Adsorption of Proteins to Hydrophobic Sites on Mixed Self-Assembled Monolayers

This paper describes a technique that uses mixed self-assembled monolayers of two alkanethiolates (-S(CH2)11(OCH2CH2)6OR, R = a hydrophobic group, and -S(CH2)11(OCH2CH2)nOH, n = 3, 6, EGnOH), in combination with surface plasmon resonance spectroscopy, to study the influence of the size and shape of R, and its density at the surface, on the hydrophobic adsorption of proteins at solid−liquid interfaces. Detailed results were obtained for β-galactosidase, carbonic anhydrase, lysozyme, and RNase A using R = C(C6H5)3, CH(C6H5)2, and CH2(C6H5). A hard-sphere model is used to rationalize the adsorption; this model, although very approximate, helps to interpret qualitative trends in the data. Using this model, the extent to which adsorbed proteins undergo conformational rearrangements appears to depend on the density of the hydrophobic groups at the surface and on the concentration of protein in solution. This paper describes the first step toward the development of a system that will allow the study of hydrophobic interactions of proteins with surfaces presenting organic groups of well-defined shape.