MALDI–TOF mass spectrometry of polyflavonoid tannins
Résumé
Matrix-Assisted Laser Desorption/Ionization Time-of-Flight OL (MALDI-TOF) appears to be a suitable method for examining polyflavonoid tannin oligomers. It appears capable to determine aspects of the structure and characteristics of polyflavonoid tannins, which are otherwise too difficult to determine by other techniques. It has been possible to determine by MALDI-TOF for the two major industrial polyflavonoid tannins which exist, namely mimosa and quebracho tannins, and some of their modified derivatives that: (i) mimosa tannin is predominantly composed of prorobinetinidins while quebracho is predominantly composed of profisetinidins, that (ii) mimosa tannin is heavily branched due to the presence of considerable proportions of 'angular' units in its structure while quebracho tannin is almost completely linear. These structural differences also contribute to the considerable differences in viscosity of water solutions of the two tannins. (iii) the interflavonoid link is more easily hydrolysable, and does appear to sometime hydrolyse in quebracho tannin and profisetinidins, partly due to the linear structure of this tannin, and confirming NMR findings that this tannin is subject to polymerisation/depolymerisation equilibria. This showed that the decrease of viscosity due to acid/base treatments to yield tannin adhesive intermediates does also depend in quebracho from a certain level of hydrolysis of the tannin itself and not only of the carbohydrates present in the extract. This tannin hydrolysis does not appear to occur in mimosa tannin in which the interflavonoid Link is completely stable to hydrolysis. (iv) sulphitation has been shown to influence the detachment of catechol B-rings much more than pyrogallol-type B-rings. (vi) the distribution of tannin oligomers, and the tannins number average degree of polymerisation obtained by MALDI-TOF appear to compare well with the results obtained by other techniques.