Trumpet plots

A mutant form of ferredoxin 1 has been obtained in which the surface aspartate-15 residue is replaced by asparagine. With this mutant protein the voltammograms are similar to the wild type, except that the anodic wave at low pH disappears at a lower scan rate and reappears at very rapid scan rates. Trumpet plots of the CV data, as shown in Fig. 13 are quite helpful in displaying these results. The fact that gating occurs at lower scan rates for the mutant than for the wild type implies that... 

Fig. 13. Representative Trumpet Plots for the [3Fe-4S]+/0 couple in native and D15N mutant forms of Azotobacter vinelandii ferredoxin I adsorbed on a PGE electrode. The plots for D15N also show the fits based on k0 t = 2.5 s-1. Note the intermediate region of the plot (pH 5.50) in which an oxidation peak is not observed because ET is gated. Data points shown in red are for the pH values indicated whereas data points shown in blue are for the uncoupled electron-transfer reaction occurring at pH > pffoiuater- Reproduced from Ref. (33) by permission of the Royal Society of Chemistry.

When the scan rate is sufficiently high to preclude sample equilibration with the electrode potential, then one, or both, of the peak currents will fail to achieve the magnitude predicted from the behavior displayed at lower scan rates for examples, see Fig. 3. In such conditions, the peaks become smeared across the potential axis in a maimer that allows for kinetic resolution of the underlying events. If the oxidative and reductive peaks are displaced equally, but in opposite directions, about the reduction potential, then interfacial electron transfer is the rate-limiting step. Fig. 3a. The variation of peak separation with scan rate, sometimes termed a trumpet plot, allows quantification of the standard heterogeneous rate constant for electron transfer. 

Figure 10 Representative Trumpet Plots for D15N and native A.v. Fdl. Those for D15N also show the fits based on = 2.5 s. Note the region (pH 5.50) in which the oxidative peak is not observed. Open symbols refer to data obtained at pH = 8.34.

Fig. 6.8. Typical plots of residual deviations random scattering (a), systematic deviations indicating nonlinearity (b), and trumpet-like form of heteroscedasticity (c)...

Homoscedasticity. Unequal variances are recognizable from residual plots as in Fig. 6.8c where frequently ey is a function of x in the given trumpet-like form. In such a case, the test of homoscedasticity can be carried out in a simple way by means of the Hartley test (Fmax test), Fmax = smax/5min> see Sect. 4.3.4 (1). 

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