Quenching of excited states of red-pigment zinc protoporphyrin IX by hemin and natural reductors in dry-cured hams

摘要

Zinc protoporphyrin IX (ZnPP), the major red pigment in hams dry-cured without nitratesitrites, is an efficient photosensitizer, which upon absorption of visible light forms short-lived excited singlet state (1ZnPP*) and by intersystem crossing yields the very reactive triplet-excited state (3ZnPP*). Using nano-second laser flash photolysis and transient absorption spectroscopy NADH, ascorbic acid, hemin and dehydroascorbic acid were each found to be efficient quenchers of 3ZnPP*. The deactivation followed, in homogeneous dimethyl sulfoxide (DMSO) or DMSO:water (1:1) solutions, second-order kinetics. The rate constant for ascorbic acid and NADH for reductive quenching of 3ZnPP* was at 25 °C found to be 7.5 ± 0.1 × 104 L mol−1 s−1 and 6.3 ± 0.1 × 105 L mol−1 s−1, respectively. The polyphenols catechin and quercetin had no effect on 3ZnPP*. The quenching rate constant for oxidative deactivation of 3ZnPP* by dehydroascorbic acid and hemin was at 25 °C: 1.6 ± 0.1 × 105 L mol−1 s−1 and 1.47 ± 0.1 × 109 L mol−1 s−1, respectively. Oxidized glutathione did not act as an oxidative quencher for 3ZnPP*. After photoexcitation of ZnPP to 1ZnPP*, fluorescence was only found to be quenched by the presence of hemin in a diffusion-controlled reaction. The efficient deactivation of 3ZnPP* and 1ZnPP* by the metalloporphyrin (hemin) naturally present in meat may accordingly inherently protect meat proteins and lipids against ZnPP photosensitized oxidation.