OXIDATION AND REDUCTION OF CHLOROPHYLL 



459 



atoms in positions 5 and 6, so that the net result is a hydrogen iodide- 

 catalyzed internal oxidation-reduction. This interpretation implies that 

 the oxidation-reduction potential of the system porphin-dehydroporphin 

 is more positive than that of the system vinyl chlorin-mesochlorin. 



The oxidation of the H atom in position 10 is supposed to play an 

 important part in the so-called allomerization of chlorophyll and its de- 

 rivatives. This transformation occurs when alcoholic solutions of the 

 pigments are exposed to air. The solution color remains the same, and 

 the absorption spectrum is unchanged (at least in the first approximation, 

 c/. Conant, Dietz, Bailey, and Kamerling 1931; no rehable extinction 

 curve of allomerized chlorophyll has yet been published). The main 

 difference between allomerized and intact chlorophyll is in the behavior 

 towards alkali. Ether solutions of intact chlorophyll give the so-called 

 "Molisch phase test" — transient apparition of a brownish yellow color 

 (in chlorophyll a) or a brownish red color (in chlorophyll h) upon the 

 addition of alcohoUc alkah (in the cold and in the presence of air). 

 The color soon reverts to green : and the final result of the phase reaction 

 turns out to be the severance of the cyclopentanone ring between carbon 

 atoms 9 and 10, by alcoholysis, i. e., the conversion of a phorbin into a 

 chlorin. According to Fischer, Elser, and Plotz (1932) and Fischer and 

 Siebel (1933), the phase test is initiated by the enolization of the carbonyl 

 group in position 9, induced by alkali: 



N — ^ 



-/ 



V 



Thus, a double bond is formed between C(10) and C(9); it causes a 

 strain in the cyclopentanone ring, so that this ring becomes subject to 

 disruption by alcoholysis. The alkah salt of the enolized form is supposed 

 to be present in the brown phase. If the alkah is extracted at once by 

 shaking the brown ether-alcohol solution with water, chlorophyll can be 

 regenerated from the ether fraction, and is found to be unchanged. 

 The first stage of the phase test is thus reversible; but after the green 

 color has returned, the reaction cannot be reversed except possibly by 

 special methods by which chlorins can be converted into phorbins. 



Although attributing the "brown phase" to an enolate of chlorophyll 

 seems plausible from the chemical point of view, a certain difficulty is 

 encountered in the interpretation of the color change. The spectrum of 

 the brown phase is unknown, but there can be no doubt that the main 

 red absorption band of chlorophyll is either absent or strongly reduced 



