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1Collagen and Skin Structure
In order to understand the principles that underpin the conversion of hide or skin to leather, it is necessary to know the fundamental structure of the raw material and how that structure might be modified chemically. The chemistry of collagen defines not only the sequencing of its amino acid constituents but also the physical nature of its structure and how it creates levels of structure or a hierarchy. This depends on the chains creating a triple helix as the basic unit of structure. The chemical properties of collagen are defined by the sidechains on the helices, which may be charged, depending on the pH; in this way, collagen can undergo a wide range of covalent or electrostatic reactions, which are the basis for tanning processes. At the heart of the chemistry of collagen is the relationship with water, which is an integral feature of structure: the supramolecular matrix of water around the triple helices provides the region for chemical modification, leading to tanning technology.
1.1 Introduction
At the heart of the leather-making process is the raw material, hides and skins. As the largest organ of the body of mammals, the skin is a complex structure, providing protection against the environment and affording temperature control, but it is also strong enough to retain, for example, the insides of a 1 tonne cow. Skin is primarily composed of the protein collagen and it is the inherent properties and potential for chemical modification of this protein that offer the tanner the opportunity to make a desirable product from an unappealing starting material. It is part of the tanner's job and skill to purify this starting material, allowing it to be converted into a product that is both desirable and useful in modern life.
Collagen is a generic name for a family of at least 28 distinct collagen types, each serving different functions in animals, importantly as connective tissues. 1,4 The major component of skin is type I collagen – hence, unless specified otherwise, here the term ‘collagen’ will always refer to type I collagen. Other collagens do feature in leather making, however, and their roles are defined later.
Collagens are proteins, that is, they are made up of amino acids. They can be separated into the α-amino acids and the β-amino acids, as shown in Figure 1.1. Each one features a terminal amino group and a terminal carboxyl group, which become involved in the peptide link (see later), and a sidechain attached to the methylene group in the centre of the molecule. When the amino acids are linked together to form proteins, they create an axis or ‘backbone’ to the polymer, from which the sidechains extend. It is the content and distribution of the sidechains that determine most of the properties of any protein. In the case of collagen, it is the sidechains that largely define its reactivity and its ability to be modified by the stabilising reactions of tanning. In addition, the chemistry of the backbone, defined by the peptide links, offers different reaction sites that can be exploited in some tanning processes.
Figure 1.1 Amino acid structures, α and β.
All the common amino acids are found in skin or skin components. There are two notable aspects of the amino acid content of collagen. Hydroxyproline, represented in Figure 1.1, is almost uniquely present in collagen compared with other proteins, therefore offering the basis of measuring the collagen content in any skin or skin derivative. Tryptophan, shown in Figure 1.2, is absent, therefore making collagen deficient as a foodstuff.
Figure 1.2 Tryptophan.
In terms of leather making, some amino acids are more important than others, since they play defined roles, set out in Table 1.1: the roles of importance are either in creating the fibrous structure or involvement in the processing reactions for protein modification. Other amino acids, not included in the table, are important in defining the properties of the collagen, but play less defined roles in the leather-making processes.
Table 1.1 Amino acids of importance in leather making
| Name | Abbreviation | Type | Sidechain: R = | Importance in leather making |
| Glycine | Gly | α, neutral | –H | Collagen structure |
| Alanine | Ala | α, neutral | –CH3 | Hydrophobic bonding |
| Valine | Val | α, neutral | –CH(CH3)2 | Hydrophobic bonding |
| Leucine | Leu | α, neutral | –CH2CH(CH3)2 | Hydrophobic bonding |
| Isoleucine | Ileu | α, neutral | CH3CH2CH(CH3) | Hydrophobic bonding |
| Phenylalanine | Phe | α, neutral | –CH2C6H5 | Hydrophobic bonding |
| Serine | Ser | α, neutral | –CH2OH | Unhairing |
| Cysteine | CySH | α, neutral, S containing | –CH2SH | Unhairing |
| Cystine | CyS–SCy | α, neutral, S containing | –CH2SSCH2– | Unhairing |
| Aspartic acid | Asp | α, acidic | –CH2CO2H | Isoelectric point (IEP),a mineral tanning |
| Asparagine | Asn | α, neutral | –CH2CONH2 | IEP |
| Glutamic acid | Glu | α, acidic | –(CH2)2CO2H | IEP, mineral tanning |
| Glutamine | Gln | α, neutral | –(CH2)2CONH2 | IEP |
| Arginine | Arg | α, basic | –(CH2)3NHC(NH)NH2 | IEP |
| Lysine | Lys | α, basic | –(CH2)4NH2 | IEP, aldehydic tanning, dyeing, lubrication |
| Histidine | His | α, basic | | Aldehydic tanning, dyeing, lubrication |
| Proline | Pro | β, neutral | See Figure 1.1 | Collagen structure |
| Hydroxyproline | Hypro | β, neutral | See Figure 1.1 | Collagen structure, hydrogen bonding |
Amino acids create macromolecules, proteins such as collagen, by reacting
via a condensation process as shown in the following equation, where the amide or peptide link is in bold:
The condensation reaction, removing the elements of water, can be reversed by hydrolysis, by adding the elements of water. Clearly, hydrolysis, as set out in this equation, cannot be fast, nor does the equilibrium lie to the left, otherwise the protein would be unstable and useless as the basis of life. On the other hand, the hydrolysis reaction is catalysed by general acid and general base; importantly for leather making, it is catalysed by H+ and OH−. The impact on process...
