Acylated β-Caseins: Electrostatic Interactions and Aggregation

Abstract

Abstract 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 CaCl 2 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.