MOLECULAR AND MACROMOLECULAR STRUCTURE 163 



the terminal residues of the chains. The end with a free — NH 2 is referred 

 to as N-terminal; that with a free COOH as a C-terminal. 



N-terminal groups are determined by reacting the protein with Sanger's 

 reagent dinitrofluorobenzene (DNF) which attaches to the free amino 

 group producing a substituted amino acid easily identified after hydrolysis 

 by its yellow colour. The C-terminal groups are found by reacting the 

 protein with hydrazine (N 2 H 4 ) when the chain residues are converted into 

 hydrazides with the exception of the C-terminal acid. 



Several workers have examined wool and are in good agreement as to 

 which acids form the end groups although estimates of numbers vary. 

 Blackburn (1950) reports the same groups for a N.Z. coarse wool as Kerr 

 and Godin (1959) for human hair and horse hair. The latter refrain from 

 citing their quantitative findings, accepting Thompson's (1957) criticism 

 that these methods are not reliable for materials such as solid keratins. 



Middlebrook (1951) rounded off his figures to give the following twenty- 

 seven N-terminal groups: glycine 8, alanine 2, valine 4, serine 2, threonine 

 8, aspartic acid 1, and glutamic acid 2 in 10 6 g wool keratin. He calculates 

 a chain weight of the order of 60,000 and thus a total molecular weight 

 for " wool keratin " of 27 x 60,000 ~ 1,600,000 assuming homogeneity. 

 Estimates for chain weight from the figures in Table 8 vary from 50,000- 

 90,000 approximately. 



The C-terminal groups of wool were also found by Blackburn and Lee 

 (1954) to be glycine, alanine, serine, threonine, aspartic and glutamic acid. 

 These same six residues were found by Kerr and Godin in hair. Alexander 

 and Smith (1956) have determined the end groups of three fractions a, 

 and y derived from wool oxidized by peracetic acid (see p. 238) and found 

 the same groups in somewhat different proportions. 



The N-terminal end groups of a soluble derivative of feather keratin 

 have been determined by Woodin (1954a and b, 1955 and 1956). The 

 same groups as for wool and hair were found in very small quantities 

 indicating a deficiency of N-terminal groups which Woodin thinks may 

 mean that feather keratin is a cyclic polypeptide. Krimm and Schor 

 (Schor, 1958), who favour a much larger unit (see p. 208) than Woodin's 

 monomer (m.w. 10,000), think that in solublizing his feather, he hydrolyses 

 the peptide bonds linking the frequently-occurring proline residues and 

 thus produces shorter chains with proline end groups which are not then 

 detected by the procedure used. 



Molecular structure 



Methods of Partial Degradation 



Amino acid analyses and other analytical procedures show that keratin 

 consists of polypeptides and that, if constituents other than amino acids 

 are present, they are there in very small quantities. Methods of partial 



