Electrochemical Studies of the Effect of Temperature and pH on the Adsorption of α-Lactalbumin at Pt
The adsorption behavior of holo- and apo-a-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 a -LA onto a Pt surface was found to be an entropically 
governed process, suggesting structural unfolding of the protein at the 
electrode surface.