ELECTRON-TRANSFER PRINCIPLES IN AMPEROMETRIC BIOSENSORS - DIRECT ELECTRON-TRANSFER BETWEEN ENZYMES AND ELECTRODE SURFACE

The most promising approach for the development of reagentless enzyme electrodes is to establish a direct electrical communication between t he enzyme and the electrode surface. We could demonstrate for monolaye r-immobilized enzymes catalyzing the reduction of H2O2. (e.g. cytochro me c, microperoxidase MP-11 and horseradish peroxidase) that their cat alytic activity in solution is not correlated with their abilities for direct electrochemical communication with the electrode when immobili zed at thio-monolayers. In this case, the distance between the active site of the enzyme and the electrode surface is by far more important for electron-transfer processes with high rate constant. To achieve th e smallest possible distance it is advantageous to use the biocatalyst with the best access to its active site and hence the smallest molecu lar weight. Using monolayer-immobilized microperoxidase MP-11 instead of horseradish peroxidase, the current caused by the electrocatalytic reduction of H2O2 could be increased by a factor of about 18 000 compa red with the enzymatic activity in solution. Consequently, in this spe cial electrode arrangement allowing as the only electron-transfer path way the direct electrochemical communication between monolayer-immobil ized biocatalyst and electrode surface, the most effective biocatalyst s should be the smallest molecule which still shows the envisaged cata lytic activity. These structures are called 'minizymes' (minimized enz ymes).