The triplet state of photosynthetic pigments. I. Pheophytins

Abstract

ZFS constants (in cm −1) and decay rate constants for the lowest triplet state of pheophytins have been determined by ESR: pheophytin a: D = 341 ± 3, E = 33 ± 3, K T = 1050 s −1; pheophytin b: D = 358 ± 8, E = 46 ± 5, K T = 630 s −1; bacteriopheophytin: D = 256 ± 4, E = 54 ± 5/37 ± 5, K T ≈ 4000 s −1. In addition values for the decay rate constants and relative populating rates of the individual spin levels have been obtained; these numbers turn out to be appreciably different from those for the corresponding chlorophylls. For the series pheophytin a, b and bacteriopheophytin we find parallel behaviour with the corresponding chlorophylls. The effects of side group substitution and pyrrole ring reduction on the ZFS constant D can be understood by including configuration interaction between the excited states using the 4-orbital model. The change of the mean triplet decay constant K T upon side group substitution and pyrrole ring reduction follows an energy gap law. Substitution of the central Mg-ion by two protons, however, causes K T to increase; this is attributed to the introduction of an extra promoting mode - of the NH-group - and/or to the presence of low lying nπ * states in pheophytins.