Analysis of commercial proanthocyanidins. Part 2: An electrospray mass spectrometry investigation into the chemical composition of sulfited quebracho (Schinopsis lorentzii and Schinopsis balansae) heartwood extract

The chemical changes that take place upon extraction of quebracho proanthocyanidins with bisulfite (sulfitation) were investigated with electrospray mass spectrometry. Sulfonic acid moieties are introduced on both the C-2 and C-4 positions, changing certain physical properties such as water solubility, and enhancing the suitability of the natural extract for industrial applications. C-4 sulfitation is accompanied by interflavanyl bond fission and a reduced average chain length. [Display omitted] ► An ESI investigation into the composition of sulfited quebracho proanthocyanidins. ► Extraction with bisulfite leads to the introduction of sulfonic acid moieties. ► C-4 sulfitation breaks interflavanyl bonds and reduces chain length. ► C-2 sulfitation opens the pyran ring and increases water solubility. ► A chromatographic method to separate and quantify sulfited and non-sulfited PACs. Proanthocyanidins (PACs) are natural plant-derived polymers used in leather tanning, wood adhesives, water purification, and mud additives for oil drilling. Quebracho (Schinopsis lorentzii and Schinopsis balansae) heartwood and mimosa (Acacia mearnsii) bark extracts are the major industrial sources of PACs. These commercial extracts are often sulfited via treatment with sodium hydrogen sulfite to reduce their viscosity and increase their solubility in water. An ESI-MS investigation into the molecular composition of sulfited (cold-water-soluble) quebracho heartwood extract indicates that sulfitation of the PACs occurs via SN2 attack of a sulfite ion at both C-2 and C-4 of the constituent flavan-3-ol monomer extender units. Attack at C-2 leads to the opening of the pyran ring. This releases an additional electron-donating phenolic hydroxy group on the A-ring and renders the extract more nucleophilic and suitable for the manufacturing of adhesives. Attack at C-4 leads to interflavanyl bond fission and decrease of the PAC oligomer chain length. The introduction of sulfonic acid moieties at C-2 or C-4 increases the polarity and water solubility of the hot water soluble (unsulfited) extract and transforms it into a cold-water-soluble extract.