Catalytic currents of albumin at the hanging mercury drop electrode

Abstract

Bovine serum albumin (BSA), as well as completely reduced BSA denoted by P (SH) 35, are adsorbed on the hanging mercury drop electrode (HMDE) from alkaline buffer solutions. When time is allowed, a monolayer is adsorbed from very dilute (10 −9 M) BSA solutions in ammoniacal and borate buffers. With a monolayer of adsorbed protein the voltammograms at the HMDE are then identical in a given ammoniacal or borax buffer containing cobalt(III) or (II) and different BSA concentrations. Voltammograms of P (SH) 35 are virtually identical with those of native BSA. At the HMDE the second Brdička current is proportional to concentration of cobalt(III) or (II) and the first current nearly so. Incompletely or completely adsorbed BSA or P (SH) 35 is not desorbed on keeping the HMDE for one hour in ammonia buffers. An incomplete layer of adsorbed BSA or P (SH) 35 is relatively rapidly desorbed at −1.6 V (vs. SCE) and a complete film at −1.65 V, some desorption occurring at −1.6 V. Upon desorption, the second Brdička current decreases faster than the first one; this is particularly striking in 1 M ammonia buffer. The rate of desorption is increased by calcium chloride, but the rate of adsorption is not, or only slightly, increased in the presence of calcium. Incomplete adsorption occurs at −1.60 V (vs. SCE) and no adsorption at −1.65 V. Indications are obtained that “presodium currents” yield a slight plateau at −1.67 to −1.70 V, the plateau currents being attributed to adsorbed BSA, while unadsorbed BSA yields catalytic currents without a plateau, the currents merging with the residual one of the buffer. Calcium chloride greatly increases the presodium currents. From many kinetic data obtained at the dropping mercury electrode (DME) and from results at the HMDE it is concluded that, depending on the BSA concentration, Brdička currents at the DME are partly of a kinetic and partly of a surface adsorption nature and partly diffusion-controlled. Adsorption equilibrium is not attained at the DME at 25° at concentrations of BSA smaller than 10 −6 M.