Abstract
Many biological redox systems are difficult to study by electrochemicaltechniques sice they exhibit slow or negligible rates of electron transfer at electrodes. A spectroelectrochemical technique using an Optically Transparent Thin Layer Electrode (OTTLE) has been developed for measruing formal reduction potentials ( U o′ ) and electron stoichiometries ( n ) of such biological systems. In an OTTLE a thin solution layer ( ca . 0.2 mm thick) is confined adjacent to an optically transparent electrode such as a gold minigrid. An optical beam is passed through the transparent electrode and the solution. OTTLEs which enable measurements to be made on 20 mm 3 (μl) of solution have been developed. The oxidation state of the redox system in solution is controlled by the applied potential. U o′ and n are determined from a Nernst plot of a series of applied potentials for which the corresponding ratios of oxidized to reduced forms are measured spectroscopically as illustrated for cytochrome c and myoglobin. The OTTLE is easily thermostated for measuring the temperature dependence of U o′ . A charge-potential technique has been developed for use with biological systems which have weak spectral features. The addition of a mediator-titrant is necessary for some biological species such as cytochrome c and myoglobin to facilitate the transport of electrons between electrode and redox system. A series of mediator-titrants whose U o′ values collectively span the potential range of the gold electrode have been characterized with respect to their effective potential ranges.
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