CuemIcAL STRUCTURE The chemical structure of wool fibre is the chemical structure of wool keratin. The keratin molecule is supposed to be of a polypeptide nature, composed of 15 - 20 amino acids. The amino acids, with one or two exceptions, have common feature. The molecule consists of a carbon atom joined to four different groups, three of which are common to all amino acids. Those are 1) a H-atom 2) an amino group (NH,), i.e., basic in nature 3) Anacid group (COOH). The fourth group represented by R is one which distinguishes one amino acid from another. The angle between each valency bond is approximately 109 (fig.7.1). All the amino acids are condensed with each other by means of one’s NH, group with the COOH group of another. This condensation would join molecules together by a peptide linkage, as shown in fig 9.2. The above figure shows five molecules of amino acids condensed together. They differ from each other in the size and structure of the radical R. The keratin molecule is a long polypeptide chain (NH-CH-CO) with fifteen to twenty different side chains (R). About 50% of the weight of woo! fibres is due to the side chains. AMINO ACID COMPOSITIONS Amount of each amino acid can be determined by boiling wool with alkalis. The relative proportion can be calculated by dividing the amount by molecular weight. The amino acids occur in varying proportions. Table 9.2 indicates different aminoacids with their functions. Ro Ry i i { NH, CO. Cc. NH, co, C. NH, co 7 Not NH [00% 07 NH [00 So | H | H | Ri Re Rs Fig. 9.2 Structure of a polypeptide chain in wool fibreA text book of fibre science and technology Table 9.2. Aminoacids present in wool fibre [S. Functional Name Formula of R Relative No. group of R Proportion %) 1. Hydrocarbon Glycine H 100 2» Alanine cH, 50 3. Valine (CH,),CH 42 a Leucine (CH),CHCH, 87 5 Iso-leucine CH,.CH,.CH.CH, 6 Phenylalanine C,H,,CH, 23 7. Hydroxyl Serine (CH,.OH — Bow Threonine CH,.CH.OH 55 an Tyrosine C,H,.CH,.OH 270 10. Acidic Aspartic acid. CH, COOH 73 i Glutamic acid CH,.CH, COOH 153 12. Basic Arginine 'NH,C(NHNH) 80 (chi, is Lysine ‘NH,CH,), 3.0 a. Histidine CHN,CH, os I) ‘Tryptophan CHINILCH, 06 16. Sulphur Methionine CH, S.CH,CH, 0s | Cystine CHSS.CH, 1 CH(NH COOH 18. 5member ring Proline —_ Wuen R Is HYDROCARBON ‘The first four groups have an increasing order of molecular weight. Phenyl alanine has a benzene ring in its group. Wool is fairly rich in these types of acids than other protein fibres. Due to presence of hydrocarbon side groups, these acids repel polar molecules like water, salts, acids and alkali but attract non-polar molecules like hydrocarbons. ‘When R Is HYDROXYL Three acids are present in this group. Wool is rather rich in these type of amino aeids. All these acics provide polar side groups and attract polar molecules. Wuen Rts acipic Only two acids are present, the amount of acidic group is fairly high in wool.Wuen R Is Basic Wool is rich in arginine. Arginine, Lysine and Histidine being bases, produce basic side groups in the protein chain which can combine with acid or dyestuffs which contain acidic groups in their molecule. Tryoptophon is relatively unimportant. Wuen R CONTAINS SULPHUR Methionine is relatively unimportant as wool contains only a small amount. Cystine has a special significance in the wool chemistry. In comparison, cystine is more in wool. So it makes many important contributions to the properties of wool. PROLINE This is a five member ring having the formula CH, — CH, — CH, | | co ———_—_—— NH This shows that proline is composed of four carbon atoms and one nitrogen atom in its ring. When nitrogen and the carboxyl groups are involved in peptide linkage with other amino acids, this amino acid gives aright angle bend. Thus proline is important because of the effect it gives on the structure of the protein.