1772 CHEMISTRY OF CHLOROPLAST PIGMENTS CHAP. 37B 



perature. The isomeric chlorophyllides are less adsorbable than the original 

 chlorophyllides a and h. Separated on the column and redissolved in 

 propanol, they can be (at least partially) reconverted to a and h by heating. 

 The isomerization equilibrium of the ethyl and methyl chlorophyllides 

 seems to correspond, at room temperature, to [a]: [a'] = [6]: [6'] = 3:1. 



Isomerization is not affected by the presence of air or dimethyl- 

 aniline (which, respectively, bring about and prevent allomerization). 

 It is accelerated by alkali: 0.2% KOH produces isomeric eciuilibration in 

 1-5 minutes, followed by a slower oxidation. 



In discussing the possible mechanism of isomerization. Strain pointed 

 out that its absence after allomerization points to a role of the C(10) atom, 

 and suggested that the isomers differ only by the spatial arrangement of the 

 two groups (COOCH3 and H) attached to C(10); they could then be con- 

 verted into each other via their common enol, in which the H-atom is trans- 

 ferred from C(10) to C(9) to form a hydroxyl group there. This could ac- 

 count for the catalytic effect of alkalis on the isomerization. 



Freed and Sancier (1951) noticed a variability of the spectrum of chloro- 

 phyll 6 preparations — particularly of the small peak at 481.5 ni/u. 



Subsequently, Freed, Sancier and Sporer (1954) isolated from such 

 preparations a fraction which they called chlorophyll b" . It had a spec- 

 trum very similar to that of h, but gave no phase test. Freed suggested 

 that 6, 6' and h" may be the three isomers anticipated on page 444, differing 

 only in the routing of the conjugated bond system. On the other hand, 

 Freed's b" appears similar to Holt's "fraction 3" of allomerized chloro- 

 phyll a (cf. page 1775). 



The main subject of the studies of Freed and Sancier (1951, 1952, 1953, 

 1954) was the transformation of the chlorophylls and their derivatives at 

 low temperatures. At first (1951) the observed reversible spectroscopic 

 changes were interpreted as evidence of the formation of new isomers. 

 Because of the analogy between these spectral changes and those caused by 

 various solvents, it was suggested that the solvent effect, too, may be caused 

 by isomerization. After the experiments of Livingston and co-workers, 

 Evstigneev, and others {cf. Chapter 21, section Bl; Chapter 23, sections 

 4-6, and Chapter 37C, section 2) had revealed that chlorophyll forms 

 complexes with water and other solvents which differ in their absorption 

 spectrum and capacity for fluorescence. Freed and Sancier (1954) reinter- 

 preted the temperature changes of the absorption spectra as also resulting 

 from the formation and dissociation of solvates. However, the two types 

 of transformation — isomerization and solvation — may be coupled; e. g., 

 association with a protophilic solvent is likely to favor enolization. 



It is a matter of arbitrary choice whether to discuss the solvent effects 

 and temperature effects on the chlorophyll spectrum in the present chapter 



