484 LIGHT AND LIFE 



reduced porphyrin ring — is chelated to magnesium or iron as the 

 central metal ion. Let us suppose that a magnesium chlorin (as in 

 chlorophyll) lies close to a haem iron (as in haem protein) . If the 

 magnesium chlorin is excited by absorption of a photon in the 

 characteristic red band, it will reach a singlet (or triplet) excited 

 state with an energy equivalent ^1.8 e.v. above the ground state. 

 We can expect that de-excitation can proceed by electron transfer 

 from the neighboring haem. In this process there will be produced 

 a partially reduced chlorophyll, which can be represented as a "semi- 

 chlorinogen," and a partially oxidized haem. If the iron complex is 

 originally one in which the metal is formally in the (III) state of 

 oxidation, the process can be written as in Fig. 1, in which only the 

 essential portions of the ring structure are shown. 



This process most likely yields both products in their ground 

 states. The energy difference can be expected to be close to the 

 original energy of excitation of the chlorophyll. Hence, in this sys- 

 tem, a potential difference can be considered to exist in which the 

 oxidized form of haem is -~1.8 e.v. more oxidizing than the semi- 

 chlorinogen radical. However, stabilization of the products suffi- 

 cient to prevent rapid back-reactions may require 0.2-0.3 e.v., so 

 that perhaps -~1.5 e.v. is the maximum spread in potential. 



For the present, it is possible to infer consequences of such a process 

 only by analogy because nothing is known directly about the solution 

 chemistry of the higher oxidation states of iron, either as the hy- 

 drated ion or as a chelate. George and Irvine (10) have shown that 

 metmyoglobin treated by a variety of oxidizing agents (peroxide, 



C. Fe^'* C Fe^^** 



/^/ \ r... //V 



\ 



^ -C- N OH"^" -C N OH" 



I / ' / 



I 



C Mg 



^ / v./ 



-C- N 

 ' / 



-> 



scheme; Chl + Fe*** -^ Chr + Fe'"** 



hp hp 



lig. 1. Sclicinc lor tlic rtadion of thloiopliyll witli a liacm piolciii. Alllioiigh 

 OH- is shown as ligand, it is more likely that water or protein supplies tho 

 co-ordinating group. 



