HEMATIN COMPOUNDS IN PHOTOMETABOLISM 161 



of the hematin compound by inductive resonance can occur. The 

 physical disposition of the cytochrome and other hematin com- 

 ponents of the chloroplast is not known, but from the studies of vari- 

 ous investigators (see, for instance, ref. 41) it appears that the chloro- 

 plasts consist of laminae in which fatty layers alternate with the 

 aqueous phase. In the interface, the chlorophyll molecules lie close- 

 packed on end, oriented so that the phytol chain dips into the lipid 

 phase and the porphyrin-polar end binds to a protein in the aqueous 

 phase. The hematin compounds would be expected to bind to protein 

 in the aqueous phase, perhaps to the same protein as that which holds 

 the chlorophyll. Thus there could be a bridge of hematin compounds 

 connecting the chlorophyll-lipoprotein with the systems directly 

 contiguous to the chloroplast w^hich are involved in the secondary 

 processes of CO2 reduction, etc. Light energy absorbed by a given 

 chlorophyll molecule would be likely to migrate through the interface 

 until it found a cluster of hematin compounds, whereupon the whole 

 energy of the quantum would become available to the hematin 

 compounds. It is not difficult to imagine that absorption of the 

 relatively large energy equivalent to one quantum would excite the 

 heme protein to a state in which a dismutation reaction resulted. 

 The result would be the production of partially dissociated ferroheme 

 with one or more free Fe valences which could reduce H acceptors 

 such as DPN at or close to the potential of the hydrogen electrode. 

 After reduction, the ferriheme could "relax" into its original bound 

 state with an "electron deficient region" left in the hydration en- 

 velope of the hematin compound. The continuation of this process 

 would accumulate electron-deficient compounds in the phase sur- 

 rounding the protein which could act as oxidants for back reactions 

 and also as precursors of free oxygen. A fragment of such a mechanism 

 is part of a recent suggestion by Warburg invoking a "photo-dissocia- 

 tion" of a heavy-metal binding compound coupled with a back oxida- 

 tion of the dissociation products (39). 



Acknowledgment. In conclusion, it is a pleasure to acknowledge the 

 continued support of the C. F. Kettering Foundation, which in large 

 part has made possible many of the studies which provide the frame- 

 work for this report. 



