62 THE BACTERIAL PHOTOCHEMICAL APPARATUS 



nesium and iron chelates, functions which are readily apparent, such 

 as photosensitization for the chlorophylls (magnesium chelates) and 

 oxidation catalysis for heme proteins (iron chelates), are familiar to 

 all of us. 



The structural classification of heme proteins must be made on the 

 basis of imperfectly understood chemical properties and behavior, as 

 deduced from magneto- chemical, spectrochemical, and physical or- 

 ganic studies on the proteins and on model compounds. A simple 

 summary of present classification schemes can be presented, based 

 on four generally accepted criteria: 



(1) The basic chelate structure. This is the beta- substituted tetra- 

 pyrrole structure, associated with type III porphyrins. Three possible 

 variations occur. The first, and most frequent, is based on the por- 

 phyrin skeleton, in which two of the pyrrole moieties possess double 

 bonds which do not participate in the conjugated macrocyclic resonance 

 system. This we can call the "P" class. The second is the dihydro- 

 porphyrin, or chlorin, in which one of these double bonds is removed 

 by hydrogenation. This occurs primarily among the magnesium chel- 

 ates (chlorophylls), but one example is known in the heme proteins— 

 namely, the D-type cytochrome (4) (5), formerly called "cytochrome 

 a^-" This we can call the "D" class. Thirdly, we must entertain the 

 possibility that the tetrahydroporphyrin structure, in which no non- 

 participating double bonds are left, may also be found to occur for 

 heme chelates, as it does for the magnesium chelates (bacteriochloro- 

 phyll). This gives a third class, which we can term "T." 



For the present, we can assume that general structures, other than 

 type III, will not occur. One contribution from the study of bacterial 

 cytochromes is already apparent in that we are prepared to accept the 

 possibilities of classes "D" and "T." 



(2) Ligand interactions. The special effects created by the intro- 

 duction of ligand groups from the protein into the extraplanar positions 

 of the tetrapyrrole ring cannot be defined completely on the basis of 

 any single chemical criterion, but the magnetochemical behavior is as 

 adequate as any. This criterion leads to specification of three sub- 

 classes for each of the main classes established by criterion 1, 

 These are "high spin" (H), "low spin" (L), and "mixed spin" (M). The 

 M subclass is actually derivative from the other two because it is 

 probable that no absolutely pure H or L state exists. However, as a 

 practical matter, the totality of heme proteins known at present falls 

 easily into three such classes. 



(3) Binding type. The chelate may be bound to the protein by simple 

 acid-base linkages which may be split easily by the usual acid- acetone 

 treatment; or it may be bound by covalent linkages (thioether, ester, 

 carbon-carbon bonds, etc.), which resist such treatment. These two 

 cases can be distinguished by the letters "F" ("free") and "C" ("cova- 

 lent"). 



