The Haem-Globin Linkage 



179 



showed absorption increments on adding caffeine, with Soret peak at 

 392-5 mn and attachment occurring from about pH 6-0, rising to a maximum 

 at pH 7-0, and being unaffected by addition of dithionite. 



Fig. 2. Attachment of haematin and haem to caffeine. Soret absorbance incre- 

 ment curves prepared as described in the text, for haematin + caffeine at 402 m/z 

 • — • — •, and haem + caffeine at 420 m/f O — o — o. (Buffers, pH 4-0-6-2 

 phthalate, pH 6-2-7-5 phosphate, I = 0-05, T = 2rc.) 



Preliminary Experiments with Theophylline 



Theophylline has an unsubstituted imino group and therefore resembles 

 more closely the imidazole ring as it would be expected to occur as the side- 

 chain of proteins. In a single series of experiments with theophylHne, attach- 

 ment of haematin occurred from about pH 5, with a pronounced maximum 

 at pH 6-5 and a minimum at about pH 7-3, but no attachment of haem was 

 detected in this range. At lower hydrogen ion concentration, J. Keilin 

 (1943) also had not found combination of haem with theophylline, though she 

 did with caffeine. 



These experiments with the imidazoles are prehminary; more points 

 should be obtained to plot the increment curves exactly, but if the curves are 

 nearly correct, a mean increase of propionate pA^ value of about 0-9 unit 

 could be indicated on reduction, of the same order as that suggested from the 

 experiments with the albumin compounds. Further studies on the reactions 

 of haematins with imidazoles will be reported later. 



Haem Propionate - Protein Linkages in Adult Horse Haemoglobin and Horse 

 Myoglobin 



Nickel porphyrins would appear to be very useful compounds for the 

 study of the attachment of haem propionate groups to the side-chains of 

 proteins. They would be expected to be of almost identical size and shape 

 to the iron porphyrins. Haurowitz and Klemm (1935) found nickel dimethyl- 

 mesoporphyrin and Pauling and Coryell (1936) found nickel protoporphyrin 

 to be diamagnetic and concluded that these porphyrins contained no unpaired 



