Electrochemical impedance spectroscopy study of the adsorption behaviour of α-lactalbumin and β-casein at stainless steel

The adsorption behaviour of holo-a-lactalbumin (alpha-LA) and beta-casein (beta-CN) on high-purity austenitic low-carbon stainless steel and its effect on the state of the electrode surface have been studied in a pH 7.0 phosphate buffer solution over the temperature range 299-343 K. Cyclic voltammetry measurements were used to characterize the effects of polishing on the electrode surface. The electrochemical impedance spectroscopy (EIS) technique was used to investigate the interfacial behaviour of alpha-LA and beta-CN at an open circuit potential. An equivalent-electrical-circuit (EEC) was used successfully in modeling the electrode\electrolyte interface and corresponding surface processes. The charge-transfer resistance was very sensitive to the amount of adsorbed protein, indicating that the adsorption process was accompanied by the transfer of charge and influenced the mechanism and kinetics of the corrosion reaction. At all the temperatures studied, adsorption of alpha-LA and beta-CN onto the stainless steel surface was successfully described with a Langmuir adsorption isotherm. DeltaG(ADS)(0) values for alpha-LA and beta-CN (-54 and -55+/-0.5 kJ mol(-1) at 298 K, respectively) show that the molecules have a strong affinity for the stainless steel surface. The DeltaH(ADS)(0) (15.6 and 3.6+/-0.5 kJ mol(-1)) and DeltaS(ADS)(0) (236 and 192+/-1 J K-1 mol(-1)) values for alpha-LA and beta-CN, respectively, indicated that alpha-LA partially unfolds at the surface upon adsorption and for both proteins the adsorption process is governed entropically. A comparison is made for the first time of the thermodynamic values for adsorption of four proteins on the same stainless steel surface under the same experimental conditions, which show remarkable differences due to their conformational stability emphasizing the importance of these thermodynamic measurements in understanding the complex protein/metal interactions. (C) 2004 Elsevier B.V. All rights reserved.