Link-Lock theory of collagen stabilisation
Submitting InstitutionUniversity of Northampton
Unit of AssessmentAeronautical, Mechanical, Chemical and Manufacturing Engineering
Summary Impact TypeTechnological
Research Subject Area(s)
Chemical Sciences: Macromolecular and Materials Chemistry, Physical Chemistry (incl. Structural), Theoretical and Computational Chemistry
Summary of the impact
The leather industry is globally significant: the supply chain, from
leather production to leather goods manufacture is currently worth in the
order of $1 trillion per annum. Although written records show that leather
production is at least 5,000 years old, an understanding of the chemical
principles underpinning the reactions involved only began at the turn of
the 19th century, roughly corresponding to the development of
modern chemistry and, coincidentally, with the revolutionary introduction
of chrome tanning. At that time, the basis of the stabilising reaction was
assumed to be crosslinking, whereby the component strands of protein are
linked like stitches by the tanning chemical and this was the accepted
view thereafter. Furthermore, it was also accepted that the ability of
tanning reactions to confer high hydrothermal stability is a property of a
few unrelated chemistries, of which chromium(III) tanning is the best
Link-lock theory is revolutionary, insofar as it is the first new
thinking in the chemical stabilisation of collagen in over half a century.
This view of tanning was the outcome of examining the reaction with modern
analytical instrumentation and applying a weight of evidence from the
literature. The consequence has been the rejection of the accepted view of
tanning mechanism, in favour of a simpler but more powerful theory. The
principles of the theory can explain the effects of all known tanning
processes. In an applied technology, its use is most powerful as it can
predict the outcome of all, even as yet unknown reactions. Moreover, it is
a major part of the way in which practitioners can predict details of the
processes required to make leather and other biomaterials with desired
properties and performance.
The fundamental importance of the theory is that it allows the subject to
move on as it is more powerful than the alternative view and there is much
evidence to support it. By combining this thinking with other new thinking
into wider aspects of the heterogeneous chemistry, modifying collagen is
now placed on a firm basis of leather science, which means that the
outcomes of reactions can be predictable. The primary impact of this new
view of protein stabilisation lies in the ways in which the thinking has
and is informing developments of collagenic biomaterials and applications
in the global leather and associated industries.
The `link-lock' theory was developed by Covington, (who was Professor,
and then in October 2011 became Emeritus Professor of Leather Science),
over a period of about 10 years [1,2]. Here, the most important
experimental collaboration was with the Universities of Canterbury and
Manchester, when X-ray studies were undertaken on the synchrotron at
Daresbury . This was the first time the chrome tanning reaction had
been subjected to examination at the molecular level. Although it had been
expected that the observations on collagen tanned with basic chromium(III)
sulfate would confirm the prevalent view of crosslinking, unexpectedly,
the direct evidence contradicted it as follows:
(i) bound chromium is in the form of a linear tetramer, not a dimer.
(ii) sulfate is not bound to chromium species as a ligand — it functions
as a stabilising component of the reaction i.e. the chrome tanning
reaction has a combination mechanism.
(iii) if the chrome tanning reaction is dependent on the crosslinking
mechanism, the outcome would be independent of the anion: however, the
tanning reaction is highly dependent on the anion e.g. tanning with
chromium(III) chloride or perchlorate can only yield shrinkage temperature
no higher than 85oC, like any other single tanning species.
The latter observation means that chrome tanning falls into line with
other known reactions capable of conferring high hydrothermal stability,
but are characterised as combination processes e.g. plant hydrolysable
polyphenol + aluminium(III) salt.
Therefore, all tanning reactions, regardless of the chemical nature of
the tanning agents and the way they react with collagen, are fundamentally
the same, insofar as they merely physically interfere with the shrinking
mechanism when collagen is heated in water. Consequently, any single
reagent is only capable of raising the observed denaturation temperature
from 65 to no more than 85oC: this is referred to as the
linking reaction. If a second reagent is applied, it can react on the
collagen and interact with the first reagent in one of three ways:
independently, antagonistically or synergistically. In the latter case,
the reaction is designated the locking reaction, because it locks the
molecules of the first reaction together: in this way, a new tanning
system is created, a matrix or scaffolding, which stabilises the collagen
by concerted interactions/bonds. The consequence of this process is that,
when the modified collagen is subjected to wet heat, it is much more
difficult for the triple helix structure of collagen to unravel and for
the random coils of the denatured protein to collapse into the interstices
of the highly organised collagen structure: this is observed as a marked
raising of the shrinkage temperature, up to 130oC .
The theory incorporates a new understanding of the chrome tanning
reaction: more importantly, this understanding provides the basis for
making improvements not only in the properties and performance of chrome
tanned leather but also to the environmental impact of its production [5,
References to the research
1. Covington, A.D., "New tannages for the new millennium", .J. Amer.
Leather Chem. Assoc., 1998, 93(6), 168.
2. Covington, A.D., "Theory and mechanism of tanning: current thinking
and future implications"J. Soc. Leather Technol. Chem., 2001, 85(1), 24.
3. Covington, A.D., Lampard, G.S., Menderes, O., Chadwick, A.V.,
Rafeletos, G., O'Brien, P., "Leather tanning studies using Extended X-ray
Absorption Fine Structure (EXAFS)" Polyhedron", 2001, 20, 461.
4. Covington, A.D., Song, L., Suparno, O., Koon, H.E.C., Collins, M.J.,
"Link-lock: an explanation of the chemical stabilisation of collagen", J.
Soc. Leather Technol. Chem., 2008, 92(1), 1.
5. Covington, A.D., "The mechanism of chrome tanning", Global J. Inorg.
Chem., 2010, 1(2), 119.
6. Covington, A.D., "Prediction in Leather Processing: A Dark Art or a
Clear Possibility", J. Soc. Leather Technol. Chem., 2011, 95(6), 231.
For reference 3, Lampard's doctoral studies were funded by the University
of Northampton, Menderes' doctoral studies were funded by the Turkish
Government, Professor Chadwick and Mr. Rafeletos were collaborators from
the University of Canterbury, Professor O'Brien was a collaborator from
the University of Manchester.
For Reference 4, Song's doctoral studies were funded by the University of
Northampton, in a collaborative programme with Sichuan Union University of
Chengdu, P.R. China and Suparno's doctoral studies were funded by the
Indonesian Government. Professor Collins and Ms. Koon were collaborators
from the University of York.
Details of the impact
Technological progress and acceptance of new thinking in the leather
industry is conservatively slow, but the link-lock theory appears to have
been embraced surprisingly quickly. Since the first publication of the
link-lock theory in the technical leather literature in 2008, the theory
was refined for further dissemination in the technical magazine World
Leather, Oct/Nov 2010. The theory was used extensively in the argument for
predicting the processing by which desired characteristics could be
conferred to leather, published in the Journal of the Society of Leather
Technologists and Chemists, 2011, 95(6), 231, taken from the Procter
Memorial Lecture given to the Annual Conference of the Society of Leather
Technologists and Chemists of UK in 2010, and presented to the
International Union of Leather Technologists and Chemists Societies
Congress in Valencia, Spain in 2011, a biennial meeting of the technical
arm of the global leather industry.
The theory was extensively treated in the following book, the first
treatment of leather science since 1993, in which the future of tanning
was reviewed in the light of the new thinking: Covington, A.D., Tanning
Chemistry. The Science of Leather, Royal Society of Chemistry
Publishing, Cambridge, 2009 (ISBN:978-0-85404-170-1)
The link-lock theory, however, has been disseminated beyond academia into
the global leather and associated industries: it is known to be being used
in the thinking of leather scientists in the USA and Europe. In
particular, The American Leather Chemists Association (ALCA) gave
Covington the Alsop Award in 2011 for `outstanding scientific or
technological contribution to the leather industry'. It should be
noted that ALCA were founded in 1903 specifically to promote the
advancement of science and engineering in their application to the leather
and leather product industries.
In practical terms, the link-lock theory is informing the development
programme of the Lanxess company of Germany. They are currently marketing
a new tanning agent based on poly (carbamoyl) sulfonate, In order to
optimise the effect of the reagent in the market, the link-lock theory is
being used to define how the technology can be extended in terms of
combinations of other chemicals with the base reagent. In this way, a new
generation of tanning processes can be devised, allowing tanners an
organic option to the global standard of chrome tanning and a means to
avoid the environmental difficulties of using metal chemistry (contact Dr.
Dietrich Tegtmeyer, Lanxess).
Link-lock is also the basis of the thinking in trying to define the
development of a new generation of tanning chemicals being undertaken by
Buckman International, an American based chemical supply house for whom
Covington acted as Consultant in 2011. Since there is a developing market
for leather which is chromium free, they like other supply houses are
interested in supplying alternative tanning reagents to the tanning
industry. Here, the particular interest lies in organic alternatives to
chromium(III) chemistry and any successful alternative must match as many
of the properties of chrome tanned leather as possible. The only way to
achieve high hydrothermal stability, required in modern applications of
leather, is to use the principles of link-lock (contact Mr. Elton Hurlow,
The ECCO company, a multinational leather business based in The
Netherlands, Indonesia and P.R. of China, currently sends cohorts (ca.10
pa) of its brightest technologists to the University of Northampton for
the MSc in Leather Technology. The purpose is to educate them in the
latest thinking, in which link-lock theory and associated developments are
central. Graduates return to their factories equipped with the ability to
engage in development programmes to make operations more profitable: this
is the return on the investment made by the parent company (contact Mr.
Arthur Jones, ECCO).
Sources to corroborate the impact
- Brown, E.M., Dudley, R.L., "Approach to a tanning mechanism; study of
the interaction of aluminium sulfate with collagen", J. Amer. Leather
Chem. Assoc., 2005, 100(10), 401.
- Reich, G., "From collagen to leather — the theoretical background",
BASF Ludvigshafen Germany, 2007.
- Tegtmeyer, D., Tysoe, C., Reiners, J., "X-Tan an innovative organic
tanning technology with superior sustainability", Proc., IULTCS
Congress, Valencia Spain, September 2011.
Reference 1 states: `Recent proposals ... that tanning is a matter of
protein modification, not just collagen crosslinking, are supported by
our recent finding that hydrothermal stability of leather is a function
of total bound chromium, not just a fraction that forms `productive
Reference 2 is a privately published monograph written by one of the
world's foremost leather scientists in the latter part of the last
century, in which he discusses aspects of the thinking leading to the
link-lock theory — although this work was in preparation just before the
term was coined. Reich accepts the developing conclusions relating to the
importance of the matrix created by the tanning system in stabilising the
Reference 3 Is the text corresponding to the oral presentation at the
Congress of the International Union of Leather Technologists and Chemists
Societies, given by Tegtmeyer of the company Lanxess, an international
chemical supply house, during which much mention was made of analysing the
mechanism and performance of the new tanning system by the link-lock
Beneficiaries who could be contacted to corroborate the claims made.
Vice President, Head of Product Development and Application, Lanxess
Deutschland GmbH, Germany
Gobal Market Development Manager, Buckman Laboratories International,
ECCO Leather Academy Director, ECCO Tannery (Thailand) Co. Ltd., Thailand