Electronic gate

The PSII complex contains two distinct plastoquiaones that act ia series. The first is the mentioned above the second, Qg, is reversibly associated with a 30—34 kDa polypeptide ia the PSII cote. This secondary quiaone acceptor polypeptide is the most rapidly tumed-over proteia ia thylakoid membranes (41,46). It serves as a two-electron gate and connects the single-electron transfer events of the reaction center with the pool of free... 

A single channel pulse height analyser utilizes an electronic gate typically 0.1 V wide, which only accepts pulses between the preset upper and lower limits. Scintillation counters frequently employ such devices to remove small noise pulses and large pulses initiated by cosmic rays, as well... 

Ions will not be rejected by electronic gating. Mass analysis of the primary beam Is generally desirable to Insure that no Impurities are present In the beam. Such species will become Implanted Into the sample and preclude their analysis at low concentrations In a sample (2). 

Brzezinski, P. Malmstrom, B.G. (1987). The mechanism of electron gating in proton pumping cytochrome c oxidase. Biochim. Biophys. Acta 894,29-38. 

As seen in Figure 3-5, both the analysis side and the coincidence side of the scintillation counter contain discriminators, which are particularly useful in experiments that employ two or more /3 emitting isotopes. Discriminators contain electronic gates that limit the portions of an energy... 

It should be mentioned that electron transfer to the quinone pool, both by PS II and by the reaction centers of purple bacteria, now proceeds via a two-electron gating mechanism after one electron has arrived at the temporarily bound quinone Qb, the semiquinone remains in its unprotonated, negatively charged form and tightly bound to the reaction center only after a second photoreaction does its full reduction, protonation and release as a quinol take place [19]. This procedure may also have played a role in the selection of the dimeric reaction center structure, but its importance most likely has to do with the reactivity of semiqui-nones with molecular oxygen and in that case it probably appeared much later. 

The delivery of electrons from PSII-RC to the plastoquinone pool was originally proposed to occur via the formation of free semiquinone and the spontaneous dismutation to quinol [236]. It is clear now that, both in PSII and in bacterial RC, this electron transfer is controlled by a two electron gate mechanism involving bound forms of quinones, in which the semiquinone anion is stabilized for a time long enough to allow two turnovers of the same reaction center [40]. If diffusion of the quinone indeed happens, this occurs after complete reduction to quinol at the Qg site of bacterial RC or the B site of PSII-RC and dissociation from these sites [37,38]. 

Luminescence decay curves may be observed by displaying the output of the photomultiplier on an oscilloscope. Precautions must be taken to correct for instrumental distortion of fast decay curves (D13). In multicomponent systems with differing decay times, electronic gating may be used to isolate the signal due to one component (time resolved phosphorimetry) (SI). A complete emission spectrum can be observed using a spectrograph with a photographic plate or television camera tube, but these systems are as yet only of specialist interest. 

Use a pulsed laser source to discriminate against fluorescence because the lifetime of Raman scattering (10-12-10-1° s) is much shorter than that of fluorescence (10 7—10 9 s). Thus, an electron gate can be used to preferentially measure the Raman signals. 

The Qa Qb complex may be looked upon as a two-electron gate, with the two quinones, one primary and the other secondary, acting in series. The two-electron reduction of the secondary quinone results from two one-electron reduction steps by the primary quinone and uptake of two protons. This series of reactions result in a binary oscillation as seen in the accompanying absorbance changes shown in figure 3. 

CA Wraight (1988) The involvement of stable semiquinones in the two electron gates of plant and bacterial photosynthesis. In B Trumpower (ed) Function of Quinones in Energy-Conserving Systems, pp 181-197... 

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