KINETICS OF OXYGEN AND CARBON MONOXIDE REACTIONS 289 



reactions are explained by the low heats of activation rather than by the 

 entropies of activation, since the latter is small in the case of carbon monoxide 

 reaction and negative in the case of the reaction with oxygen. He suggests 

 that the negative entropy of activation of the association reaction with 

 oxygen may be due to changes in the protein absent in the carbon monoxide 

 reaction. If this is .so, such changes apparently do not include changes in 

 the ionization of the protein, since pH affects both reactions to approxi- 

 mately the same extent. 



7.3.3. Spectroscopic Differences. The similarity between the type 

 of spectrum of carboxy- and of oxyhemoglobin in the visible region 

 (due to the fact that in both iron bonds are covalent) should not divert 

 attention from the differences which exist in the infrared absorption 

 spectrum. 



In contrast to oxyhemoglobin, carboxyhemoglobin is photosensitive. 

 Photosensitivity of complexes containing carbon monoxide is not peculiar, 

 however, to the compound which this molecule forms with heme and its 

 derivatives; it is a rather general property of iron-carbon monoxide com- 

 pounds (r/., e.g., 012). 



So far the investigation of the properties of the carbon monoxide-iron 

 bond has not thrown light on this problem, or on the related problem of the 

 effect of the protein on the quantum yield of the reaction. While in some 

 of these compounds the absorption of one quantum of light of the appropriate 

 wavelength is sufficient to dissociate the carbon monoxide complex, Bucher 

 and Xegelein (S7^) have recently claimed that the number of quanta required 

 to produce dissociation depends on interaction between the hemes in the 

 molecule. While one quantum was able to dissociate myocarboxyhemoglobin, 

 two and four quanta, respectively, were required to produce dissociation of 

 one mole cf carb n monoxide from carboxyhemoglobin under conditions 

 when it contained two and four hemes per molecule. 



In emphasizing the differences between oxy- and carboxyhemo- 

 globin Holden (1317) has recently even made the tentative suggestion 

 that the oxygen and the carbon monoxide molecules are attached to 

 different portions of the hemoglobin molecule. This idea could be 

 criticized on a number of grounds. 



It would, for example, be extremely difficult to interpret the .similarity of 

 behavior of the heme-linked proton dissociation groups in the two compounds 

 if the gas molecules were not combined at the same position. In addition 

 the work of Barcroffe and collaborators (64-) on the relation between band 

 position and affinity for oxygen and carbon monoxide provides further data 

 which make Holdens suggestion unlikely. 



In the course of their investigations of the hemoglobins of a number of 

 species Barcroft and co-workers found that the shift of the a band when 

 oxygen was replaced by carbon monoxide was linearly related to the logarithm 



