Cytochromes Cooled in Luinid Nitrogen 455 



greatest interest. Since at present interpretation of the absorption bands of 

 haemoproteins is sketchy, any discussion of factors influencing these bands 

 must be largely general, by analogy with simpler systems. The appearance 

 of an absorption band such as the c\^-b?ind of heart muscle cytochrome c 

 probably represents the resolution of a vibrational component of the c^^- 

 band which may be supposed to be a single electronic transition. This and 

 other vibrational bands are not apparent at higher temperatures because, 

 as the bands broaden with the increased temperature, they overlap and 

 obscure all fine structure. The question of how various factors influence the 

 resolution, i.e. the narrowing, of an absorption band such as c^^ can only be 

 surmised. Different solvents or solutes that can interact slightly with the 

 haemoprotein molecule may affect the rate of energy loss from the vibrational 

 levels, and hence narrow or broaden the absorption lines corresponding to 

 excitation of the system to these levels. The viscosity of the medium is an 

 important factor. Changes in solvent-bonding may affect the lifetime of the 

 vibrational states. Adsorption of the molecule on crystals of the solvent or 

 ice surfaces (cracks) may, by distorting the molecule, change the degree to 

 which both weak electronic transitions and vibrational transitions are 

 forbidden. The observation that a loss of c\<^ accompanies the loss in enzymic 

 activity resulting from a modification of the protein, implies that these bands 

 are profoundly influenced by the associated protein structure surrounding the 

 haemin molecule. The modification of the protein introduces a perturbation 

 resulting in the change of planarity or symmetry of the ring. These changes 

 may be so small, however, that they could not be detected by chemical 

 analysis. 



Of interest is the shift in absorption maxima of the a- and /)-bands observed 

 with cytochrome c,^ with the retention of an associated fine band structure 

 closely resembling that seen with heart muscle cytochrome c. The somewhat 

 lower electronic energy levels observed in this instance, yet the appearance of 

 vibrational transitions the same distance apart, may indicate a small change 

 in a substitutent location resulting in the change of an electronic energy 

 level. This might be accomplished by a change in the arrangement of the 

 associated side chains on the porphyrin ring. 



As our understanding of the factors influencing the spectral characteristics 

 of less complicated molecules such as porphyrins and haemins increases, and 

 our knowledge of the chemistry of haemoproteins such as cytochrome c 

 increases, we should be able to explain better the changes observed in spectral 

 properties under a variety of conditions. In addition studies which are 

 planned at liquid helium temperatures, giving better resolution of the associ- 

 ated absorption bands, may establish more precise data for accurate spectral 

 analysis. The technique as described, therefore, is best applied to resolve 

 absorption bands of a mixture of pigments in studies of cytochrome inter- 

 action in various particulate materials. 



