Changes in protein adsorption on self-assembled monolayers with monolayer order: Comparison of human serum albumin and human gamma globulin

The role that alkyl chain packing density in a self-assembled monolayer (SA M) plays in the adsorption of protein to the SAM surface was investigated u sing in situ neutron reflectivity (NR) and total internal reflection fluore scence (TIRF) measurements of the adsorption behavior of human serum albumi n (HSA) and human immunoglobulin G (HGG). The proteins differ particularly in the fact that HSA has specific binding pockets for alkyl chains while HG G does not. NR results show that HSA adsorbs from a 1.0 mg/mL solution as a single layer on the SAMs, with the protein interpenetrating into a less de nsely packed SAM, but not into a more densely packed SAM. Likewise, the kin etics of the HSA adsorption to the SAMs varies markedly with the alkyl chai n packing. In contrast, both the structure of the adsorbed layer and adsorp tion kinetics vary little with the alkyl chain packing density in the case of HGG adsorbing from solution. HGG also does not penetrate into loosely pa cked SAMs. NR results reveal that the HGG adsorbs as two layers, with the l ayer closest to the SAM being more tightly packed. When HSA and HGG compete for adsorption sites on a SAM, HGG effectively displaces HSA from a tightl y packed SAM, but does not compete effectively with HSA adsorbed tenaciousl y onto a loosely packed SAM.