Surfactant effects on the ethanolic fractionation of dilute gelatin solutions

Abstract

Addition of a non-solvent, such as ethanol, to aqueous solutions of the heterogeneous protein, gelatin, causes progressive desolvation of the polymer. Sodium dodecyl sulphate (SDS) associates with gelatin through hydrophobic interactions involving the hydrocarbon tail, and through ionic interactions between the negatively charged headgroup of SDS and positively charged side groups on the gelatin molecule; both mechanisms cause unfolding of the protein and yield a hydrophobic complex. Thus the addition of SDS dramatically alters the desolvation behaviour of gelatin solutions, such that at pH’s at and below the IEP, a primary desolvation is observed, the extent of which increases with decreasing pH. The precipitate dissolves with increasing ethanol concentration and a secondary desolvation subsequently occurs. Above the IEP, primary desolvation of SDS-gelatin mixtures is not observed but secondary desolvation occurs to a greater extent than for gelatin solutions alone. It has been suggested that electrostatic binding of SDS to gelatin plays a key role in the primary desolvation event, whilst hydrophobic binding is responsible for the more complete desolvation of SDS above the IEP. It can thus be hypothesised that in the presence of a non-iconic surfactant, such as Tween 20, the primary desolvation effects seen in the presence of an anionic surfactant, such as SDS, will not be observed, but the secondary desolvation effects will still be present.