120 P. George, J. Beetlestone and J. S. Griffith 



low-spin forms, and in the next section this hypothesis will be put to a 

 quantitative test. 



QUANTITATIVE CORRELATION BETWEEN THE MAGNETIC 

 MOMENTS AND THE SPECTRA OF FERRIHAEMOPROTEIN 



HYDROXIDES 



Making the assumption that the hydroxides are mixtures of high- and low- 

 spin forms, the magnetic moments and extinction coefficients at each wave- 

 length should be interrelated in the following way. Denoting the moments of 

 the high- and low-spin forms by /^^ and i^ii, the moments of, say, ferrimyoglobin 

 and ferrihaemoglobin hydroxide, /^ji,, and /<h,^, are determined by the 

 equations 



/"nil) = /'j "^Jlb + /^A (1 ~ ^Mb) (8) 



f^B.h = ^h^^nh + fhX^ - ^Hb) (9) 



where ai^j and ay^, are the fractions of the low-spin form present in ferri- 

 myoglobin and ferrihaemoglobin hydroxide respectively. The reason why 

 the terms contain /t^ rather than // is that the additive magnetic property is 

 the molar susceptibility, Zjj, which is related to /« through the expression 

 /< = 2-^AVx^iT. At a given wavelength the extinction coefficients of the 

 two hydroxides, e-^^ and s^-^ are also determined by the fractions ol-^^ and 

 a^i) according to the equations 



fiMb = Hoi^i + (1 - ajrb)£/i (10) 



£Hb = ^Hb^J + (1 - ^B.h)^h (1 1) 



where e^^ and e^ are the extinction coefficients of the high- and low-spin 

 forms at this particular wavelength. Since ii^^^, i-ij^y^, e^^b ^^^ ^Hb ^^^ known 

 experimental quantities, and since reasonable values can be adopted for 

 /<;j and //„ these equations can best be used to evaluate f;, and £„ and thus 

 obtain the absorption spectra of the high- and low-spin components. 



However, there are several reasons why calculations of this kind can only 

 be approximate. First, although the high-spin hydroxide can be assigned a 

 magnetic moment of 5-92 B.M., the theoretical spin contribution of five 

 unpaired electrons, the value for the low-spin hydroxide is a matter of choice. 

 Some contribution of orbital angular momentum must be taken into account 

 in view of the values in excess of the spin contribution of one unpaired electron 

 obtained for the cyanide complexes (see below), where the very strong ligand 

 field makes it extremely unlikely that thermal mixtures exist. Secondly, 

 although a value of 5-92 B.M. is equally valid for the high-spin form of both 

 ferrimyoglobin and ferrihaemoglobin hydroxide, the moment of the low-spin 

 form could very well differ by up to 0-5 B.M., since the values obtained for 

 cyanide complexes are 2-35, 2-50, 2-66 and 2-29 B.M. in the case of ferri- 

 myoglobin, ferrihaemoglobin, ferriperoxidase and ferricatalase respectively 



