ACETONITRILE-PROTEIN INTERACTIONS - AMINO-ACID SOLUBILITY AND PREFERENTIAL SOLVATION

The solubility of amino acids and the preferential solvent interaction of hen-egg lysozyme in acetonitrile (AN)-water mixtures ( < 60 w/v% A N) were investigated by means of densimetric and refractometric method s at 25 degrees C. The free energy of transfer from water to aqueous A N was negative for most nonpolar side-chains of amino acids and positi ve for the peptide group, the extent being comparable to those for met hanol and ethanol systems. Addition of AN to an aqueous solvent was th us suggested to weaken the hydrophobic interaction and to enhance the peptide-peptide hydrogen bond therein leading to the denaturation of p roteins. A parallel examination by circular dichroism confirmed that t he conformation of lysozyme (pH 3) remains native in aqueous AN up to 40% but changes to the helix-rich form at higher AN concentrations. At all solvent compositions up to 50% AN (pH 3), however, lysozyme was p referentially hydrated probably due to a local salting-out of the AN m olecules from the charges on the protein surface, indicating the incre ase of the chemical potential of the protein. These results are discus sed in relation to the role of AN as an eluting organic solvent in rev erse-phase chromatography. (C) 1998 Elsevier Science B.V. All rights r eserved.