KINETICS OF THE FORMATION OF ZINC-COMPLEXES OF CHLOROPHYLL DERIVATIVES

Zinc complexes of chlorophyll derivatives are similar in color to chlorophylls. Unlike the parent compounds, however, they survive heat treatment as observed in the vegetable-processing industry. Little is known about the kinetics of their formation. The reaction of chlorophyll a derivatives with zinc(II) ions in 80/20 (v/v) acetone/water was monitored by HPLC. Pyropheophorbide reacted most quickly at 20-degrees-C followed by pheophorbide, methyl pheophorbide, ethyl pheophorbide, pyropheophytin, and pheophytin. Half-life values were 54, 90, 166, 171, 174, and 305 min, respectively. Similar trends were observed at 25, 30, and 35-degrees-C. The E(a) was calculated as 21.8 +/- 0.5 kcal/mol and was the same for all reactions. Results suggest that steric hindrance from peripheral groups and charge distribution on the tetrapyrrole ring had an effect on the rate of reaction of chlorophyll derivatives with zinc ions.