208 KERATIN AND KERATINIZATION 



at 1650 cm -1 reveals a perpendicular dichroism. The NH stretching 

 mode at 3315 cm -1 is also perpendicularly dichroic. By exchanging the 

 H of the OH and NH groups in the accessible (non-crystalline) regions 

 with deuterium, Parker (1955) removed their absorption from these 

 regions and showed that the perpendicular dichroism of the well-oriented 

 region rose to 4*8. 



The absorption bond of the CO stretching frequency has a double peak 

 (Fig. 83) and, on the basis of their empirical rule (see p. 198), Ambrose 

 and Elliott (1952) concluded that this meant that a mixture of a- and 

 j8-forms were present. We have already indicated (p. 198) that this deduction 

 would not necessarily follow from our present-day views of the meaning 

 of these frequency shifts. 



Recently a new examination of the diffraction data has been made by 

 Krimm and Schor (1956) who, while confirming that the structure is of a 

 /?-type, consider that none of the hitherto-proposed arrangements is 

 correct. On the grounds of the difficulty of fitting in all the axial reflections 

 in Tables 10 and 11 as orders of 94*5 A they consider that the true spacing 

 is 2 X 94*5 = 189 A. The 3*07 A spacing usually regarded as meridional 

 is stated by them to be an off-meridional 3*15 A layer line spacing. A 

 meridional reflection at 2*9 A is interpreted as the amino acid repeat (or 

 rise) and there are thus sixty-four residues in the master period (189 A). 



They have proposed a model which might be described as a modified 

 ^S-helix which consists of ten polypeptides, each of sixty-four residues, 

 wrapped helically around a cylinder (Fig. 89). The helices are right- 

 handed and every eighth residue is a prolyl, the side chain being on the 

 inside of the cylinder. The non-proline sequences form a modified 

 j8-pleated sheet. The ten chains aggregate coaxially by hydrogen bonding, 

 the prolyl residues of neighbouring chains coming out at about the same 

 level, and the strong 23*4 A meridional reflection arises from the planes 

 containing these residues (see Fig. 89). The R groups project both inside 

 and outside the cylinder. The particles of molecular weight ^ 10,000 

 isolated by Woodin and which appeared to have no (or very few) end 

 groups, they believe arise by a fission of the cylinder adjacent to the prolyl 

 planes thus producing prolyl end groups which are not estimated by the 

 usual methods. 



A fairly satisfactory prediction of the strong equatorial reflections at 33*5 

 1 1*2 and 55 A (Schor's figures) was obtained by assuming that compound 

 cylinders, each consisting of seven unit cylinders, were placed in hexagonal 

 array. 



It will be seen that the Schor-Krimm model is essentially a return to the 

 idea that the large axial spacings represent distances over which a sequence 

 of amino acids is repeated. In this particular instance the prolyl residues 

 are supposed to recur at intervals of ten residues. What little chemical 



