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Tannin Fingerprinting in Vegetable Tanned Leather by Solid State NMR Spectroscopy and Comparison with Leathers Tanned by Other Processes
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK
Department of Chemistry, University of the Free State, 205 Nelson Mandela Drive, Park West, Bloemfontein 9301, South Africa
* Author to whom correspondence should be addressed.
Received: 6 December 2010; in revised form: 26 January 2011 / Accepted: 27 January 2011 / Published: 28 January 2011
(This article belongs to the Special Issue Tannins
Abstract: Solid state 13C-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 27Al-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.
Keywords: vegetable tannins; polyphenols; chromium; aluminium; glutaraldehyde
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Romer, F.H.; Underwood, A.P.; Senekal, N.D.; Bonnet, S.L.; Duer, M.J.; Reid, D.G.; Van der Westhuizen, J.H. Tannin Fingerprinting in Vegetable Tanned Leather by Solid State NMR Spectroscopy and Comparison with Leathers Tanned by Other Processes. Molecules 2011, 16, 1240-1252.
Romer FH, Underwood AP, Senekal ND, Bonnet SL, Duer MJ, Reid DG, Van der Westhuizen JH. Tannin Fingerprinting in Vegetable Tanned Leather by Solid State NMR Spectroscopy and Comparison with Leathers Tanned by Other Processes. Molecules. 2011; 16(2):1240-1252.
Romer, Frederik H.; Underwood, Andrew P.; Senekal, Nadine D.; Bonnet, Susan L.; Duer, Melinda J.; Reid, David G.; Van der Westhuizen, Jan H. 2011. "Tannin Fingerprinting in Vegetable Tanned Leather by Solid State NMR Spectroscopy and Comparison with Leathers Tanned by Other Processes." Molecules 16, no. 2: 1240-1252.