Succinylation improves structure and hydrophobicityof yak casein micelles

Abstract: Yak caseins are special materials whose structures are different from cow caseins. Chemical modification is commonly applied to improve the functional properties of food proteins, such as acylation, succinylation, esterification, oxidation, reduction, glycosylation, phosphorylation, and alkylation. Succinylation is a useful method for significantly improving the functional properties of protein; iexhaustive succinylation induces large changes in physio-chemical properties of caseins, particularly in their electrostatic nature. However, very little literature is available about the structures, physio-chemical, and functional properties of yak caseins to direct industry processing of yak caseins. The present work tested the influence of succinylated modification on the structure and hydrophobicity of yak casein micelles. The spatial structure of modified and unmodified yak casein micelles were studied by Fourier transformation infrared spectroscopy. The diameter of modified and unmodified casein micelles was detected by dynamic light scattering and its surface morphology was observed by scanning electron microscope.The results showed that the spatial structure contents of β-turn, irregular, β-sheet, and large loop in yak casein micelles were 33.1%, 23.2%, 32.0%, and 11.7% respectively, while α-helix was not detected. After succinylated modification, a certain amount of amino changed into the amide linkage, the charge of side chain changed from positive charge into negative charge, the repulsion between side chain groups decreased, and α-helix was formed with content of 12.4%. The modification has slight influences on the other spatial structures except α-helix. Succinylation treatment induced the dissociation of caseins from the micelles, the size of yak casein micelle was decreased, and the shape of casein micelles was changed from irregular spherical into a sphere. After succinylation, lysine changed to succinamoyl groups and the electrostatic ionic interactions among ANS and caseins decreased, resulting in decreasing numbers of ANS molecules bounded to casein micelle. Thus, the ?uorescence intensity of ANS decreased in casein solution. The maximum emission wavelength (λmax) was very useful in estimating the hydrophobicity of the microenvironment around tryptophan residues. By succinylation, many -NH2 in caseins were effectively changed to carboxylic groups from succinic anhydride, and the hydrogen bonds formed by -NH2 were destroyed. At pH of 7.0, the deprotonation of the carboxylic groups gave rise to many changes in casein micelle formation, including electrostatic repulsions and hydrogen bonds and the destruction of salt bridges, eventually leading to the loose structure of the casein micelle. Therefore, the tryptophan residues were located at less hydrophobic micro domain areas in casein micelles with succinylation, which led to the increase of the maximum emission wavelength and the decrease of ?uorescence intensity in steady-state ?uorescence emission spectra. To sum up, the hydrophobicity of yak casein declined significantly by succinylation. The results could provide references for the chemical modification of yak casein.