Thomas Buch-Rasmussen

Study of the new electron transfer mediators in glucose oxidase catalysis

The steady-state oxidation of glucose oxidase from Aspergillus niger by phenothiazines, phenoxazines, Wursters salts, dithia- and tetrathiaaromatic compounds, and nickelocene was investigated spectrophotometrically and electrochemically. At pH 7.0 the determined oxidation rate constants (TN/Km) vary in the range 103 to 108 M−1·s−1. For phenothiazines, phenoxazine and Wursters salts oxidation constants depend on the redox potential of the electron acceptors, and results were interpreted in the framework of the outer sphere electron transfer theory (Marcus and Sutin, Biochim. Biophys. Acta, 811 (1985) 265). The interpretation of the kinetic results concerning thiaaromatic compounds and metallocenes are complicated due to complex formation with the enzyme active center and aggregation of their oxidized form in buffer solution.

Glucose biosensor based on the incorporation of Meldola Blue

Abstract The phenoxazine compound Meldola Blue (MB) is shown to efficiently mediate the electron transfer from reduced glucose oxidase to a conventional carbon paste electrode. The mediation process is exploited for developing an amperometric biosensor for glucose, which yields a linear response to 0–25 mM glucose at an operating potential of 50 mV (vs. SCE), where interfering reactions do not occur. Experimental variables such as enzyme loading or operating potential are explored. A stable response is observed over several months.

Glucose Oxidase Electrode Based on Graphite and Methylthio Tetrathiafulvalene as Mediator

Abstract A glucose sensor based on glucose oxidase and a new mediator - 4,5-dimethyl-4′-methylthio-Δ 2,2′-bi-1,3-dithiole (MTTTF) is described. The background for sensor action is the effective MTTTF cation interaction (apparent bimolecular constant (2.0+/-0.5)∗106 M−1 s−1 at 25°C and pH 7.0) with reduced glucose oxidase and the high electrochemical rate of mediator transformation. A glucose sensor was prepared by adsorbing mediator (MTTTF) and glucose oxidase on graphite rods. The sensor responds to glucose at electrode potentials higher than 50 mV vs SCE, but the maximal activity is obtained at a potential of 250 mV. In air saturated solution the electrode shows a non-linear calibration curve with a half-saturation concentration 10.4 mM and Hill coefficient 2.08 at 250 mV. Sensor response changes little at pH 6.5–8.0. The energy of activation of the sensor response calculated from the Arrhenius equation was 64.5 kJ/mol, and the temperature coefficient at 25°C was 9.2%.

Effect of whole blood and plasma on the permeability of glucose through different cellulose and cellulose acetate membranes

Abstract Model experiments were made with a flow-through dialysis cell to study how glucose in whole blood, plasma and aqueous standards is transferred through different membranes. Detection was via a glucose dehydrogenase reaction and NADH monitoring with a chemically modified electrode. The detector response was linear with the glucose concentration in all cases. The non-Newtonian properties of whole blood seem to be of great importance for the response. Theoretical models are proposed both for the effect of plasma viscosity and the red cell volume fraction (haematocrit) dependence on the glucose transfer rate. The models could only partially explain the differences between measurements in aqueous solutions and body fluids. Better agreement and less haematocrit dependence were obtained with low-permeability membranes.