Understanding oxidation processes in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 21

Table 1.4 Second-order rate constant for initial step in oxidation of amino-acids,
peptides, proteins and plant phenols by triplet-state riboflavin in pH 6.4 aqueous
solution at 25 ëC a

k 2 (l mol ÿ1 s ÿ1 ) Comment Substrate

1.6 10 6

Cystein (Cys) Low pH, hydrogen atom transfer

1.2 10 9 Cystein anion High pH, electron transfer

Cystin ± No reaction

6.4 10 7 Methionine Electron transfer

5.2 10 7 Histidine Electron transfer ± rate decreases for

higher pH due to protonation

Phenylalanine ± No reaction

1.4 10 9 Tyrosine (Tyr) Electron transfer

1.8 10 9 Tryptophan Electron transfer

1.9 10 9 H-Cys-Tyr-Cys-Tyr-OH Electron transfer

H-Cys-Gly-OH b 6.7 10 7 Electron transfer

1.3 10 9 H-Tyr-Gly-OH Electron transfer

1.6 10 9 H-Gly-Tyr-OH Electron transfer

2.1 10 9 H-Gly-Tyr-Gly-OH Electron transfer

3.6 10 8

-lactoglobulin Electron transfer

2.3 10 8

Bovine Serum Albumin Electron transfer

1.7 10 9 (ÿ) Epigallocatechingallate Hydrogen atom transfer/electron transfer

1.4 10 9 () Catechin Hydrogen atom transfer/electron transfer

1.0 10 9 Rutin Hydrogen atom transfer/electron transfer

a

From Cardoso et al. (2004), and Becker et al. (2005).

b

Gly is glycine.

mechanisms for oxidative changes of aqueous phase components in foods and
making distinction between electron transfer and hydrogen atom transfer as rate
determining (Cardoso et al., 2007). Proteins seem to be oxidized by initial
electron abstraction, while some plant phenols rather transfer a hydrogen atom

Fig. 1.11 Rate of electron transfer from histidine and other reducing nitrogen
heterocycles to triplet riboflavin as photosensitizer shows a linear free energy relationship
(LFER) indicative of a common reaction mechanism (from Huvaere and Skibsted, 2009).