Optically transparent thin layer electrode techniquesfor the study of biological redox systems

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.