E. M. JOPE 



visible, is due to the haem group (Figure 1). These two absorbing 

 parts of the molecule exert their effects in a large measure independently 

 of each other. 



JO 20 



v'-H. (cm x IO' J )'' 



200 



250 



300 400 500 1000 



A (m/i) 



Figure 1. The ultraviolet and visible spectral 

 absorptions of haemoglobins. 



The band system due to the haem groups is responsible for the 

 colour of haemoglobins, and consists of several bands in the visible, of 

 molar extinctions of the order 10 to 20 x 10 3 *, and a very intense 

 band of molar extinction of the order 100 to 200 x 10 3 in the extreme 

 violet at about 400 to 425 mpi. This is called the y, or Soret band 1 , 

 and is an essential property of the 16-membered porphyrin ring of 

 conjugated double bonds. Both the visible and Soret bands depend 

 in wavelength position upon the valency of the central iron atom of 

 the haem, on combination with O a or CO, and on changes in the 

 double bond systems of the side-chains conjugated to the 16-membered 

 central ring, as for instance in protoporphyrin with its vinyl side 

 chains, and mesoporphyrin with its ethyl side chains in those positions 2 . 

 The bands due to the haem are also dependent to some extent upon 



* For convenience in comparisons molar extinctions of haem — protein compounds are 

 usually stated in terms of molecular units containing 1 haem group and 1 atom of Fe, 

 usually associated with a protein unit of molecular weight about 17,000. 



206 



