270 VI. HEMOGLOBIN 



5.1.8. Hemoglobin-Hemiglobin Equilibrium. The relation be- 

 tween h and a has also found application to this system. Modern 

 work on the equilibrium commences with the work of Conant and 

 collaborators (479), who showed that the system was thermodynam- 

 ically reversible. Subsequent work has measured the equilibrium 

 under various conditions of pll, temperature, and ionic strength 

 {171477 ,479,1187 ,1190, 2 U6, 27 50, 27 51) and in solutions of urea 

 (2750), and in addition has investigated the related equilibrium 

 between niyohemoglobin and myoheniiglobin (2753). The system is 

 electromotively sluggish, but stable potentials are reached on the 

 addition of electroactive substances. The value of E„ at 30° C. and 

 2?H 7 lies between + 0.144 and + 0.152 v. E'JpYi = between pH 

 5 and 6, while, at pH 8 to 9, the slope is 0.06. 



Difficulties are presented, however, by the interpretation of the electrode 

 equation Eh = E'o + RT/nF In (Hi/Hb). The value of n obtained from 

 the experimental results lay between 1 and 'i, and the equation could there- 

 fore be interpreted neither as a one-step oxidation of one iron atom nor on 

 the basis of the oxidation of hemoglobin of molecular weight 68,000 containing 

 four iron atoms. Conant {4"^!) pointed out the similarity of this problem to 

 the problem of the equilibrium between oxygen and hemoglobin, and sug- 

 gested that the interpretation might be based on the existence of intermediate 

 compounds between Hhi and Hii. 



Coryell {40S) reanalyzed the data on the equilibrium from the point of 

 view of heme-heme interaction. He showed tliat the electrode equation was 

 capable of interpretation in the same way as was the sigmoid coefficient in 

 Hill's equation, and ol)tained values of a: varying from 1.9 to 5.0, correspond- 

 ing to a variation in the interaction energy from 1.500 to 3800 cal. per mole. 



The causes for the variations of the sigmoid coefficient have not as yet 

 been specifically investigated. It would be of interest to see if the interaction 

 constant for the hemoglobin-hem/globin equilibrium is sensitive to changes 

 in the method of preparation of the pigment, or to specific ion effects, in the 

 same way as is the equilibrium l)etween oxygen and hemoglobin (cf. 4-4-) • 

 The value for £o which Taylor and Hastings (2751) obtain on horse hemo- 

 globin purified by recrystallization agrees excellently with those which Have- 

 mann (US'?) obtained on the pigment from the same species after purification 

 by electrodialysis. The former workers, however, find that the value of n 

 increases from 1 to '-2 as the oxidation proceeds {27o0,2751), while Havemann 

 finds a symmetrical curve throughout. 



Taylor {27 oO) has recently investigated the hemoglobin-hemiglobin system 

 when the pigments of horse and dog are dissolved in 4 M urea. £o is found 

 to be -f 0.108 V. at 30° C, while n has a value of 2 throughout the oxidation. 

 Under these conditions horse hemoglobin is known to be split into half 

 molecules. Taylor and Morgan (2753) found that the £[ at pH 7.0 and 

 30° C. for horse myohemoglobin was -|- 0.046 v., and that n equalled 1, the 

 value expected for a system containing only one heme. The system was 



