Abstract
The influence of Ca and Mg addition on the secondary structure of αS1-, αS2-, β- and κ-CN in solutions of individual and binary mixtures of caseins was investigated using FTIR spectroscopy. Both in individual and their binary mixtures, addition of Ca and Mg resulted in increase in β-sheet structures and decrease in triple helices and turns, implying binding of cations to similar sites. Binding of cations with phosphoseryl clusters with loop-helix-loop motifs explained the reduction in helical element. In addition, the binding of cations to electronegative regions reduced electrostatic repulsion, resulting in an increase in hydrophobic interactions accounting for increase in sheet structures. Compared with Mg, it seemed that Ca had more affinity for caseins, especially when they were in a binary mixture. The information presented here expands the present understanding of casein interactions.
Highlights
Casein micelles are colloidal complexes of four types of caseins held together by amorphous calcium phosphate, electrostatic and hydrophobic forces (Huppertz, Fox, & Kelly, 2018)
FTIR spectra were acquired in the range of 4000 to 600 cmÀ1 at 25 C using a PerkinElmer Frontier FTIR spectrometer (PerkinElmer, Boston, MA, USA) with a resolution of 4 cmÀ1 and averaging 16 scans for each spectrum
0.5 mL of sample was added onto an attenuated total reflectance (ATR; PerkinElmer Universal ATR Accessory, single reflection) cell
Summary
Casein micelles are colloidal complexes of four types of caseins (aS1-, aS2-, b- and k-CN) held together by amorphous calcium phosphate, electrostatic and hydrophobic forces (Huppertz, Fox, & Kelly, 2018). Casein micelles carry approximately two thirds of the total milk calcium, half the inorganic phosphate, one third of magnesium, and smaller proportions of citrate and the other small ions (Bijl, Huppertz, van Valenberg, & Holt, 2018). Some differences exist in distribution of calcium in the micelle and its interaction with different caseins. This is mainly due to a lack of information as caseins could not be crystallised and complete secondary and tertiary structure is not available. Spectroscopic techniques (Raman, Fourier transform infrared spectroscopy, Circular dichroism) and molecular modelling have given some interesting insights on secondary structure of caseins and their interactions with calcium, sodium and potassium (Curley, Kumosinski, Unruh, & Farrell, 1998; Farrell, Brown, & Malin, 2013; Huppertz, 2013)
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