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

Zinc protoporphyrin IX (ZnPP), the major red pigment in hams dry-cured without nitrates/nitrites, is an efficient photosensitizer, which upon absorption of visible light forms short-lived excited singlet state ((ZnPP)-Zn-1*) and by intersystem crossing yields the very reactive triplet-excited state ((ZnPP)-Zn-3*). 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 (ZnPP)-Zn-3*. 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 (ZnPP)-Zn-3* was at 25 A degrees C found to be 7.5 +/- A 0.1 x 10(4) L mol(-1) s(-1) and 6.3 +/- A 0.1 x 10(5) L mol(-1) s(-1), respectively. The polyphenols catechin and quercetin had no effect on (ZnPP)-Zn-3*. The quenching rate constant for oxidative deactivation of (ZnPP)-Zn-3* by dehydroascorbic acid and hemin was at 25 A degrees C: 1.6 +/- A 0.1 x 10(5) L mol(-1) s(-1) and 1.47 +/- A 0.1 x 10(9) L mol(-1) s(-1), respectively. Oxidized glutathione did not act as an oxidative quencher for (ZnPP)-Zn-3*. After photoexcitation of ZnPP to (ZnPP)-Zn-1*, fluorescence was only found to be quenched by the presence of hemin in a diffusion-controlled reaction. The efficient deactivation of (ZnPP)-Zn-3* and (ZnPP)-Zn-1* by the metalloporphyrin (hemin) naturally present in meat may accordingly inherently protect meat proteins and lipids against ZnPP photosensitized oxidation.