Bromide does not bind to the Mn4Ca complex in its S1 state in Cl(-)-depleted and Br(-)-reconstituted oxygen-evolving photosystem II: evidence from X-ray absorption spectroscopy at the Br K-edge.

Chloride is an important cofactor in photosynthetic water oxidation. It can be replaced by bromide with retention of the oxygen-evolving activity of photosystem II (PSII). Binding of bromide to the Mn(4)Ca complex of PSII in its dark-stable S(1) state was studied by X-ray absorption spectroscopy (XAS) at the Br K-edge in Cl(-)-depleted and Br(-)-substituted PSII membrane particles from spinach.

Characterization of a semi-stable, charge-separated state in reaction centers from Rhodobacter sphaeroides.

In reaction centers from Rhodobacter sphaeroides, subjected to continuous illumination in the presence of an inhibitor of the Q(A) to Q(B) electron transfer, the oxidation of P870 consisted of several kinetic phases with a fast initial reaction followed by very slow accumulation of P870(+) with a halftime of several minutes. When the light was turned off, a phase of fast charge recombination was followed by an equally slow reduction of P870(+). In reaction centers depleted of Q(B), where forward electron transfer from Q(A) is also prevented, the slow reactions were also observed but with different kinetic properties.

Conformational regulation of charge recombination reactions in a photosynthetic bacterial reaction center.

In bright light the photosynthetic reaction center (RC) of Rhodobacter sphaeroides stabilizes the P(+)(870).Q(-)(A) charge-separated state and thereby minimizes the potentially harmful effects of light saturation. Using X-ray diffraction we report a conformational change that occurs within the cytoplasmic domain of this RC in response to prolonged illumination with bright light.

Spectroscopic characterization of a semi-stable, charge-separated state in Cu(2+)-substituted reaction centers from Rhodobacter sphaeroides.

In reaction centers from Rhodobacter sphaeroides exposed to continuous illumination in the presence of an inhibitor of the Q(A)(-) to Q(B) electron transfer, a semi-stable, charge-separated state was formed with halftimes of formation and decay of several minutes. When the non-heme iron was replaced by Cu(2+), the decay of the semi-stable, charge-separated state became much slower than in centers with bound Fe(2+) with about the same rate constant for formation. In Cu(2+)-substituted reaction centers, the semi-stable state was associated with an EPR signal, significantly different from that observed after chemical reduction of the acceptor-side quinone or after illumination at low temperature, but similar to that of an isolated Cu(2+) in the absence of magnetic interaction.

Lipidic cubic phase crystal structure of the photosynthetic reaction centre from Rhodobacter sphaeroides at 2.35A resolution.

Well-ordered crystals of the bacterial photosynthetic reaction centre from Rhodobacter sphaeroides were grown from a lipidic cubic phase. Here, we report the type I crystal packing that results from this crystallisation medium, for which 3D crystals grow as stacked 2D crystals, and the reaction centre X-ray structure is refined to 2.35A resolution.

The function of the chloride ion in photosynthetic oxygen evolution.

The involvement of Cl(-) and several other monovalent anions in photosynthetic oxygen evolution was studied using photosystem II membranes depleted of Cl(-) by dialysis. The results of these studies differ significantly from results obtained using other depletion methods. Binding studies with glycerol as a cryoprotectant confirm our previous observations with sucrose of two interconvertible binding states of photosystem II with similar activities and with slow or fast exchange, respectively, of the bound ion.