Oxidation in 3 types of bovine milk with different fatty acid profiles obtained through manipulation of feed was evaluated by analytical methods quantifying the content of potential antioxidants, the tendency of formation of free radicals, and the accumulation of primary and secondary oxidation products. The milk samples were evaluated in parallel by descriptive sensory analysis by a trained panel, and the correlation between the chemical analysis and the descriptive sensory analysis was evaluated. The fatty acid composition of the 3 types of milk was found to influence the oxidative and lipolytic changes occurring in the milk during chill storage for 4 d. Sensory analysis and chemical analysis showed high correlation between the typical descriptors for oxidation such as cardboard, metallic taste, and boiled milk and specific chemical markers for oxidation such as hexanal. Notably, primary oxidation products (i.e., lipid hydroperoxides) and even the tendency of formation of radicals as measured by electron spin resonance spectroscopy were also highly correlated to the sensory descriptors for oxidation. Electron spin resonance spectroscopy should accordingly be further explored as a routine method for detection of early events in lipid oxidation in milk to predict shelf-life.
Kinetics of reduction of iron(IV) in ferrylmyoglobin by chlorogenate in neutral or moderately acidic aqueous solutions (0.16 M NaCl) to yield metmyoglobin was studied using stopped flow absorption spectroscopy. The reaction occurs by direct bimolecular electron transfer with (2.7 +/- 0.3) x 10(3) M(-)(1).s(-)(1) at 25.0 degrees C (DeltaH( )(#) = 59 +/- 6 kJ.mol(-)(1), DeltaS(#) = 15 +/- 22 J. mol(-)(1).K(-)(1)) for protonated ferrylmyoglobin (pK(a) = 4.95) and with 216 +/- 50 M(-)(1).s(-)(1) (DeltaH( )(#) = 73 +/- 8 kJ. mol(-)(1), DeltaS( )(#) = 41 +/- 30 J.mol(-)(1).K(-)(1)) for nonprotonated ferrylmyoglobin in parallel with reduction of a chlorogenate/ferrylmyoglobin complex by a second chlorogenate molecule with (8.6 +/- 1.1) x 10(2) M(-)(1).s(-)(1) (DeltaH( )(#) = 74 +/- 8 kJ.mol(-)(1), DeltaS( )(#) = 59 +/- 28 J.mol(-)(1).K(-)(1)) for protonated ferrylmyoglobin and with 61 +/- 9 M(-)(1).s(-)(1) (DeltaH( )(#) = 82 +/- 12 kJ.mol(-)(1), DeltaS( )(#) = 63 +/- 41 J. mol(-)(1).K(-)(1)) for nonprotonated ferrylmyoglobin. Previously published data on ascorbate reduction of ferrylmyoglobin are reevaluated according to a similar mechanism. For both protonated and nonprotonated ferrylmyoglobin the binding constant of chlorogenate is approximately 300 M(-)(1), and the modulation of ferrylmyoglobin as an oxidant by chlorogenate (or ascorbate) leads to a novel antioxidant interaction for reduction of ferrylmyoglobin by ascorbate in mixtures with chlorogenate.
Reduction of iron(IV) in ferrylmyoglobin by ascorbate (and
d-isoascorbate) is fast; at neutral or
moderately acidic pH there are three parallel reactions: (i) a
bimolecular reaction with 2.7 ± 0.8
M-1·s-1,
ΔH
# = 45 ± 13
kJ·mol-1 , and
ΔS
# = −85 ± 46
J·mol-1·K-1;
(ii) a bimolecular reaction
between ascorbate and protonated ferrylmyoglobin (28 ± 5
M-1·s-1,
ΔH
# = 31 ± 8
kJ·mol-1, and
ΔS
# = 113 ± 28
J·mol-1·K-1);
and (iii) an intramolecular electron transfer from ascorbate bound
to
protonated ferrylmyoglobin (7.5 ± 0.1 s-1,
ΔH
# = 89.6 ± 0.9
kJ·mol-1, ΔS
#
= 72 ± 3
J·mol-1·K-1);
rate constants and activation parameters are for aqueous 0.16 M NaCl
solutions at 25.0 °C. Binding
of ascorbate has ΔH
⊖ = −43 ± 3
kJ·mol-1 and
ΔS
⊖ = −100 ± 9
J·mol-1·K-1,
and dissociation of
protonated ferrylmyoglobin pK
a = 5.34 ± 0.02
(ΔH
⊖ = 11 ± 2
kJ·mol-1,
ΔS
⊖ = −66 ± 7
J·mol-1·K-1).
One-electron transfer dominates to yield iron(III)
metmyoglobin, and ascorbyl radical was detected
by ESR spectroscopy. d-Isoascorbic acid was found to
be slightly more acidic (pK
a = 3.706 ±
0.003,
ΔH
⊖ = 5.7 ± 0.5
kJ·mol-1,
ΔS
⊖ = −52 ± 2
J·mol-1·K-1)
than ascorbic acid (pK
a = 3.772 ±
0.003,
ΔH
⊖ = 5.8 ± 0.4
kJ·mol-1,
ΔS
⊖ = −53 ± 1
J·mol-1·K-1),
while d-isoascorbate had reducing properties
similar to those of ascorbate (cyclic voltammetry and stopped-flow
spectroscopy).
Keywords: Ferrylmyoglobin; ascorbic acid; d-isoascorbic acid;
protein binding; electron transfer
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