Acylated β-Caseins: Electrostatic Interactions and Aggregation

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Abstract
B[beta]-Casein was both acetylated and succinylated to investigate alterations of selected properties brought about by removal of positive charges and addition of negative charges in the case of succinylated [beta]-casein. The role of electrostatic interactions in the aggregation of [beta]-casein was evaluated. Mobility during alkaline poly-acrylamide gel electrophoresis of both derivatives was greater than [beta]-casein, succinylated [beta]-casein having the greatest mobility. Succinylated [beta]-casein and acetylated [beta]-casein, in that order, required a higher concentration of NaCl than [beta]-casein for elution from diethyl-aminoethyl (DEAE)-cellulose at pH 7.0. The Ca ion sensitivity of acetylated [beta]-casein was decreased in comparison to [beta]-casein. Succinylated [beta]-casein was insensitive to Ca ions at pH 7 in 0.1 M CaCl2-at a 0.3% concentration. Sedimentation patterns at pH 6.86, 20 C, revealed that succinylated [beta]-casein did not form a fast-sedimenting peak usually associated with aggregation. The fast-sedimentating peak of acetylated [beta]-casein had a lower sedimentation coefficient than [beta]-casein. These alterations of [beta]-casein may result from an increase in the net negative electrostatic charge of the modified protein in neutral solution. For succinylated [beta]-casein, the net negative electrostatic charge is sufficient to prevent precipitation by Ca ions and to prevent aggregation under conditions that favor aggregation of [beta]-casein.