Surface forces between protein A adsorbed mica surfaces

Direct measurement of interactions between adsorbed vitronectin layers: The influence of ionic strength and pH

Vitronectin (Vn) is an adhesive protein in the plasma serum and plays an important role in cell attachment, spreading, and proliferation. The interactions between protein bovine vitronectin layers adsorbed onto a silica probe and a mica surface have been investigated with the use of atomic force microscopy (AFM). Adsorption of vitronectin was confirmed by XPS surface analysis. The force-separation curves and pull-off forces were measured as a function of ionic strength and solution pH. The pull-off force (adhesion force) decreased as the salt concentration increased, which suggests that some binding domains of this protein may associate with the ionic species and reduce its binding ability. Discrete jumps, or discontinuities, in the separation force curve were observed to extend to a maximum of 300 nm, evidence that the protein molecules bridge between the surfaces. As a function of pH, the adhesion force on separation of the protein-coated surfaces showed a maximum at pH 5 (i.e.p. of vitronectin), decreasing in magnitude at lower and higher pH values. At pH 5, the approaching curves illustrated a jump-in force; whereas for pH values away from 5, the approaching force curves were repulsive. Correlation of the interaction forces with Vn conformational changes in different pH environments, directly visualized with the use of AFM imaging, was developed. In its i.e.p. region, the Vn molecular conformation appeared to be dense and compact. Significantly, at wounds/injured sites the pH is low (approximately 5) which this study discovered to facilitate adsorption and formation of vitronectin aggregates, known to trigger their subsequent biological functions.

Direct Measurement of Interactions between Adsorbed Protein Layers Using an Atomic Force Microscope

An atomic force microscope (AFM) in conjunction with the colloid probe technique has been used to measure directly the interaction of adsorbed layers of the protein bovine serum albumin (BSA). The BSA was adsorbed on both a silica colloid probe and a silica surface. Measurements of force-distance curves were made at various salt concentrations and pHs. The measured force-distance curves were in good quantitative agreement with predictions based on the DLVO theory using zeta potentials (outer Helmholtz plane potentials) calculated for BSA from an independently validated site-binding-site-dissociation surface model. The paper hence provides a direct confirmation that the AFM colloid probe technique can provide a useful means of directly quantifying the interaction of biological macromolecules. Copyright 1998 Academic Press. Copyright 1998Academic Press

Correlation between surface morphology and surface forces of protein A adsorbed on mica

We have investigated the morphology and surface forces of protein A adsorbed on mica surface in the protein solutions of various concentrations. The force-distance curves, measured with a surface force apparatus (SFA), were interpreted in terms of two different regimens: a "large-distance" regimen in which an electrostatic double-layer force dominates, and an "adsorbed layer" regimen in which a force of steric origin dominates. To further clarify the forces of steric origin, the surface morphology of the adsorbed protein layer was investigated with an atomic force microscope (AFM) because the steric repulsive forces are strongly affected by the adsorption mode of protein A molecules on mica. At lower protein concentrations (2 ppm, 10 ppm), protein A molecules were adsorbed "side-on" parallel to the mica surfaces, forming a monolayer of approximately 2.5 nm. AFM images at higher concentrations (30 ppm, 100 ppm) showed protruding structures over the monolayer, which revealed that the adsorbed protein A molecules had one end oriented into the solution, with the remainder of each molecule adsorbed side-on to the mica surface. These extending ends of protein A overlapped each other and formed a "quasi-double layer" over the mica surface. These AFM images proved the existence of a monolayer of protein A molecules at low concentrations and a "quasi-double layer" with occasional protrusions at high concentrations, which were consistent with the adsorption mode observed in the force-distance curves.