198 KERATIN AND KERATINIZATION 



ester (normally giving two bands at 1658 cm" 1 and 1628 cm" 1 ) is cast from 

 formic acid, a pure /3-form results giving only a band at 1629 cm -1 . 



Generalizing from these observations the Courtauld group proposed 

 that as a simple test an a-configuration could be diagnosed, for example, 

 by a carbonyl stretching frequency round about 1665-1660 cm -1 as 

 opposed to the corresponding bond of the /^-configuration at about 1630 -1 . 

 Not all authorities accepted this test (Darmon and Sutherland, 1949; 

 Sutherland, 1952) and recent reports by Elliott, Hanby and Malcolm 

 (1954, 1956 and 1958) show that these workers themselves have abandoned 

 its rigid application. 



Table 12. Wave-Numbers (cm -1 ) of Infra-Red 

 Absorption Bonds in a- and /^-Configurations. 



CO stretching 

 NH deformation 

 NH stretching 



a-configuration 



1650-1660 

 1540-1550 

 3290-3305 



/^-configuration 



1630 (J.)* 

 1520-1526 (||) 

 3287-3301 (J_) 



* The dichroic sense in the jS-form is indicated by the symbols (]|) 

 parallel and ( J_) perpendicular to the fibre axis. 



Recently developments (p. 194) have shown that a-helices can be 

 diagnosed more certainly by observations on the dispersion of the optical 

 rotation, and Elliott, Hanby and Malcolm, having measured the infra- 

 red absorption spectra and the optical rotation of several polypep- 

 tides and protein films, have come to the conclusion that the carbonyl bond 

 near 1660 cm -1 is not necessarily associated with the a-helix. They 

 describe several polypeptide preparations (alkaline salts of poly-L-glutamic 

 acid and copolymers of L- and D-polyalanine) and a soluble fibroin in 

 which the dispersion of the optical rotation shows conclusively that 

 a-helices are absent, but which also have the carbonyl absorption bond 

 near 1660 cm -1 earlier proposed as a test for the presence of the a-form. 

 The present position would seem to be that the /^-configuration shows a 

 characteristic absorption at 1630 cm -1 and that almost any departure from 

 this ranging from an a-helix to a random coil may show a bond at 1660 

 cm -1 . 



The spectra of proteins of course show many other bands to be expected 

 from our knowledge of chemical composition of the proteins, but identi- 

 fication is not always easy (Bellamy, 1954). The measurement of dichroic 

 ratios is valuable in excluding some proposed structures. The dichroic 

 ratios found are small and, when these are found in specimens giving a 



