Epicatechin-induced conformational changes in beta-lactoglobulin B monitored by FT-IR spectroscopy

The interaction between whey carrier protein beta-lactoglobulin B and (-)-epicatechin, a major dietary flavonoid with a wide range of health-promoting biological activities, was investigated by Fourier transform infrared spectroscopy in physiological conditions. Amide I spectra of epicatechin - beta-lactoglobulin complexes, in D2O buffer solutions, pD = 6.8, at molar ratios from 0.5:1 to 15:1, were measured by using a cell device specifically created. Changes in secondary structure elements at increasing epicatechin concentrations were quantified. Two different trends were observed for the intensities of beta-sheet, random coil, and side chain contributions. At molar ratios <= 2 the beta-exposed strand contributions (1625 cm(-1)) increased at the expence of the beta-antiparallel sheet band (1637 cm(-1)). At molar ratios >2 the intensities of both beta structures slightly decreased. The same behaviour was observed for the side chain contributions (band around 1610 divided by 1620 cm(-1)). In addition, a conformational transition to a slightly opened structure, followed by aggregate formation at the highest molar ratios, were revealed. The results suggest that binding of epicatechin to beta-lactoglobulin in physiological conditions occurs at the surface of the protein molecule, resulting in protein dissociation at molar ratios <= 2 with minor changes in secondary structure. This finding provides further evidence for the possibility of successful use of the protein as a carrier of flavonoids, epicatechin included.