependence on low lumenal pH, the ligand residue changes between LHCII and the minor CPs, the preferential distribution of components of the xanthophyll cycle in the minor CPs, the inhibition of q E-quenching by DCCD, and the specific binding of DCCD to the minor CPs.

Cytochrome b-561 of the ubiquinol-cytochrome c2oxidoreductase complex of Rhodopseudomonas sphaeroidesis reduced after flash illumination in the presence of myxothiazol in an antimycin-sensitive reaction. Flash-induced reduction was observed over the redox range in which cytochrome b-561 and the Q-pool are both oxidized before the flash. The extent of reduction increased with increasing pH, and was maximal at pH > 10.0 where the extent approached that observed in the presence of antimycin following a group of flashes. Reduction of cytochrome b-561 in the presence of myxothiazol showed a lag of ~ 1 ms after the flash, followed by reduction with ( t12~ 6 ms; by analogy with the similar kinetics of the quinol oxidase site, we suggest that the rate is determined by collision with the QH 2produced in the pool on flash excitation.

A new method of measuring the rate of the back reaction from the state Z +P680 Q A−in Tris-washed chloroplasts is described. By using ratios of back reaction rates we demonstrate a Tris-induced change in the equilibrium between Z and P680 and attribute this change to an alteration of the midpoint potential of Z by Tris treatment. We also demonstrate that the previously observed inhibition of the back reaction by ADRY reagents can be localized at Z and understood in terms of electron donation to Z +by ADRY reagents.

When tyrosine‐Z of the D1‐polypeptide of the photosystem II from Chlamydomonas reinhardtiiwas changed to phenylalanine, the rapid donor to P680+was lost, and P680+accumulated on illumination. The rapid donation from tyrosine‐Z was replaced by a slow electron transfer from an endogenous donor. Spectrophotometric measurements showed that carotenoids and chlorophylls were bleached by the P680+either directly or indirectly upon illumination. The carotenoid bleaching was inhibited in the presence of SOD or catalase, but the reaction did not require molecular oxygen as an electron acceptor. These observations led us to conclude that active oxygen radicals, possibly hydroxyl radicals, take part in the destruction of carotenoids in the Y161F mutant. Possible mechanisms for the destruction are discussed.

A quantitative study of the kinetics of electron transfer under coupled conditions in photosynthetic bacteria has so far been prevented by overlap of the electrochromic signals of carotenoids and bacteriochlorophyll with the absorbance changes of cytochromes and reaction centers. In this paper a method is presented by which the electrochromic contribution at any wavelength can be calculated from the electrochromic signal recorded at 505 nm, using a set of empirically determined polynomial functions. The electrochromic contribution to kinetic changes at any wavelength can then be subtracted to leave the true kinetics of the redox changes. The corrected redox changes of the reaction center measured at 542 and 605 nm mutually agree, thus providing an excellent test of self-consistency of the method. The corrected traces for reaction center and of cytochrome b-566 demonstrate large effects of the membrane potential on the rate and poise of electron transfer. It will now be possible to study the interrelation between proton gradient and individual electron transfer reactions under flash or steady-state illumination.

The redox potential of the Rieske Fe‐S protein has been investigated using circular dichroism (CD)‐spectroscopy. The CD features characteristic of the purified bc1complex and membranes of Rhodobacter sphaeroideswere found in the region between 450 and 550 nm. The difference between reduced and oxidized CD‐spectra shows a negative band at about 500 nm with a half of width 30 nm that corresponds to the specific dichroic absorption of the reduced Rieske protein (Fee, J.A. et al. (1984) J. Biol. Chem. 259, 124–133; Degli Esposti, M. et al. (1987) Biochem. J. 241, 285–290; Rich, P.R. and Wiggins, T.E. (1992) Biochem. Soc. Trans. 20, 241S). It was found that the redox potential at pH 7.0 for the Rieske center in the isolated bc1complex and in chromatophore membranes from the R‐26 strain of Rb. sphaeroidesis 300±5 mV. In chromatophores from the BC17C strain of Rb. sphaeroides, the Emvalue measured for the Rieske iron‐sulfur protein (ISP) was higher (315±5 mV), but the presence of carotenoids made measurement less accurate. The Emvaried with pH in the range above pH 7, and the pH dependence was well fit either by one pKat ∼7.5 in the range of titration, or by two pKvalues, pK1=7.6 and pK2=9.8. Similar titrations and pKvalues were found for the Rieske Fe‐S protein in the isolated bc1complex and membranes from the R‐26 strain of Rb. sphaeroides. The results are discussed in the context of the mechanism of quinol oxidation by the bc1complex, and the role of the iron sulfur protein in formation of a reaction complex at the Qo‐site.