Solubility and reactivity of caseins and beta-lactoglobulin in protic solvents

The study of the solubility of unstructured proteins (alpha s1-, beta-, and kappa-casein) and well-structured globulin (beta-lactoglobulin) in low water binary solvent systems demonstrated the crucial importance of solvent polarity and neutralization of protein polar functions on the final outcome of solubility experiments. The solubilities up to 38, 56, and 96% in CHCl3/CH3OH (1/1, v/v) acidified with HCl and up to 5, 10, and 25% in CHCl3/CH3OH (1/1, v/v) in the presence of triethylamine (TEA) were obtained for kappa-, alpha s1-, and beta-casein, respectively. The importance of protein charge neutralization was apparent when the solubilization was performed in basified CHCl3/CH3OH media, giving the optimal results when the studied proteins were brought before to their isoionic point. The maximum solubility of beta-casein at its pI in 30-70% methanol in CHCl3 was reaching 50-60% with triethylamine (TEA) added. beta-lactoglobulin could be solubilized up to 70% in CHCl3/CH3OH (7/3, v/v) acidified with HCl and up to 40% in CHCl3/CH3OH (3/7, v/v) in the presence of TEA. The observed yield of reductive alkylation of beta-lactoglobulin was much higher (98%) when performed in studied solvent system than in aqueous conditions (75%). Apparently, steric hindrance of the well-folded beta-barrel (in aqueous conditions) structure masks the portion of epsilon-NH2 groups. In the case of unstructured aqueous media beta-casein, 90% alkylation yields were obtained in organic and aqueous conditions.