Charge translocation

Just as for the ground-state radical anion dissociation discussed in Section 3.7.2, we treat the model excited state PSB isomerization problem in terms of three key coordinates to describe the motion and two VB states to describe the electronic structure aspects. For two of these coordinates, as well as for insight into which VB states are the essential ones, we have benefited from extensive previous vacuum theoretical studies by Robb, Olivucci and coworkers [16,83-89] and also by Martinez and coworkers [16,88], In particular, the former authors have shown that upon an initial Franck-Condon (FC) excitation from the ground state to the excited state, there is a significant charge translocation in a PSB,... 

The term 6 ms,de v> n Equation (7.15) computes the total charge translocated across the membrane for the transport process. Note that since a chemical reaction does not create or destroy charge,... 

H are agreed to be released on the C side of the membrane, and one electrical charge equivalent translocated, per transferred electron when ubiquinol is oxidised by ferricytochrome c. In mitochondria this is thermodynamically equivalent to translocation of IH /e because release of the second proton to the well-buffered C phase is thermodynamically futile (without corresponding charge translocation Table 3.1). 

Comparatively few papers describe the properties of Complex III after reconstitution into liposomal membranes [198,279,280]. The found stoicheiometries of proton and electrical charge translocation agree well with the data for mitochondria. 

HPTS is a pH-sensitive fluorophore (pk, 7.3) [6]. The opposite pH sensitivity of the two excitation maxima permits the ratiometric (i.e. unambiguous) detection of pH changes in double-channel fluorescence measurements. The activity of synthetic ion channels is determined in the HPTS assay by following the collapse of an applied pH gradient. In response to an external base pulse, a synthetic ion channel can accelerate intravesicular pH increase by facilitating either proton efflux or OH influx (Fig. 11.5c). These transmembrane charge translocations require compensation by either cation influx for proton efflux or anion efflux for OH influx, i.e. cation or anion antiport (Fig. 11.5a). Unidirectional ion parr movement is osmotically disfavored (i.e. OH /M or X /H symport). HPTS efflux is possible with pores only (compare Fig. 11.5b/c). Modified HPTS assays to detect endovesiculation (Fig. 11.1c) [16], artificial photosynthesis [17] and catalysis by pores [18] exist. 

Forward electron transfer through the reaction center. Note that two charge translocations must occur in order for the (labile) quinol QbH2 to be produced. Once QbH2 dissociates from the RC and is replaced by another, oxidized, Qg, the cycle can begin anew. 

It is known that the D-pathway transfers the pumped protons, but it also transfers substrate protons to be consumed at the binuclear site, at least those consumed during the Pm F - Oh reactions. Mutation of the lysine in the K-pathway (Figure 3a) hardly affects these reactions, but strongly inhibits reduction of heme 03 and blocks the reversal of the E R reaction step. Measurements of charge translocation in reconstituted oxidase vesicles suggested that also the Oh — Eh reaction is blocked by this mutation. One important problem, already alluded to, is indeed why two proton transfer pathways are required, especially since the two do not discriminate between pumped and substrate protons, as first assumed. ... 

Although potentiometry provide most valuable data that reflect charge transfer processes in the enzyme, getting molecular insights from the experiments proved to be difficult because of lack of theory for proper quantitative interpretation of the data. Until recently, it has not been possible to directly relate the potentiometric data to a specific molecular mechanism of charge translocation in CcO. 

Eq.(l) shows that the full power of Stark spectroscopy is exploited only if the method is used with oriented samples. One obvious way to accomplish this aim is to incorporate the protein into LB structures. From a materials science view, this comprises the first step toward the design of a composite system, incorporating a functional entity into a lipid matrix. More important, this will facilitate an independent measurement of static and induced dipole moment changes. The directionality of the charge translocation can be determined in oriented samples, whereas, at present, it is only evaluated as a cone angle, 5, around the axis defined by the transition moment. 

As pointed out by Mitchell, all uncouplers are weak acids (HW) with relatively high solubility in the lipid phase of the membrane. In BLMs, weak acids act as carriers for H+ and the charged translocating species is either the anionic form of the weak acid, W, or the complex of the anion, HW2 and the undissociated acid, HW. Thus, the concentration of the undissociated HW and W should be governed by the pH of the outer and inner bathing solutions . At a steady state in the membrane phase... 

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