CONSTRUCTING A PROTON TITRATION CURVE FROM ION-STEP MEASUREMENTS, APPLIED TO A MEMBRANE WITH ADSORBED PROTEIN

A new measuring method is described for obtaining a proton titration c urve. The curve is obtained from a microporous composite membrane, con sisting of polystyrene beads in an agarose matrix, with lysozyme molec ules adsorbed to the bead surface. The membrane is incorporated into a sensor system by deposition on a silicon chip with a pH-sensitive ion -sensitive field effect transistor (ISFET) located in the middle of a Ag/AgCl electrode. The actual measurement is performed by creating a s tepwise change in the salt concentration of the bathing electrolyte (t he ion step) and measuring the ISFET potential versus the Ag/AgCl elec trode. This potential shows a transient change in the ion step, which indicates a transient pH change in the membrane. This procedure is rep eated at a series of pH values. Equations are presented to calculate t he proton titration curve of the membrane from the amplitude and durat ion of the measured transients. Measurements show qualitative agreemen t between the curves obtained and equilibrium titration experiments on the same system. (C) 1997 Academic Press.