Protein-Surfactant Interactions Phase Behaviour and Phase Structure

Sammanfattning: Protein-surfactant interactions in water are investigated by following the phase equilibria for protein-surfactant pairs whose constituents carry both opposite and the same sign of charge. The concentration ranges studied are: 0-20 wt% protein, 0-20 wt% surfactant, and 80-100 wt% water. The oppositely charged lysozyme-sodium dodecylsulphate (SDS)-water system forms three main regions: a precipitation region, a solution phase, and a gel phase. The influence of the surfactant hydrophobic effect on the phase behaviour is examined by determining phase diagrams for the lysozyme-Na alkylsulphate-water systems, with surfactant chain lengths of 12, 10, 8, and 6 carbons. Similar phase behaviour is obtained for all systems. However, the stability of the phase regions is dependent on the surfactant hydrophobicity. Protein-surfactant interactions in water are investigated by following the phase equilibria for protein-surfactant pairs whose constituents carry both opposite and the same sign of charge. The concentration ranges studied are: 0-20 wt% protein, 0-20 wt% surfactant, and 80-100 wt% water. The oppositely charged lysozyme-sodium dodecylsulphate (SDS)-water system forms three main regions: a precipitation region, a solution phase, and a gel phase. The influence of the surfactant hydrophobic effect on the phase behaviour is examined by determining phase diagrams for the lysozyme-Na alkylsulphate-water systems, with surfactant chain lengths of 12, 10, 8, and 6 carbons. Similar phase behaviour is obtained for all systems. However, the stability of the phase regions is dependent on the surfactant hydrophobicity. The oppositely charged systems b-lactoglobulin (BLG)-dodecyltrimethylammonium chloride (DOTAC)-water and ovalbumin-DOTAC-water also exhibit similar phase behaviour. However, for the bovine serum albumin (BSA)-DOTAC-water system no precipitate or gel is detected. In the similarly charged lysozyme-DOTAC, ovalbumin-SDS, and BSA-SDS systems, only a solution phase is obtained. In the BLG-SDS-water system a gel is found in addition to the solution phase. The phase diagram studies show that different protein-surfactant systems exhibit unique properties and that no generalised phase behaviour may be inferred neither for similarly nor for oppositely charged systems. Structural studies of the precipitation region, the gel phase, and the solution phase are presented. The precipitation region is studied by standard gravimetric methods and the transition between the gel phase and solution phase is investigated with nuclear magnetic resonance (nmr) relaxation and cryogenic-transmission electron microscopy (cryo-TEM) techniques. From the precipitate studies, it is observed that surfactants are sparingly soluble in protein solutions. The composition of the protein-surfactant precipitate corresponds to surfactant charge neutralising the protein. The redissolution of the precipitate is dependent on the surfactant aggregation behaviour. 2H nmr relaxation of deuterated surfactant and cryo-TEM images demonstrate the formation of large protein-surfactant aggregates in the gel regions and small aggregates in the colourless isotropic solution phase.