Hydrogen-Deuterium exchange kinetics in β-lactoglobulin (−)-epicatechin complexes studied by FTIR spectroscopy

Hydrogen-Deuterium exchange kinetics of β-lactoglobulin and β-lactoglobulin (−)-epicatechin solutions has been investigated through the analysis of the amide I absorption band at 1650 cm−1 in the FTIR spectrum. H-D substitution in NH amides and residues of the protein results in a slight red-shift and in intensity changes of the amide I components: either these effects have been inspected in the framework of the Principal Components Analysis methods. The present analysis allowed to unveil three H-D kinetics at the timescale of the oligomeric fluctuations of the protein. A fast mechanism (lifetime from 5 to 10 min) can be ascribed to the dynamics of protein oligomers and aggregates at the scale of the quaternary structure variations, and it is not observed in the complexes β-lactoglobulin (−)-epicatechin. The other slowest kinetics, whose lifetimes are in the range 1-10 h, are here associated to dynamics of high-molecular weight complexes that hamper the proton exchange. The role of (−)-epicatechin as an enhancer of the formation of stable high-molecular weight aggregates from β-lactoglobulin is also discussed by comparison of the lifetimes at different protein concentrations.