290 MARTIN D. KAMEN 



producing systems may be a consequence of the need to funnel more than 

 one exciton to a given reaction site to produce molecular oxygen. It is 

 reasonable to suppose that in a process involving multiple electron dona- 

 tion, as in the production of molecular oxygen, a mechanism for delivering 

 the energy of more than one quantum to an active site may be required. 



Perhaps the puzzling inability of the green sulphur bacteria to produce 

 molecular oxygen, despite their utilization of quanta with energies as high 

 as those absorbed effectively by green plants, is owing to relatively low 

 chlorophyll content. 



Returning to haem protein function in photosynthesis, the failure to 

 observe shifts in spectra in the chloroplast upon illumination which can 

 be interpreted as oxidation of haem, can be rationalized on the basis of the 

 reaction scheme of Fig. i. The spectroscopic methods employed at present 

 permit only observation of changes associated with the ferrous to ferric 

 transition. Transitions from ferric haem to ferryl or pentavalent iron haem 

 do not involve changes in characteristic maxima in difference spectra 

 which are sufficient to allow detection by present procedures. If the cyto- 

 chrome f (chloroplast cytochrome c) is in its ferric state to begin with, 

 then the photo-oxidation may proceed to the higher valence state of iron, 

 required for generation of the system which oxidizes water, without being 

 accompanied by a visible shift in absorption. 



Leaving sheer speculation for the more solid ground of physical 

 chemistry, it should be emphasized that our knowledge of the chemical 

 potentialities of haem proteins is limited ; it is derived solely from studies 

 of specimens obtained from a restricted set of unique biochemical struc- 

 tures — the mitochondrial respiratory systems. As discussed elsewhere [60] 

 haem proteins derived from a variety of bacterial and plant sources, where 

 metabolism is in no way associated with obligatory reduction of oxygen, 

 exhibit a great diversity of physico-chemical properties quite unexpected 

 on the basis of the classical cytochrome preparations. There is a great 

 urgency to isolate in pure form in sufficient quantities as many of these 

 haem proteins as possible to enable intensive chemical studies. 



As an example. I may cite the unusual haem protein we know as 

 "RHP", which appears to be present only in the purple photosynthetic 

 bacteria [19]. R. J. P. Williams has presented some elegant studies on 

 haem models from which he has been able to make some remarkable 

 correlations between oxidizing potential, spectra, magnetic proper- 

 ties, and haem binding and structure in the haem proteins [61]. RHP 

 represents a class of haem protein, hitherto unknown, which can be 

 rationalized in the Williams scheme, provided one of the ligands in the 

 co-ordination position out of the porphyrin plane is a group with a rela- 

 tively high proton affinity (e.g. carboxyl, hydroxyl, etc.). RHP is a myo- 

 haematin protein with a typical myoglobin-like spectrum and electro- 



