Potential-dependent spectral changes

Figure 2. Apparatus for measuring potential-dependent spectral changes from a stained spherical lipid bilayer. (Reproduced with permission from reference 21. Copyright 1989 Biophysical Society.)...

Figure 4.9 Time-dependent UV-Vis spectral changes obtained during the controlled-potential reduction of [(T/ TP)SbCl2] in CH2CI2/O.2 M TRAP (a) during the first 10 s at -0.40 V (vs. SCE), (b) at longer time scales at -0.40 V and (c) at —0.80 V (vs. SCE). With permission from the American Chemical Society, ref. 33.

The slow dyes that change their spectral characteristics as a result of potential-dependent accumulation are well suited for cells in suspension. However, the complexity of these mechanisms makes it very difficult to interpret the change in dye fluorescence within a single cell. Accordingly, we set out to find dyes that display a rapid, reversible, potential-dependent intracellular accumulation with no accompanying spectral perturbation (44). As previously discussed, the dyes TMRE (Chart III) and TMRM emerged from this investigation and have proven useful in a wide variety of cultured cells. 

The rate of anion diffusion can be measured in various ways. The conventional way is to use classical electrochemical methods, e.g., chronoamperometry or chronocoulome-try. The measurement of electrochemical impedance is also sometimes used. However, the electronically conducting polymers have a special property, potential-dependent absorption spectrum, which can be advantageously used to monitor the oxidation state of the polymer. In addition to the neglect of capacitive current, monitoring of the spectral change gives additional information. For instance, the presence of an isosbestic point shows that most likely... 

Simple Potential Step Difference Manipulations As was stated above, in order to pick out the potential-dependent, weak absorptions due to near-electrode and/or adsorbed species, those contributions to the signal at the detector that do not change with potential or time such as the solvent, detector response, source emission envelope, and so on must be annulled. This is generally achieved by adopting a difference protocol for the spectral data collection, the simplest of which was employed in the first paper... 

Spectroelectrochemistry as a combination of electrochemistry and UVA IS spectroscopy is self-evident because of the direct relation of electron transfer with changes in electronic orbitals and thus with spectral changes. In redox proteins, with a few exceptions, the redox activity is caused by cofactors such as hemes, quinines, iron sulfur centers or metal centers, depending on the function and on the potential range. The role of the protein backbone and the amino acid side chains is the binding and precise orientation of these cofactors and the fine-tuning of their optical and redox properties with polarity or charges. In this context, the protein must able to react to redox transitions with conformational... 

Thus, in the modified Lewis (or Lux-Flood) concept, pure alkali halides represent the highest degree of basicity as the solvent composition changes from alkali halide-rich to alkali halide-deficient melts, the solvent becomes acidic. Acid-base properties of molten halides may be used to explain stabilization of unusually low (or high) oxidation states, the differences in stability of the same oxidation state in related melts, and the effects on coordination observed spectrally for certain metal ions. Or, restating the idea in other terms, the redox potentials depend on melt basicity. Thus, the systematic variation of melt composition is a useful technique in the arsenal of the molten salt electrochemist who is interested in the chemistry of solute species in molten salt solvents. In this respect, it is important to note that variation of temperature may be used to serve the same purpose for example, it has been shown that in neutral chloro-aluminates C1- decreases with temperature. 

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