Physico-Chemical Evidence on Structure 



if n were given a value in the neighbourhood of 2-6. This is the 

 equation for the reaction 



Hb + n0 2 = Hb(0 2 )„ 



Such an equation implies the simultaneous combination of one 

 molecule of haemoglobin with n molecules of oxygen. With n = 2-6 

 however the equation was devoid of physical meaning. In answer to 

 this objection it was therefore suggested that haemoglobin existed as 

 a mixture of polymers, in each of which the reaction occurred with the 

 simultaneous acceptance of the full quota of oxygen molecules. The 

 observed value of n therefore represented kind of average degree of 

 polymerization of the monomer containing one haem. These views 

 were put forward before the accurate determinations of molecular 

 weight by osmotic and ultra-centrifugal methods had demonstrated that 

 haemoglobin solutions were monodisperse and that the molecules all 

 contained four haems. With the advent of that discovery the Hill 

 hypothesis was dropped, and equation (6) remains simply as a useful 

 empirical equation giving an approximate description of the facts. 

 Actually it does not describe the best data very accurately ; one trouble 

 being that it yields a symmetrical plot of Y vs log p, whereas the most 

 accurate results show that the curve is not exactly symmetrical. Never- 

 theless there is an important element of truth in the Hill hypothesis, 

 in so far as it implies the existence of an interaction between the haems, 

 though it is wrong to assume the interaction energy to be infinite as 

 would be required to account for the simultaneous acceptance of two 

 or more molecules or oxygen by a single molecule of haemoglobin. 



It should be pointed out here that any reaction occurring in stages 

 by a molecule containing more than one reacting group will be 

 describable with more or less approximation by an equation having 

 the form of (6) with a suitable choice of n, though the approximation 

 will be increasingly poor as n drops below unity. This is so if the 

 reacting groups are unlike and independent, or if, whether alike or 

 unlike, they are subject to destabilizing interactions so that the reaction 

 of one group leads to a decrease in the reactivity of one or more of 

 the others, as in polybasic acids generally. Only if some at least of 

 the groups show a stabilizing interaction will n be greater than unity, 

 and a value of n greater than unity is a sure sign that there is a 

 stabilizing interaction between some at least of the groups. It is in 

 this case that the equation is generally a useful approximation. The 

 equation is of course without direct physical significance, but it is a 

 simple matter to relate the value of n, given by the slope of the curve 

 of Y vspor log/? at its midpoint (Y —- 1) to the various overall constants 



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