Proanthocyanidins, known also as polyflavan-3-ols or condensed tannins, are the second most abundant natural phenolics in plants and consist of mixtures of oligomers made of flavan-3-ol monomeric units linked mainly through C-4 and C-8 or C-6 bond, being procyanidins (PCs) the most widespread group. Beyond contributing to organoleptic characteristics of foods they are responsible for some bioactive properties such as cardiopreventive, anti-inflammatory, antioxidant, antiallergic, antithrombotic, antibacterial and anticarcinogenic activities, among others. Rapid structure determination of these compounds is needed, notably for the more complex PCs. Matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF MS) has been proved to be effective in the structural determination of these polymers related to grape seeds, in spite of the presence of other food components which may affect signal intensities. Power ultrasound (US) (>1 W/cm2) has been pointed out as an adequate environmental friendly physical technique to improve the miscibility of solution and suspensions and depolymerisation of biomacromolecules. To the best of our knowledge, no investigation has been done on a previous treatment of complex polymeric procyanidins before MALDI-TOF MS structural elucidation. In this work, we have subjected to US (45 and 20 kHz) a purified macromolecular fraction (more than 85% of polymeric PCs) from grape seed extract obtained by preparative tangential-flow pressure-driven ultrafiltration and diafiltration with water. MALDI-TOF MS analysis showed the occurrence of 6 B-type and 4 A-type procyanidin series in this fraction. The presence of 1.2% (w/w) of protein by colorimetric assay of Bradford, as well as minor concentrations of sorbitol, fructose, glucose and myo-inositol were also determined by GC-FID. US treatments gave rise to a substantial increase (<50%) of catechin and oligomeric and polymeric procyanidin content. US at 45 kHz produced also an important rise in the amount of PCs with higher molecular masses (up to decamers) and a broad increase in the mass signal intensities in most of the detected B-type procyanidin series. In general, US produced certain deliverance of monomeric, oligomeric and polymeric procyanidins that most probably proceed from disaggregation of linkages with other biopolymers, such as proteins and/or polysaccharides. The most practical application of this study was that US could be a very useful procedure for improving the analysis of polymeric PCs by MALDI-TOF MS.

Trabajo presentado al XVIII Scientific Meeting of the Spanish Society of Chromatography and Related Techniques (SECyTA), celebrado en Granada del 2 al 4 de octubre de 2018.

MICINN of Spain, Project AGL2014-53445-R.

Peer reviewed