3.2. Separation and characterization of procyanidins from grape seed extract Ultra-high performance liquid chromatography combined with electrospray ionization tandem mass spectrometry (UHPLC-ESI-MS) was used to tentatively identify the major procyanidins present in the grape seed extract used. The commercial grape seed extract was labeled as having greater than 40% procyanidins and the base peak chromatogram (Fig. 1A) confirmed the presence of flavan-3-ols and oligomeric procyanidins. Identification of the procyanidins in the HPCCC fractions was used to deduce structure–activity relationships regarding their antimicrobial activity. The base peak chromatograms of the three obtained fractions (Fig. 1B-D) show their respective composition. Several compounds were identified as oligomeric procyanidins in the grape seed extract (Table 3). The most abundant peaks at m/z 289 correspond to the monomer catechin (compound 13) or its isomer epicatechin (compound 22), both of which were identified by comparison with commercial standards. Less abundant were an epicatechin dimer PC2 (m/z 577- compound 18) and an flavanol galloyl ester (epi)catechin gallate (m/z 441- compound 28) (Fig. 1A)