We describe the characterization of the B-type procyanidins in wine and the B-type dehydrodicatechins (dimeric flavan-3-ols) obtained for the autoxidation of (+)-catechin and (-)-epicatechin by tandem mass spectrometry (MS/MS) coupled to reversed-phase high-performance liquid chromatography (HPLC). The MS/MS analysis demonstrates that the interesting major fragments derive from the dissociations of the C-ring on the catechin or epicatechin unit, such as retro-Diels-Alder reactions. The two kinds of dimers give completely different fragmentations because of the striking effect of the C-C interflavan linkage (IFL). For the natural dimers in wine, a catechin or epicatechin unit linking to the C-4 position stabilizes the product ions by forming a large pi-pi hyperconjugated system, whereas a similar pi-pi system is formed within dehydrodicatechin B through the C-C IFL. Thus dissociation in MS/MS experiments was inhibited. Apparently, the fragmentations of the dimers differ from that of the monomer, which is very important in the study of the gas-phase ion behaviour of the polymeric flavan-3-ols. In addition, two specific fragment ions at m/z 451 for native dimers and at m/z 393 for autoxidation species in HPLC/MS/MS were found to be very useful for analysing mixtures of B-type procyanidins and B-type dehydrodicatechins in food and beverages.
Using electrospray ionization mass spectrometry (MS) combined with sequential tandem MS(ESI-MSn), two major steroidal saponins extracted from Tribulus terrestris were studied, and considerable useful structural information was obtained. The structure of the proposed known steroidal saponin was verified, and the structure of the unknown saponin was investigated using MSn experiments. Some special fragment ions were also observed, and the corresponding fragmentation mechanisms were investigated which are characteristic for steroidal saponins and can give some information on the linkage position of some sugar groups in saponins. This methodology has been established as a powerful tool for the rapid, comparative analysis of mixtures such as crude plant extracts.
Alditols, which have a sweet taste but produce much lower
calories
than natural sugars, are widely used as artificial sweeteners. Alditols
are the reduced forms of monosaccharide aldoses, and different alditols
are diastereomers or epimers of each other and direct and rapid identification
by conventional methods is difficult. Nanopores, which are emerging
single-molecule sensors with exceptional resolution when engineered
appropriately, are useful for the recognition of diastereomers and
epimers. In this work, direct distinguishing of alditols corresponding
to all 15 monosaccharide aldoses was achieved by a boronic acid-appended
hetero-octameric Mycobacterium smegmatis porin A (MspA) nanopore (MspA-PBA). Thirteen alditols including
glycerol, erythritol, threitol, adonitol, arabitol, xylitol, mannitol,
sorbitol, allitol, dulcitol, iditol, talitol, and gulitol (l-sorbitol) could be fully distinguished, and their sensing features
constitute a complete nanopore alditol database. To automate event
classification, a custom machine-learning algorithm was developed
and delivered a 99.9% validation accuracy. This strategy was also
used to identify alditol components in commercially available “zero-sugar”
drinks and healthcare products, suggesting their use in rapid and
sensitive quality control for the food and medical industry.
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