pH influence on the mechanisms of interaction between chitosan and ovalbumin: a multi-spectroscopic approach

The forces behind supramolecular structures (SS) formation need to be deeply understood, to allow identifying conditions under which their production can be optimized. In this study, a multi-spectroscopic approach was used to study SS formation upon ovalbumin (OVA) and chitosan (CHS) interaction in HCl solution, at pH 4.0 (both biomacromolecules positively charged) or 6.0 (biomacromolecules oppositely charged). UV-Vis and fluorescence spectroscopies showed that the biomacromolecules interacted and formed complexes without suggesting major modifications in the protein’s structure. The increase in CHS concentration progressively augmented the UV-vis absorption of OVA-CHS structures at 278 nm, whereas reduced the fluorescence emission of OVA, at both pH values, without promoting obvious shift of the maximum fluorescence wavelength, with an estimated center of spectral mass of <λ> = 341.41 ± 0.31 nm (at pH 4.0) and <λ> = 342.30 ± 0.40nm (at pH 6.0). Additionally, a conjoint analysis of CD and FT-IR spectroscopies revealed that the presence of CHS promoted alterations on the secondary structure of OVA. Indeed, CHS may have affected the hydrogen bonding network that stabilized the structure in the native protein, increasing the stability of α-helix and random coil structures, which was reflected in the loss of β-sheet. Results suggested that the OVA-CHS interaction, at pH 6.0, are driven mainly by electrostatic forces (attractive) and H-bonds. At pH 4.0, these macromolecules are more likely to interact through a balance of electrostatic forces (repulsive), hydrophobic interactions and H-bonds.