This paper presents the results of constant-current (chronopotentiometric) measurements of the egg yolk phosphatidylcholine (PC) bilayer membrane without and with cholesterol. The experiments were performed on planar bilayer lipid membrane (BLM) formed by the Mueller-Rudin method. It is demonstrated that the constant-intensity current flow through bilayer membranes generated fluctuating pores in their structure. The presence of cholesterol in the membrane caused an increase in the value of the breakdown potential. It is postulated that greater stability of the bilayer with cholesterol can result from an increased critical pore radius (at which the bilayer would undergo irreversible rupture). This confirms that cholesterol has a stabilizing effect on BLM. Besides, our results suggest that addition of cholesterol causes shift in the distribution of pore conductance towards a smaller value. It is suggested that this can be connected with the phenomenon of domain formation in the membranes containing high concentration of cholesterol. Moreover, it is shown that chronopotentiometry with programmable current intensity is a promising method for observation of the membrane recovery process.

A method was developed for assessing ascorbic acid concentration in commercial fruit juice by cyclic voltammetry. The anodic oxidation peak for ascorbic acid occurs at about 490 mV on a Pt disc working electrode (versus SCE). The influence of the potential sweep speed on the peak height was studied. The obtained calibration graph shows a linear dependence between peak height and ascorbic acid concentration in the domain (0.1–10 mmol·L−1). The equation of the calibration graph was y = 6.391x + 0.1903 (where y represents the value of intensity measured for the anodic peak height, expressed as μA and x the analyte concentration, as mmol·L−1, r2 = 0.9995, r.s.d. = 1.14%, n = 10, Cascorbic acid = 2 mmol·L−1). The developed method was applied to ascorbic acid assessment in fruit juice. The ascorbic acid content determined ranged from 0.83 to 1.67 mmol·L−1 for orange juice, from 0.58 to 1.93 mmol·L−1 for lemon juice, and from 0.46 to 1.84 mmol·L−1 for grapefruit juice. Different ascorbic acid concentrations (from standard solutions) were added to the analysed samples, the degree of recovery being comprised between 94.35% and 104%. Ascorbic acid determination results obtained by cyclic voltammetry were compared with those obtained by the volumetric method with dichlorophenol indophenol. The results obtained by the two methods were in good agreement.

The constant-intensity current chronopotentiometric measurements of egg yolk phosphatidylcholine bilayer membranes (BLM) are presented. It is demonstrated that a constant-intensity current flowing through the bilayer membranes generates the pores in their structures. For the current intensity from 0.1 to 2.0 nA, the generated pores open and close cyclically. The frequency of oscillations depends on the current intensity: the higher current intensity, the higher frequency of pore oscillations. It is suggested that the presented method may allow to create one pore in BLM and to observe its dynamical behaviour. Based on chronopotentiometric curves, a method of pore conductance calculations is presented. It is demonstrated that the value of obtained conductance can be applied for pore diameter estimation. The hypothetical application of constant-current method as a biotechnological tool for selective and controlled incorporation of molecules into microorganisms is discussed.

This paper presents the application of chronopotentiometry in the study of membrane electroporation. Chronopotentiometry with a programmable current intensity was used. The experiments were performed on planar bilayer phosphatidylcholine and cholesterol membranes formed by the Mueller-Rudin method. It was demonstrated that a constant-intensity current flow through the bilayer membranes generated voltage fluctuations during electroporation. These fluctuations (following an increase and decrease in membrane conductance) were interpreted as a result of the opening and closing of pores in membrane structures. The decrease in membrane potential to zero did not cause the pore to close immediately. The pore was maintained for about 200 s. The closing of the pore and recovery of the continuous structure of the membrane proceeded not only when the membrane potential equalled zero, but also at membrane potentials up to several tens of millivolts. The fluctuations of the pore were possible at values of membrane potential in the order of at least 100 mV. The size of the pore changed slightly and it closed after some time below this potential value.