628 



Rufus Lumry 



chlorophyll and a more direct connection between the particular confornnation 

 of the chlorophyll porphyrin tr system and the electronic properties of the 

 nnagnesium ion which depend on p orbitals may appear. It will be interesting 

 to see what the Faraday effect is in the several bands of chlorophyll. Con- 

 ceivably one of these bands contains an admixture of a charge transfer process 

 in which an electron nnoves from the porphvrin ir system to magnesium ion. 

 According to the analysis of Witt, et al. , ^^^' the P700 pignaent is photo - 

 oxidized on illximination and magnesium could be the catalyst for this process. 



This kind of consideration is still based on too few observations to allow us 

 to eliminate porphyrin distortion as important in determining the electronic 

 properties of henne iron. Nevertheless it does concentrate our attention on the 

 fifth -position and sixth -position ligands of the iron complex as being primarily 

 responsible for determining this electronic state. Thus aside from porphyrin 

 effects the oxidation -reduction potential of cytochromes is not determined sol- 

 ey by the nature of the fifth- and sixth -position ligands, but also by the posi- 

 tioning of these ligands achieved in the tug of war between the directed valence 

 of the 3d orbitals of iron and the tendency of the protein to fold to a state of 

 lowest free energy. We have carried out several experiments to show that the 

 oxidation-reduction potential and paramagnetic susceptibility of iron can be. ,_ 

 altered by alteration of the protein without change in nature of ligands; ' ' 

 and there is little reason to doubt that this is due to the distortion mechanism 

 which thus plays an important role in establishing the necessary electronic 

 properties for this particular physiological reaction. The mechanisnn also 

 explains how the electronic properties of iron can be mutationally altered to 

 become more suitable for evolving organism since changes of amino acid re- 

 sidues at key positions can alter the positions of the fifth and sixth ligands and 

 thus alter the functional electronic properties of the iron atom. ^^^> 1°) 

 Electronic evolution is thus seen to be no more complicated than morphologi- 

 cal evolution since it is based on the same mechanism. Experiments with 

 chemical modification of single side -chains or pairs of side chains in chymo- 

 trypsin' °' convince us that the rate parameters of enzymes are evolved to 

 their high levels of efficiency in the same way. Similarly the varying affin- 

 ities of hemoglobins from different organisms for oxygen appear to be estab- 

 lished in this way. ( ^ ^ ) 



We have been concerned with the possibility that dramatic changes in the 

 folding of cytochrome c nnight occur on reduction of iron. Such changes 

 might be expected either as a result of the elimination of the charge on heme 

 iron or as a result of ligand reorientation since either factor could upset the 

 order of conformational stability of the possible folded forms. We have men- 

 tioned that such refolding or shifts in folding might explain the shifts in the 

 peaks of the visible spectrum. Reasonably satisfactory evidence, particularly 

 from Okunuki' s group, ^ °' shows that there is a difference in character of the 

 protein in the sense that one form is much more resistant to proteolytic 

 enzyme attack and to denaturation. However, we have been unable to provide 

 definite evidence from viscosity experiments that any major change in size or 

 shape occurs.' ' The viscosity experiments are, however, complicated and 

 we have recently turned to dielectric -dispersion experiments. It has been 

 possible to eliminate electroviscous effects which have made the method un- 

 reliable heretofore, and under such circumstances Yue has found that the 

 oxidized and reduced forms of cytochrome c are very similar. The differ- 

 ences are small and about the magnitude expected from sedimentation experi- 

 ments. ^20) It is probable that there are differences in the proteins but thexe 

 can be no large change in volume or shape. Somewhat similar conclusions 

 can be reached with hemoglobin under physiological salt concentrations. A 



