460 



CHLOROPHYLL 



CHAP. 16 



in intensity. From what we know about the occurrence of this band in 

 chlorins and phorbins {cj. Vol. II, Chapter 21), one would not expect it 

 to disappear as long as the three chromophoric factors — the conjugated 

 double bond system, the hydrogenated nucleus IV, and the magnesium 

 atom — remain intact. Enolization in a side chain can scarcely be 

 expected to affect the spectrum to such an extent. 



One could suggest that the alkaU enolate is ionized, and recall that ionization often 

 changes the color of dyes (acid-base indicators) ; but this occurs only when ionization 

 affects the resonance in the chromophoric system, and there is no reason why any such 

 effect should be produced by the ionization of chlorophyll in the cyclopentanone ring. 

 It looks as if something must happen to the two hydrogen atoms in nucleus IV during 

 the "brown phase." Perhaps these atoms can slide from positions 7 and 8 to positions 

 9 and 10, thus creating a tautomeric equiUbrium between the structures: 



and 



B 



Form (B) may be responsible for the brown color, and form (A) (which needs only to be 

 present in a small proportion) for the gradual conversion into chlorins and consequent 

 disappearance of the brown phase. Another hypothesis, described on page 493, postu- 

 lates that the brown phase is a product of a reversible dismutation of enolized and 

 ordinary chlorophyll (c/. Eq. 18.11 a, b, c), or of a reversible oxidation-reduction reaction 

 between enoUzed chlorophyll and the solvent. 



The phase test has often been used to control the freshness of chloro- 

 phyll preparations (although in its usual qualitative form, it obviously 

 does not detect 'partial allomerization). Apart from incapacity to give 

 the "brown phase," allomerized chlorophyll is characterized by the 

 different products it gives in the "cleavage test" (c/. page 457). In the 

 case of the chlorophyllides, allomerization also causes the loss of good 

 crystallizability. 



If the "brown phase" is caused by an enolization of the carbonyl 

 group in position 9 involving the hydrogen atom from the neighboring 

 atom C(10), it is natural to attribute the nonoccurrence of this phase in 

 allomerized chlorophyll to the removal or binding of this hydrogen atom, 

 i. e., to an oxidation of the CH — group. Conant, Hyde, Moyer, and 

 Dietz (1931) were the first to interpret allomerization as an oxidation, 

 and Conant, Dietz, Bailey, and Kamerling (1931) proved that one mole 

 of oxygen is consumed in the slow allomerization of one mole of chloro- 



