Ph induced structural changes of the complex formed between carotenoids from sea buckthorn (Hippophae rhamnoides l.) and bovine β-lactoglobulin

Abstract

Detailed information regarding the pH-dependent conformational state of βlactoglobulin in a complex formed with sea buckthorn extract was attained by fluorescence spectroscopy investigations. β-lactoglobulin is a small globular protein (MW ≈18 kDa) with a very well characterized structure that reveals several possible binding sites for ligands, whereas sea buckthorn has gained importance as a versatile nutraceutical, due to its high nutritive value in terms of carotenoids. The stability of the complex was tested at pH 4.5, 5.2, 6.5 and 8.8.
Higher fluorescence intensity was observed at acidic conditions compared to neutral and alkaline pH, regardless of the excitation wavelength, whereas the ANS fluorescence intensity was slightly higher at pH 8.8. Based on the maximum emission wavelengths values, it was suggested that Trp residues are buried while Tyr residues are partially exposed to the solvent at every tested pH. The synchronous spectra and the excitation-emission matrices did not reveal any significant pH induced conformational changes. It seems that β-lactoglobulin is
stable within the pH range considered in our experiments, with no significant conformational changes within the complex.