Protein-Tannin Interactions of Tryptic Digests of α-Lactalbumin and Procyanidins.

Protein-tannin interactions on a molecular level were investigated by using a model system containing peptides of α-lactalbumin and berry tannins (procyanidins). Oxidation of isolated tryptic peptide LDQWLCEK (m/z 1034) with procyanidin B2 or procyanidin fraction (PF) isolated from aronia juice was monitored by LC-ESI-MS. Procyanidin B2 and PF showed radical scavenging activities toward oxidation of the peptide with the peptide also preventing procyanidin B2 from degradation. Oxidation enhanced the cleavage of peptide between tryptophan and glutamine. Interaction products arising from WLCEK or WLCE residue and degradation product of procyanidin B2 were also identified using both size exclusion chromatography and LC-MS. Tryptophan and lysine were the amino acids most prone to interact with procyanidin B2. The study shows that protein-tannin interaction takes place during oxidation leading to both degradation of the parent compounds and formation of interaction products. This may in turn affect the quality of protein and tannin containing food.