Leather 97
The presence of the ring-structured imino acid residues imparts kinks into the
protein chain and the fact that there are so many results in the formation of
long, coiled molecules. These have a left-handed twist. The large quantity of
small glycine residues enables these coils to be wound tightly. It has been shown
that three such helical chains then twist together to give a right-handed helix.
The whole structure is stabilised mainly by hydrogen bonding, both within the
individual coils, and between the three elements of the helical macromolecule.
The collagen molecule then is a stable, rod-like, triple helix. It is about 300 nm
long with a diameter of about 15 nm and has appreciable amounts of potentially
reactive free acidic and basic groups on its outer surface. It has been shown
that these triple helices then twist together further and cross-link with a quarter
stagger to form long, five-stranded, rope-like fibrils with diameters of about
100 nm and these then group and coil to make fibres. A group of fibres are
known as fibre bundles.
In addition to the hydrogen bonding which stabilises the triple helix, a variety
of links are formed between adjacent molecules within the fibrillar structure.
In particular, covalent bonds are formed between the ends of one collagen
macromolecule in what is termed the telopeptide region and the helical structure
of an adjacent one.
When the animal dies, various mechanisms, which in life protect against the
effects of proteolytic bacteria, cease and within hours enzymatic and autolytic
reactions commence and the skin begins to putrefy. It has been shown that
among the first bonds to be broken are the covalent cross-links binding adjacent
molecules together. As a result, the fibrils split apart allowing enzymatic action
to cause the triple helices to unwind. This is followed by cleavage along the pro-
teinchain itself. As has been mentioned, various methods have been employed
to inhibit this bacterial action temporarily. However, to stabilise the skin fully,
new cross-links will have to be introduced into the macromolecular structure and
these will need to be stable to bacterial attack and not be affected by water.
This addition of artificial cross-links is achieved by the tanning process.
It has been generally accepted that hydrothermal shrinkage is a result of
the disruption of the hydrogen bonding within and between the polypeptide
chains caused by an increase in the molecular vibration of the macromolecule,
generated by the introduction of energy to the system in the form of heat. The
addition of extra chemically-stable cross-links into the protein complex imparts
increased resistance to this molecular vibration, requiring higher levels of
energy before these linkages are broken and the structure is disrupted. This is
reflected in increased shrinkage temperatures. Recent work on the kinetics of
this process has refined this concept and explained for instance why oil tannage
produces a true leather without an increase in shrinkage temperature. Tanning,
therefore, is the process of introducing additional artificial biochemical-resistant
cross-links into the macromolecular protein structure of collagen.