Intermolecular interactions at early stage of protein/detergent particle association induced by salt/polyethylene glycol mixtures

Mixtures of neutral salts and polyethylene glycol are used for various purposes in biological studies. Although the effects of each component of the mixtures are theoretically well investigated, comprehension of their integrated effects remains insufficient. In this work, their roles and effects as a precipitant were clarified by studying dependence of precipitation curves on salt concentration for integral membrane protein/detergent particles of different physicochemical properties. The dependence of precipitation curves was reasonably related to intermolecular interactions among relevant molecules such as protein, detergent and polyethylene glycol by considering their physicochemical properties. The obtained relationships are useful as basic information to learn the early stage of biological macromolecular associations. •Precipitation curve defined on the basis of kinetics and reproducibility of protein concentration in supernatant.•Electric screening effects of various ions evaluated from dependence of a precipitation curve on salt concentration.•Entropically unfavorable polar–nonpolar interaction between charged species and PEG and its role in particle association.•Role in particle assembly of interparticle attraction induced by augmentation in motional freedom of PEG molecule.•Surface tension of interparticle solvent within the particle association region acting against bulk osmotic pressure.