Acylated β-Caseins: Electrostatic Interactions and Aggregation

β-Casein was both acetylated and succinylated, to investigate alterations of selected properties brought about by removal of positive charges and, in the case of succinylated β-casein, addition of negative charges. The results were used to evaluate the role of electrostatic interactions in the aggregation of β-casein. Mobility during alkaline polyacrylamide gel electrophoresis of both derivatives was greater than β-casein, succinylated β-casein having the greatest mobility. Succinylated β-casein and acetylated β-casein, in that order, required a higher concentration of NaCl than did β-casein for elution from DEAE-cellulose at pH 7.0. The calcium ion senstivity of acetylated β-casein was decreased in comparison to β-casein. Succinylated β-casein was insensitive to calcium ions at pH 7 in 0.1 m CaCl2 at a 0.3% concentration. Sedimentation patterns at pH 6.86, 20C, revealed that succinylated β-casein did not form a fast-sedimenting peak, usually associated with aggregation. The fast-sedimenting peak of acetylated β-casein had a lower sedimentation coefficient than did β-casein. These alterations of the above properties of β-casein are interpreted to result from an increase in the net negative electrostatic charge of the modified protein in neutral solution. For succinylated β-casein, the net negative electrostatic charge is sufficient to prevent precipitation by calcium ions and to prevent aggregation under conditions that favor aggregation of β-casein.