Nonturnover condition

Such a scheme lends itself to several alternative descriptions of the oxidative reaction 117, 118). However, since the AT-Compound I reaction is pH-invariant 115), the pX, of distal histidine could be atypical or, more likely, its modification is not rate determining in the reaction sequence of Elq. (4). It is uncertain, however, whether fci or fcj represents the slow step of the reaction. Kinetic or analytical demonstration of a Compound I-AT complex is also lacking. Thus, under nonturnover conditions using preformed Compound I, the redox reactions are first order in Compound I and AT when [AT] <70 mM. 

Second, under nonturnover conditions, with preformed compound I, the oxidation of semicarbazide is accompanied by uptake of oxygen (118b), suggesting the formation of an intermediate such as diimide (123, 124), followed by its autoxidation [cf. Eq. (6)]. In a parallel reaction with AT, no uptake of oxygen is to be expected, and none is obtained (118b). 

Only recently, however, has spectroelectrochemistry been used for in situ measurements of the redox state of cytochromes in thick, pregrown EABs in which extracellular electron transfer through the biofilm matrix was studied. It was shown that the c-type cytochromes inside thick G. sulfurreducens biofilms probed under nonturnover conditions were completely reduced at polarization potentials below -350 mVsHE and completely oxidized at potentials above -ElOO mVsHE> demonstrating long-range extracellular electron transfer through the cytochrome network [66]. 

One underutilized approach to CV in EAB research that is particularly powerful under nonturnover conditions is to vary the initial potential and the potential window to study anodic and cathodic peak coupling. Figure 5.16c shows three curves, indicated by (1), (2), and (3), superimposed to form a complete curve. Curve 1 was taken for a potential range of -0.35 to -0.7 V range of -0.4 to -0.3 V... 

Figure 5.18 Scan rate analysis of a G. sulfurreducens biofikn under nonturnover conditions, (a) CVs at increasing scan rates, (b) Peak currents at each scan rate plotted against the square root of the scan rate, (c) Nonturnover CVs at 1 and 10 mV s showing the superposition (arrows) of multiple anodic and cathodic peaks.

Here, we have demonstrated the differences between turnover and nontumover CVs for G. sulfurreducens biofilms. Turnover and nonturnover CVs can be used to correlate catalytic current under turnover conditions to redox peaks under nonturnover conditions. Scan rate analysis can provide a qualitative understanding of the biofilm electron-transfer mechanisms occurring within a biofilm. However, caution should be used when applying the Randles-Sevcik criterion to biofilms, as it was derived for a single-step, reversible electron-transfer step for diffusing mediators. 

Figure 5.30 Cyclic voltammograms and corresponding frequency shift responses of G. sul-furreducens biofikn under (a) turnover and (b) nonturnover conditions. Reproduced with permission from Ref. [59]. Copyright 2014 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim.

Figure 6.1 Schematic depiction of trans-biofilm electron transport by incoherent multistep electron hopping (also referred to as redox conduction) under nonturnover condition.

Two Electrodes under Nonturnover Condition Versus One Electrode under Turnover Condition... 

Figure 6.1 depicts the proposed conduction process under nonturnover condition when acetate is not present [9, 10]. Here, electrons enter the biofilm from the source electrode, are conducted across the biofilm, and exit the biofilm by the drain electrode... 

Rationale for Measuring Electron Transport Rates Using Two Electrodes Under Nonturnover Condition... 

The sigmoid-shaped dependency of catalytic current on anode potential, observed for Geobacter biofilms under turnover condition [23], also results directly from the Nernst Equation. This indicates that electron transfer between a Geobacter biofilm and an electrode is fast and not the limiting factor in catalytic current generation that is comparable in magnitude to that observed for conducted current under nonturnover condition for the same biofilm [6,7],)... 

Figure 63 Schematic depiction of concentration profile of reduced cofactor across the hiofihn under nonturnover condition for the source/biofilm/drain geometry depicted in Figure 6.1. The slope of the profile is the redox gradient. The rate of electron transport through any vertical plane sliced through the biofilm is the same and proportional to the redox gradient. The profile is, therefore, a straight line (i.e., has a constant slope) across the biofilm.

Combination of Equations 6.5-6.8 yields Equation 6.9, an expression for the rate of electron transport across the source/biofilm interface through the biofilm and across the drain/biofilm interface at steady-state condition as a function of the potentials applied to the source and drain under nonturnover condition. Here, the rate of electron transport is expressed as a current, commonly referred to as the source-drain current ( sd)-... 

Figure 8.7 (a) CV of a Geobacter biofilm grown at 0.2 V versus Ag/AgCI on a graphite rod electrode in substrate depleted (nonturnover) conditions to e[ indicates formal potentials of the four detected redox couples of the biofilm, (b) CV of the same biofilm in... 

The pH dependence of catalytic currents under conditions of nonturnover-limiting substrate delivery. 

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