Conformational aspects of proteins at the air/water interface studied by infrared reflection-absorption spectroscopy

From absorption spectra obtained with infrared reflection-absorption spectroscopy (IRRAB), it is possible to obtain information on conformational changes at a secondary folding level of proteins adsorbed at the air/water interface. In addition, information on protein concentration at the interface can be retrieved by means of spectral simulation. In this paper, we studied the adsorption behavior of beta-casein, beta-lactoglobulin, and (soy) glycinin at the air/water interface and the conformational changes that may take place during adsorption. The adsorbed amount was determined as a function of time, and the values found for the three proteins agree well with ellipsometry data. Only limited conformational changes in terms of secondary structure were found. Upon adsorption at the air/water interface, loss of beta-sheet structure was observed for (beta-lactoglobulin whereas the amount of unordered structure increased. For glycinin (pH 3), aggregation at the interface was observed by the appearance of an absorption band at 1630 cm(-1), which involves the formation of beta-sheet structures. For beta-casein, no conformational changes were observed at all. By comparison of IRRAS spectra of adsorbed and spread protein layers, it was found that spreading of protein at an air/water interface leads to a conformational state that is somewhat different from that when adsorbed from solution.