Abstract
Based on HPLC analyses of high resolution, the kinetic and mechanistic features of chlorophyll (Chl) a ⇄ a′ epimerization (reversible stereoisomerization on the cyclopentanone ring) have been studied for the first time in several organic solvents, and in diethyl ether containing a series of basic compounds. The epimerization-promoting effect of a base well parallels its p K a value, suggesting that epimerization proceeds via general base catalysis. Under favorable conditions the epimerization is completed within 1 min to give an equilibrium composition [ Chl a′] [ Chl a] ≈ 1 3 . With imidazole (Im) and its derivatives as bases, the epimerization rate constant is proportional to the squared base concentration, presumably due to the occurrence of a ‘tandem general base mechanism’ involving an Im dimer with enhanced basicity of one heteronitrogen atom. Chl a a′ epimerization is promoted also by histidine suspended in diethyl ether. These results are discussed referring to previous qualitative reports and to the epimeric composition of Chl-type pigments in photosynthetic apparatus.
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