PROTEIN SOLUBILITY: PHASE SEPARATION IN ARACHIN-SALT-WATER SYSTEMS

Solutions of arachin, a groundnut protein, show liquid‐liquid phase separations in certain conditions of ionic strength and pH. Phase diagrams show upper, lower and closed boundaries corresponding to salting in and salting out curves of globulins and a hitherto undescribed solubility behaviour. The protein rich liquid phases and protein ‘precipitates’ are similar. As conditions approach the phase boundary, arachin undergoes self‐association, as revealed by the concentration dependence of the sedimentation coefficient and gel chromatography, while in D2O, association is more extensive and the phase boundaries are correspondingly shifted. Oligomer formation could lead to thermodynamic instability of the solutions and hence phase separation. At high protein contents (60%) the protein rich phase produces deformed helical crystalline spherulites. By electron microscopy the protein rich phase contains predominantly oligomers of about 400 Å diameter, but no evidence for long range order could be found.