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

Abstract

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 (e-29 mN/m, -22 to -73kBT/protein), while smaller adhesive energies were observed on the COO --terminated SAM surface (e -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. IntroductionThe interaction between the surface of an implanted artificial medical device and blood typically results in nonspecific, noncovalent surface adsorption of blood plasma proteins followed by platelet adhesion and activation, initiation of the coagulation cascade, and thrombus formation. 1,2 In the absence of transport limitations, the interaction potential between the protein and the surface as a function of separation distance, U(D), will determine whether a protein will adsorb and at what rate. U(D) is typically a superposition of numerous nonspecific repulsive (e.g. electrostatic counterion double layer, steric, hydration, etc.) and attractive (e.g. van der Waals, hydrophobic, H-bonding, ionic, etc.) components that can lead to complicated functional forms that vary with the strength and range of the constituent interactions. 3,4 Generally, improved protein resistance can be achieved by maximizing repulsive interactions and minimizing attractive ones. Subsequent stages of protein adsorption become increasingly complex and depend on the conformation, orientation, and mobility of the adsorbed proteins, the time-scale of conformational changes, protein exchange and desorp...

Synthesis and Single Molecule Force Spectroscopy of Graft Copolymers of Poly(2-hydroxyethyl methacrylate-g-ethylene glycol)

“…Previously, we approximated the net approach surface interaction of polymer mushrooms end-grafted on Au as a linear addition of the polymer and Au interactions taken separately. 44 The bare polycrystalline Au used in this study has been found previously 43 to have a net negative charge under these solution conditions due to adsorbed anions with a surface charge per unit area of σ(Au) ) 0.0144 C/m 2 , and the Si 3 N 4 probe tip is known [45][46][47] to also have a net negative charge due to ionized silanol (SiO -) and silylamine (SiN -) groups of σ (Si 3 N 4 ) ) -0.032 C/m 2 , yielding an electrostatic double-layer (EDL) repulsion with an electrical interaction Debye length (calculated from the solution IS ) 0.15 M), κ -1 of ∼1 nm in PBS and ∼39 nm in DI H 2 O. 48 The EDL repulsion solely due to the Si 3 N 4 vs Au interaction in PBS (i.e., for 90% and 86% of total surface area for the poly(HEMA-g-EG) 16K and poly(HEMA-g-EG) 120K samples, respectively) was modeled using a constant surface charge approximation based on the numerical solution to the nonlinear Poisson-Boltzmann (PB) equation including divalent ions, [49][50][51] and these simulated curves are shown in Figure 8 as solid lines.…”

The Effect of Nanotopography on Modulating Protein Adsorption and the Fibrotic Response

“…The longer and thinner nanofeatures may resist protein adsorption due to steric hindrance and entropic repulsive forces. 19 Although others have observed differential cellular responses to nanotopography, most research has focused on cell-surface interactions and have not investigated the intermediate effect that nanostructures have on protein adsorption. 9,10,[20][21][22] Unlike microtopography, whereby the cells are mechanically constrained between features, nanotopography may work by influencing how proteins are presented to cells.…”

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…Rixman et al [385] compared the results obtained by the JKR and DMT theory. Drelich et al [353] have collected the data of several articles and calculated again the adhesion with the Maugis theory.…”