1. Division of Chemical Enzymology, Department of Chemistry, M.V. Lomonosov, Moscow State University, Moscow 119899 Russia, Department of Analytical Chemistry, Lund University, P.O. Box 124, S-221 00, Lund, Sweden, and Department of Electrical Measurements, Lund Institute of Technology, Lund University, P.O. Box 118, S-221 00, Lund, Sweden