Electrochemical studies of the effect of temperature and pH on the adsorption of α-lactalbumin at Pt

The adsorption behavior of holo- and apo-alpha -lactalbumin at a Pt/electrolyte interface was studied in an acidic, neutral, and alkaline medium over the temperature range 273 to 353 K, using the cyclic voltammetry technique. It was shown that the surface charge density, resulting from protein adsorption, is directly proportional to the amount of adsorbed protein (surface concentration), indicating that adsorption is accompanied by the transfer of charge. A significant difference in the amount of adsorbed protein (between the two types of protein) was obtained at pH 7 because of the difference in conformation of the molecule in the presence/absence of bound calcium. On the other hand, the resemblance in behavior of the two types of proteins observed at pH 2 and pH 11 suggested that the protein is in its molten globule state and is depleted in calcium at these pHs. The adsorption process was modeled using the Langmuir adsorption isotherm. The values of the Gibbs free energy of adsorption indicated that the protein molecules strongly adsorb onto the Pt surface via chemisorption. The protein expressed the highest affinity toward adsorption at pH 2 and the lowest at pH 11. The adsorption process was found to be endothermic, resulting from the excess energetics required for the breaking of intramolecular interactions relative to those involved in the formation of protein-metal bonds. The adsorption of alpha -LA onto a Pt surface was found to be an entropically governed process, suggesting structural unfolding of the protein at the electrode surface.