What’s in it for you? – Collagen
2 February 2009This is the second in a series of articles that will explain the principle components that are found in hides and skin, what their functions are in life and what their implications are for the tanner. In this issue we will discuss, in simple terms, what is probably the most important component, collagen, which forms the basis for all leather produced.
Collagen is a protein, but not all proteins can be made into leather because proteins come in many guises. There are liquid proteins, known as globular proteins, such as egg albumen (difficult to make leather from egg white!) and there are insoluble fibrous or structural proteins. Collagen is a fibrous protein and fits into the same category as muscle tissue and keratin (hair/hoof protein).
The word Collagen comes from the Greek Kolla meaning glue and relates to the ancient practice of boiling skin and bones to produce glue. In life, its key role is to provide a supporting framework into which cells grow and function. It provides the skin with structure; without it we would literally fall apart.
Collagen is the most abundant of the animal proteins; more than a quarter of the body's protein is collagen and it is not only found in skin; it also occurs in large quantities in tendon, cartilage and bone. Fresh unprocessed hide or skin contains around 33% protein, of which 29% is collagen1. Currently there are thought to be at least sixteen different types of collagen that have different functions within the body. In skin, the most abundant types are I and III with type IV forming what is essentially the grain surface.
It is the intricate structure of collagen that enables us to produce leather that is incredibly strong and durable yet highly flexible. Looking at it at a molecular level, collagen is made of thin strands of amino acids, largely glycine, proline and hydroxyproline, joined together to form a poly-peptide chain. The arrangement of the amino acids is unusually regular in collagen with glycine taking up every third space in the chain.
Three of these chains twist together to form a helix which is now called a collagen molecule that is only 1.5 nanometres in diameter. Several collagen molecules twist together to form micro-fibrils. Microfibrils twist together to form fibrils (typically 100 nm in diameter). Fibrils group together to form fibres (1 micron in diameter) and then the fibres group together to form fibre bundles. This complex structure is shown more clearly in Figures 1 and 2 where it can be seen that the structure is very similar to that of a rope.
It is the twisting together of the many sub-divisions that creates the combination of strength and flexibility. To use an analogy, let us consider a steel bar; it is very strong but lacks flexibility. If we split the steel bar into numerous thin strands of wire and then twist them into a steel rope, we now have strength plus flexibility. Interestingly, gram for gram, collagen is actually stronger than steel!
The regularity seen at the molecular level continues up the structure hierarchy; within the fibrils, the ends of adjacent collagen molecules are displaced from one another by a distance of 67 nm, which produces a characteristic striped appearance when examined at very high magnification using an electron microscope (Figures 3 and 4)2,3.
Collagen type IV forms what is essentially the grain surface of leather after the hair and epidermis have been removed. The structure of this type of collagen is slightly different to that of types I and III in as much as there are intermittent areas in the molecular structure that do not twist into a triple helix thus giving additional flexibility. Also, instead of forming a very three-dimensional fibrous matrix like types I and III, collagen type IV forms a flat two dimensional sheet.
So, other than enabling the tanner to produce leather that is strong and flexible, what other implications does collagen's structure hold in store? Well, despite its structure being its strength, it can also be its worst enemy when it comes to leather processing. Until the collagen has been stabilised by tanning, the violent treatment that tanners inflict on the collagen matrix in skin can have a severely detrimental effect unless carefully controlled.
Like all matter, collagen molecules are made from atoms that have either a positive or negative charge. Like the ends of a magnet, negative charges are attracted to positive ones, whereas similar charges repel one another. It is this attraction between negative and positive charges that hold adjacent collagen molecules together. In unprocessed hide or skin, the space between these molecules is filled with water.
Difficulties arise with this when we alter the pH of the skin during processing, eg liming and pickling. Without delving too deeply into the chemistry of the reaction, when we add alkalis to collagen, the atoms in the collagen molecules become more negatively charged. Consequently, instead of attracting one another, the molecules now repel each other, forcing themselves apart.
When the collagen molecules are pushed apart in this way, more water is taken up to fill the now larger gap between them, ie swelling occurs. This is why hides become thick and turgid during liming. Because the molecules have been pushed apart by the swelling effect, they are now much less stable and, as such, much more easily damaged.
Therefore, during processing when hides and skins are at extremes of pH they must not be exposed to high temperatures, otherwise the normally robust collagen will be irreversibly damaged. At liming pH the shrinkage temperature of collagen can be as low 35oC.
When acids are added, the molecules become more positively charged and, again, they repel one another. Different acids and alkalis will cause more or less swelling, eg caustic soda (sodium hydroxide) causes more swelling in collagen than calcium hydroxide (lime).
Generally, the swelling effect of acids is greater than that of alkalis which is why we add some extra negatively charged atoms in the form of salt (sodium chloride) during pickling to negate the effect of the positively charged acid.
Collagen is probably the most important protein of all, without it there would be no higher life forms - and no leather either!
References
1. Leather Technicians Handbook, J H Sharphouse, 1983 revised edition
2. http://www2.mcdaniel.edu/Chemistry/CH3321JPGs/Proteins
3. http://www.imagecontent.com/Lucis/applications/bio/tem1/side/tem1-side.html
