(401b) The Potential of Mean Force of Colloidal Particles Having Protein-Like Surface Chemistry

Extracting details of the potential of mean force for proteins or peptides from virial coefficients, even if they have been correctly determined, is a complicated task because of two main reasons: 1) the intrinsic complexity of the interactions between protein molecules and between proteins and peptides and 2) the relative insensitivity of the virial coefficient to the actual shape of the potential. One problem associated with the interpretation of second virial coefficients is the fact that researchers in their pursue of better fitting of experimental data have added so many terms than it has become difficult to single out important and less important contributions. There are some components of the potential of mean force, however, that seem to be unavoidable. These are: long range electrostatic interactions between electrical double layers, and structural/dispersion/hydration forces that dominate the interaction at very short ranges. Most of these contributions have been widely studied but some are marginally understood. For example, although progress has been made on the understanding of univalent ions the more practically important case of multivalent ions remains obscure.

In this work we have measured second virial coefficients using static light scattering for a series of poly(styrene) latexes having different surface chemistry in a variety of solvent conditions. The measurements were done in a research goniometer (Brookhaven) equipped with a 2W ion/argon laser (Spectra Physics). Four different 100 nm latexes were used: PS-SO3-, PS-OH, PS-COOH and PS-NH2. The sulfate containing latex was used as a standard. The other three were chosen because their surface mimics three common groups on protein surfaces. The experiments were performed at, below and above the isoelectric point of the latexes, in the presence and absence of methanol and glycerol, and in the presence and absence of 0.2M NaCl. Methanol and glycerol were chosen as cosolvents because of their known (and opposite) effect on hydrophobic interactions and dispersion forces. The potential of mean force was extracted from the measured virial coefficients but no attempt was done to decompose it into various contributions. Instead, the dependence of the potential of mean force between the various latexes on the environmental variables was analyzed. A complex dependence of the potential of mean force with cosolvent type and concentration was observed particularly at the latexes isoelectric points. Away from the isoelectric points electrostatic repulsions dominate the potential of mean force. The different polarity of the three functional groups was reflected in the range of the potential of mean force.