DETERMINATION OF THE PH DIFFERENCE ACROSS A CELL-MEMBRANE USING A METHYLAMMONIUM-SELECTIVE MEMBRANE-ELECTRODE

A method was developed for determining pH differences across cell memb ranes using a methylammonium-selective membrane electrode, based on mo nitoring of the pH gradient-induced uptake of methylammonium in situ. The methylammonium electrode was constructed using calix[6]arene-hexaa cetic acid hexaethyl ester as a neutral carrier and bis(2-ethylhexyl) sebacate as a membrane solvent in a poly(vinyl chloride) membrane matr ix. This electrode exhibited a near-Nernstian response to methylammoni um in the concentration range 2 x 10(-5)-1 x 10(-2) M with a slope of 58 mV per concentration decade in a buffer solution of 150 mM choline chloride-10 mM TRIS-HCl (pH 7.5). The limit of detection was 5 X 10(-6 ) M. In experiments using liposomes, the uptake of methylammonium into liposomes occurred effectively when the pH of the outside suspension medium was alkaline, and the determination of changes in methylammoniu m concentrations in the outer medium was quantitatively related to cha nges in the pH differences across the liposomal membrane. The transmem brane pH differences in Escherichia coli cells were also determined by this method.