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



kind used in Section IV to calculate the individual spectra of the high- and 

 low-spin hydroxides, was considered. Since azide gives a high-spin complex 

 with ferricatalase and a low-spin complex with ferrihaemoglobin, it is quite 

 likely that the ferrimyoglobin derivative would contain a significant fraction 

 of the high-spin form. Preliminary calculations tend to confirm this. Never- 

 theless, until temperature variations of the magnetic moment and absorption 

 spectra have furnished direct experimental evidence for the existence of a 

 thermal mixture, as in the case of ferrimyoglobin hydroxide, it is perhaps 

 better to leave it as an open question with regard to the other derivatives. 



Acknowledgements 



The work reported above forms part of a research programme on haemo- 

 proteins supported by grants from the National Science Foundation (G2309 

 and G7657). We wish to thank Dr. Britton Chance for making the facilities 

 of the Department of Biophysics and Physical Biochemistry available to 

 carry out the spectrophotometric study described in Section V. 



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