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Solid state 13 C-NMR spectra of pure tannin powders from four different sources -mimosa, quebracho, chestnut and tara -are readily distinguishable from each other, both in pure commercial powder form, and in leather which they have been used to tan. Groups of signals indicative of the source, and type (condensed vs. hydrolyzable) of tannin used in the manufacture are well resolved in the spectra of the finished leathers. These fingerprints are compared with those arising from leathers tanned with other common tanning agents. Paramagnetic chromium (III) tanning causes widespread but selective disappearance of signals from the spectrum of leather collagen, including resonances from acidic aspartyl and glutamyl residues, likely bound to Cr (III) structures. Aluminium (III) and glutaraldehyde tanning both cause considerable leather collagen signal sharpening suggesting some increase in molecular structural ordering. The 27 Al-NMR signal from the former material is consistent with an octahedral coordination by oxygen ligands. Solid state NMR thus provides easily recognisable reagent specific spectral fingerprints of the products of vegetable and some other common tanning processes. Because spectra are related to molecular properties, NMR is potentially a powerful tool in leather process enhancement and quality or provenance assurance. OPEN ACCESSMolecules 2011, 16 1241

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Key indicatorsSingle-crystal X-ray study T = 100 K Mean (C-C) = 0.003 Å R factor = 0.024 wR factor = 0.065 Data-to-parameter ratio = 20.6For details of how these key indicators were automatically derived from the article, see