Quebracho (Schinopsis lorentzii and Schinopsis balansae) heartwood and black wattle
(Acacia mearnsii) bark extracts are important renewable industrial sources of
proanthocyanidins (PACs). These extracts are used industrially in leather tanning and
adhesive manufacturing. These applications are derived from their chemical properties. The
poly hydroxy groups of PACs complex with proteins via hydrogen bonds and thus transforms
raw skin into leather. The phloroglucinol or resorcinol type A-rings are nucleophilic and
polymerise with aldehydes to form natural adhesives. The ortho hydroxy group on the B-ring
form insoluble complexes with heavy metals and can be used in water purification
applications. The extracts are often treated with sodium hydrogen sulphate (sulfitation) to
enhance their industrial usefulness. From a literature search and discussions with role players
in the black wattle and quebracho PAC extract manufacturing industry, it became evident that
knowledge on the composition of commercial PACs extracts and chemical changes that takes
place during sulfitation is unsatisfactory.
These PAC extracts are complex due to variable hydroxylation patterns of the constituent
flavan-3-ol aromatic rings, different configurations of the C-2, C-3 and C-4 stereogenic
centres, different degrees of polymerisation, and the existence of angular oligomers. Gel or
paper chromatography fractionations of the complex extracts are hampered by poor
resolution due to their hydrophilic polyphenolic nature and efforts to isolate pure compounds
have been restricted to the isolation of mainly monomers and a few dimers and trimers.
PACs of the commercially important quebracho (Schinopsis lorentzii and Schinopsis
balansae) and black wattle (Acacia mearnsii) extracts have a strong and stable interflavanyl
bond. This stability is important from an industrial point of view as it leads to durable leather
and adhesive products. It is attributed to the absence of 5-OH groups in the aromatic moieties
of the extender fisetinidol and robinetinidol flavan-3-ols units. However, from an analytical
point of view it is not advantageous. The high temperatures thus required to hydrolyse the
interflavanyl bonds with weak acids; leads to decomposition of the intermediate monomers
that renders conventional thiolysis and phloroglucinolysis based analytical methods
unreliable. In this thesis we used electrospray mass spectrometry (ESI-MS) to investigate the
composition of PACs in black wattle extract and the changes that takes place in the chemical
composition of quebracho PACs during sulfitation. We furthermore use all the information
available from literature on the phytochemistry of flavan-3-ols and PACs and the syntheses
of flavan-3-ol oligomers to guide us in our ESI-MS interpretations.
Previous research in our group established that quebracho PACs always consist of a catechin
starter unit to which one, two or more fisetinidol extender units are attached. The first and
second extender units are always attached to the relatively reactive phloroglucinol A-ring of
the catechin starter unit to form predominantly dimers and angular trimers. Further extender
units are attached to the relatively less reactive resorcinol A-rings of already incorporated
fisetinidol extender units. This explains the relatively short degree of polymerisation of
quebracho PAC extracts and their popularity as a tanning agent. Large PACs will not
penetrate the spaces between skin proteins and cannot act as a tanning agent.
In this thesis we established that black wattle PACs have, in addition to catechin starter units,
also gallocatechin starter units and, in addition to fisetinidol extender units, also robinetinidol
extender units. Acacia PACs are thus more complex combinations of catechin, gallocatechin,
fisetinidol and robinetinidol monomers. This contrasts with quebracho PACs that only
contain catechin and fisetinidol monomers. The higher degree of hydroxylation of
gallocatechin and robinetinidol explains the higher water solubility of black wattle PACs and
the less frequent need for sulfitation.
We also established that during sulfitation of quebracho PACs, a sulfonic acid moiety is
introduced in both the C-2 and C-4 position of the pyran heterocyclic C-ring. In the case of
C-2 sulfitation, the heterocyclic ring is opened. This enhances the reactivity of the A-ring
towards the reaction with formaldehyde (adhesive formation) and increases water solubility
due to removal of rigidity and introduction of a polar sulfonic acid group. In the case of C-4
sulfitation, the interflavanyl bond is broken. Polarity and water solubility is thus not only increased via an additional sulfonic acid moiety, but due to the presence of shorter oligomers
and a smaller average chain length. We also developed a chromatographic method to estimate
the degree of sulfitation of quebracho PAC extract.
We believe that we have made a valuable contribution towards a better understanding of the
composition of black wattle and sulfited quebracho PAC extracts and have identified a
number of misconceptions.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-07232013-111240 |
| Date | 23 July 2013 |
| Creators | Jordaan, Maryam Amra |
| Contributors | Prof JH van der Westhuizen |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-07232013-111240/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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