Abstract
Effects of peripheral substituents in the A- and B-rings of the chlorin macrocycle on demetalation of zinc chlorophyll derivatives were examined. The demetalation rate constants of zinc 8-formyl-chlorin (1) were comparable to those of zinc 3-formyl-chlorin (2). Both 1 and 2 exhibited significantly slower demetalation kinetics than the corresponding zinc chlorin 3, which had no formyl group. Comparison of demetalation kinetics between zinc 3-ethyl-chlorin (3) and zinc 3-vinyl-chlorin (4) indicated that the substitution at the 3-position of the chlorin macrocycle from a vinyl to an ethyl group accelerated removal of the central zinc. Activation energies of demetalation of zinc chlorins 1, 2, and 5 possessing a formyl group in the A- or B-ring of the chlorin macrocycle were larger than those of zinc chlorins 3 and 4 lacking a formyl group. These results indicate that the formyl groups in the A- or B-ring provide tolerance to demetalation of chlorin molecules due to their electron-withdrawing effects.
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