Partial specific volume and solvent interactions of amphipol A8-35

Abstract

Amphipols are small amphiphilic polymers that can stabilize and keep soluble membrane proteins in aqueous solutions in the absence of detergent. A prerequisite to solution studies of membrane protein/amphipol complexes is the determination of the partial specific volume v ¯ 2 and effective charge z of the polymer. The ratio ( R) of the buoyant molar masses of particles in D 2O and H 2O solutions, obtained from sedimentation velocity ( s H/ s D method) and sedimentation equilibrium experiments, and their contrast match point (CMP), determined in small-angle neutron scattering experiments, depend on v ¯ 2 and z. When z is known, v ¯ 2 can be estimated from R with a good accuracy as long as v ¯ 2 is close to 1. The effects of labile H/D exchange and of polyelectrolyte counter-ion dissociation in general cannot be neglected. The accuracy, advantages, and limits of the s H/ s D method have been studied in details using model macromolecules (DNA, protein, and polysaccharide). The s H/ s D method appears particularly advantageous for the study of heterogeneous samples. Measurements of density, s D/s H buoyant molar masses in H 2O, D 2O, and D 2 18O, and CMP of hydrogenated and partially deuterated A8-35, a polyacrylate-based amphipol containing 35 underivatized carboxylates per 100 monomers, led to a consistent description of its buoyancy and charge properties.