NON-PROTEINOUS SKIN COMPONENTS
Glycosaminoglycans:
They are typical polyelectrolytes of cellular and exracellular organic fluids. They control the viscosity of those fluids, act as buffers in tissue, participate in transport of ions and influence the water economy of the system due to their hygroscopicity.
Soaking and liming of skin are probably controlled by function of glycoaminoglycans. These substances have a characteristic skeleton of molecules typical for carbohydrates , functional groups such as -NH+,-COO- and -SO3- and their specific distribution.
From physiochemical point of view , glycosaminoglycans are representatives of polyelectrolytes ; ie polymers in which ionizing functional groups are
(-CH2-CH-)n
I
SO3 –H+
In glycoaminoglycans monosaccharide molecules occur bound by a and b glycoside bonds . This structure gives the molecule some stiffness . The only possibility of rotation of the molecule is around these bonds. In the compounds discussed, many intramolecular and inter molecular H-bonds occur. Glycosaminoglycans usally occur in extra cellular spaces where they fulfill a structural function imparting plumpress and flexibility to animal tissue .
Glycosaminoglycans are polysaccharides of animal origin which contain hexosamines. These compounds usually form complexes with proteins (mucoids).
It seems that proteins are bonded to saccharide part through sugar hydroxylic groups as well as to serine and tryptophane side chain hydroxyles . The group of glycosaminoglycans includes both acidic and neutral polysaccharides .
All glycosaminoglycans of animal origin have in common the fundamental hyalobiuronic acid link composed of the D-glucuronic acid residue connected with 2-deoxy-D-glucose by a 1,3-b-glycoside bond . The residues of hyalobiuronic acid are connected in a chain by 1,4-b-glycoside bonds .
Characteristic substituents of connective tissue glycosaminoglycans:
| Glycosaminoglycans | T | U | V | W | X | Y | Z |
| Chondroitin | H | COOH | OH | H | OH | OH | NHCOCH3 |
| Chondroitin sulfate A | H | COOH | OH | H | OSO3 – | OH | NHCOCH3 |
| Chondroitin sulfate C | H | COOH | OH | H | OH | OSO3 – | NHCOCH3 |
| Keratan sulfate | H | CH2OSO3 – | NHCOCH3 | H | OH | OSO3 – | NHCOCH3 |
| Hyaluronic acid | H | COOH | OH | OH | H | OH | OH |
| Chondroitin sulfate B (Heparine, Dermatan sulfate) |
CH2OH | H | OH | H | OSO3 | OH | NHCOCH3 |
The hyalouronic acid molecule is long , nonbranching polysaccharide chain with a considerable degree of hydration .The hydrodynamic volume of the molecule ie. the volume occupied by it in an aqueous solution, is almost twice as large as the real one .
For leather producing operations , especially for soaking and liming , the most important is its interaction with water. Important from the techonological point of view are some glycosaminoglycans of plant origin , which are applied as thickening agents in leather finishing.Pectins, derivatives of a-D-galactopyranosyluronic acid esterified in various extents , are biopolymers which occur frequently . Pectins are components of cell sap, cell walls and intercellular spaces in higher plants .
We do not know exactly the behaviour of glycosaminoglycans in leather making as it has not yet been experimented .
Fats:
From the point of view of tanning chemistry , fat in the skin is a component giving it flexibility, softness and stability . Natural fat is removed from skin in leather making processes , then it is necessary to apply fat to it in the finishing process .
A significant amount of fat in the raw skin makes their processing difficult, because hydrophobic spaces are then formed , repelling water during soaking and because insoluble calcium soaps are formed during liming . Raw skins containing much fat have to be degreased before processing .
Inorganic components and their significance:
Lyotropic Hofmeister series:
Hofmeister found that cations and anions can be arranged according to their influence on protein solubility . This regularity is observed in many cases, not only as related to protein solubility. Such series can also be set up with respect to other properties such as solution viscosity, electrophoretic mobility, enzymatic reactions and others.
Anionic series:
citrate > tatrate > sulfate > acetate > Cl– > NO3– > Br– > I – > CNS–
Cationic series (the order is less definite):
Al+3>H+>Ba+2>Sr+2 >Ca+2>K+>Na+>Li+
The reason for such an order is the intensity of electrostatic field around the ions; small ions have more intensive fields than large ions of same valency . The intensity of the field of small ions is the reason of greater hydration , which is an immediate reason of ordering .
The tanner should remember that the ability of particular ions to solubilize proteins is equal to their peptidizing ability in leather making . This rule is important in soaking , liming , and bating as a part of non-collagenous proteins becomes dissolved in a process which is parallel to softening and swelling, if the ionic strength and the kind of ions are appropriate. Peptidizing in this case is not equivalent to dissolving only: In this process a part of the weaker peptide bonds is split and the native proteins are thus converted into peptides with smaller molecules, which are easier soluble. This is due to the properties of the ions introduced.
