Interactions between chitosan and casein in solution

The combination of chitosan (CHI) and bovine casein (CAS) is interesting for food formulation [1] or scaffolds [2]. In this study, the interactions between CHI and CAS was investigated at pH 3 (NaAc/HAc) and 5.5 (MilliQ water) in solution. CAS contains four protein families (κ-, β-, αS1-, and αS2-caseins) that have low levels of α-helices in comparison to typical globular proteins [3]. The CAS content in solution was either 1% (CHI/CAS1%) or 10% (CHI/CAS10%). In solution, circular dichroism (CD) analysis at pH 3.3 showed no conformational changes of CAS in the presence of CHI, probably because at this pH, both CHI and CAS are positively charged. However, at pH 5.5, the system CHI/CAS 10% presented a reduction of 1.5% and 2.2% of helix and parallel conformation, respectively, in comparison to pure CAS. At this pH, CAS carries a negative charge (pI = 4.7), while CHI chains are positively charged (pKa ~6), leading to electrostatic interactions between them. These trends were confirmed with fluorescence spectroscopy measurements. At pH 5.5, an emission peak observed at 335 nm for CAS was assigned to the phenylalanine (Phe) and tryptophan (Trp) amino acids. In the presence of CHI, the emission peak intensity increased considerably, indicating that the interactions might have favored the exposition of Phe and Trp to the medium. On the other hand, at pH 3.3, the presence of CHI reduced the emission peak intensity, indicating that the hydrophobic residues may be partially buried to avoid interactions. Ongoing dynamic light scattering (DLS), zeta potential, and rheological measurements of CHI/CAS1% and CHI/CAS10% at pH 3 and pH 5.5 will provide more insight about the intermolecular interactions. 

[1] Ma, N.; Wang, L.; Zhou, L.;  Wan, Y.; Ding, S.; Qian, W. Food Hydrocol 2023, 137, 108386. https://doi.org/10.1016/j.foodhyd.2022.108386

[2] Mishra, B.; Hossain, S.; Mohanty, S.; Gupta, M.K.; Verma, D.  Int. J. Biol. Macromol. 2021, 185, 525–534, https://doi.org/10.1016/j.ijbiomac.2021.06.116

[3] Runthala, A.; Mbye, M.; Ayyash, M.; Xu, Y.; Kamal-Eldin, A. Molecules 2023, 28, 2023. https://doi.org/10.3390/ molecules28052023