Nanoscale Intermolecular Interactions between Human Serum Albumin and Alkanethiol Self-Assembled Monolayers

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To study the molecular origins of hemocompatibility, the blood plasma protein human serum albumin (HSA) was covalently grafted to a nanosized probe tip at the end of a soft, microfabricated cantilever force transducer. The net force versus separation distance between the HSA-modified probe tip and three different model surfaces, including (1) gold; (2) a hydrophobic, CH3-terminated alkanethiol self-assembling monolayer (SAM); and (3) a hydrophilic, COO--terminated alkanethiol SAM in aqueous sodium phosphate buffer solution (PBS, ionic strength (IS) = 0.01 M, pH = 7.4), was recorded and compared to the values of various theoretical models. The approach interaction of the HSA probe tip on the COO--terminated SAM and Au substrates was found to be purely repulsive for D < 15 nm, nonlinear with decreasing separation distance, and consistent with electrostatic double layer repulsion. The approach interaction of the HSA probe tip on the CH3-terminated SAM substrate was found to be purely attractive, long range (D < 80 nm), nonlinear with decreasing separation distance, and much greater in magnitude and range than that known for van der Waals interactions between hydrocarbon SAMs terminated with hydrophilic chemical groups on Au. Large adhesive energies were observed for the HSA probe tip on both the CH3-terminated SAM and Au surfaces (≤−29 mN/m, −22 to −73kBT/protein), while smaller adhesive energies were observed on the COO--terminated SAM surface (≤ −4.9 mN/m, −5.3 to −12kBT/protein). It was shown that short-range adhesive contacts between the HSA chain segments and these surfaces give rise to energy dissipating mechanisms, such as HSA entropic molecular elasticity and enthalpic unfolding forces (deformation and rupture of noncovalent intramolecular bonds) and noncovalent bond rupture of the HSA chain segments adsorbed to the surface.