Selective binding of potassium and calcium ions to novel cyclic proanthocyanidins in wine by high-performance liquid chromatography/high-resolution mass spectrometry
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Rationale: Cyclic B-type (referred also as crown) proanthocyanidins (PACs) were recently identified in wines. A high-performance liquid chromatography/high-resolution tandem mass spectrometry (HPLC/HRMS/MS) method was applied to study the binding of cyclic and non-cyclic PACs to potassium and calcium ions, which affect the chemico-physical stability of wines. Different binding affinities suggest that cyclic and non-cyclic analogues, despite the equal number of monomer units, influence the colloidal stability of wine and are related to the grape variety or winemaking conditions. Methods: Nineteen red and white wines were analysed by HPLC/HRMS/MS with positive electrospray ionization to investigate the distribution of novel cyclic PACs and their calcium, potassium and sodium adducts. Principal components analysis was used to study the distribution of the wines and the relationships among PACs with and without cation complexes. Results: A dependence on specific isomers (and conformations) was found for the non-cyclic procyanidin (PC) trimer whereas the cyclic tetrameric PACs were shown to bind better to potassium than their non-cyclic analogues. The binding to these metals appeared to be influenced not only by the number of monomer units, but also by the conformation assumed by the molecules. Statistical analysis evidenced that the distribution of adducts in different wines is less influenced by the grape variety used for winemaking than their associated [M+H](+) species studied earlier. Conclusions: Wines from 19 grape varieties were investigated in order to identify potassium and calcium complexes of non- and cyclic B-type PACs that were recently discovered. The results showed a dependence of the distribution of metal complexes according to the cyclic or non-cyclic geometry of PACs. The multivariate analysis of the mass spectrometric results showed a relationship with the grape variety; however, this was not as straightforward as evidenced for the non-complexed species.