The effect of sugar stereochemistry on protein self-assembly: the case of β-casein micellization in different aldohexose solutions

Protein self-assembly applications, such as nanoencapsulation of drugs and nutraceuticals, require deep understanding of the parameters governing the micellization process, including the effects of ionic and non-ionic co-solutes, like salts and sugars respectively, which is often overlooked. Herein, with the aim of shedding light on the effect of nonionic cosolute stereochemistry on protein self-assembly, we studied the ternary system of water–protein–sugar by examining the concentration-dependent effects of three aldohexoses, D-glucose (Glu), D-galactose (Gal) and D-mannose (Man) and that of urea, on the micellization of beta casein (β-Cas), using pyrene as a fluorescent probe for the formation of hydrophobic domains. Pyrene's excitation spectra were recorded for several sets of samples with rising protein concentration (0–5 mg ml−1), each set with a different co-solute type and concentration. Critical micellization concentration (CMC) and cooperativity of micellization were evaluated according to changes in pyrene spectra as it partitioned from the aqueous environment to the hydrophobic cores of β-Cas micelles. All sugars examined lowered the CMC of β-Cas with increasing sugar concentration and with a diminishing degree of effectiveness (Glu > Gal > Man) which correlated well with the sugars' dynamic hydration number, defined by Uedaira, and correlated negatively with their hydrophobic to hydrophilic molecular surface ratio. These results support the hypothesis that sugars affect protein self-assembly through both changes in water structure and by hydrophobic interactions, both of which are evidenced to be highly sensitive to sugar stereochemistry.