Understanding oxidation processes in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 19

Fig. 1.9 Metmyoglobin is a moderate catalyst for lipid oxidation in an oil-in-water
emulsion as followed electrochemically by oxygen depletion, the catalytic effect being
strongly enhanced by proteolysis of metmyoglobin under stomach conditions (based on
Carlsen and Skibsted, 2004). The three chromatograms (A, B and C) indicate hydrolytic
cleavage of myoglobin and correspond to the oxygen depletion curves.
proteins are involved forming disulfide bridges and may also be involved in
formation of dityrosine cross-links through a free radical mechanism (Baron and
Lund, 2010). For the whey protein -lactoglobulin, heat or pressure denaturation
leads to polymerization through thiol/disulfide exchange reactions, while
oxidation of denaturated -lactoglobulin, leads to formation of inter-molecular
ditryosine (éstdal et al., 1996).
Besides becoming cross-linked through oxidation, protein side-chains may be
modified oxidatively or undergo fragmentation as illustrated in Fig. 1.10 (Baron
and Lund, 2010). The initial step in protein oxidation is the abstraction of an
electron or a hydrogen atom by the actual oxidant. Using triplet-state riboflavin
generated photochemically using a laser-flash technique, the second-order rate
constants for electron transfer from a number of amino acids, peptides, and
proteins to this strong aqueous oxidant have now become available (Cardoso et
al., 2004). As may be seen from Table 1.4, tryptophan, tyrosine, and the cystein
anion are reacting with comparable rates approaching the diffusion limit for
electron transfer. Cystein at physiological pH reacts much slower and by
hydrogen atom transfer. Proteins are accordingly easily oxidized at the specific
side chains with thiol groups or phenol groups leading to cross-linking, as is
further supported by the insensitivity of the rate of oxidation to the incorporation
of these amino acids in peptides (Table 1.4). The rate of electron transfer as the
primary step in oxidative modification of proteins depends on the driving force
for the electron transfer reaction (difference in redox potentials) as has been
shown for histidine and closely related N-heterocycles as protein models
(Huvaere and Skibsted, 2009). Such linear free energy relationships (Fig. 1.11)
are together with demonstration of so-called isokinetic behaviour (activation
entropy depends linearly on activation enthalpy) most valuable when assigning