Structure and properties of adsorption layers of beta-casein formed from guanidine hydrochloride rich solutions

Adsorption layers of beta -casein formed at the interface between air and a buffer including various concentrations of guanidine hydrochloride (GuHCl) were studied by neutron reflectivity and by bubble tensiometry. A transiti on in the structure and in the properties of the adsorption layer seems to occur around a GuHCl concentration of 1.5 M. The data are interpreted assum ing that the adsorbed protein molecules behave like multiblock copolymers w ith alternating hydrophilic and hydrophobic sequences. Below the transition , the hydrophilic coils and the hydrophobic two-dimensional blocks have a f ractal dimension larger than that beyond the transition where they have the features of either two-dimensional or three-dimensional excluded volume co ils. The effect of temperature on these phenomena indicates that they are n ot dominated by hydrophobic interactions. Thus, the attractions between ami no acids which are broken by GuHCl might be hydrogen bonds which are freque ntly encountered in the secondary structure of polypeptide chains. These re sults show that even with the flexible polypeptide chain of beta -casein, i nteractions between amino acids contribute significantly to the structure o f the adsorption layer formed from a buffer devoid of denaturing agent.