Milk oligopeptide inhibition of (α)-tocopherol fortified linoleic acid oxidation

Figures & data

Figure 1. Effect of ferrous ion concentration and oxidation duration on the formation of malondialdehyde in Fe²⁺-vitamin C-induced linoleic acid oxidation model. (a) Ferrous ion concentration and (b) oxidation duration. Data are the mean ± standard deviation of triplicate tests. Different letters represent significant difference in the means at a significant level of 0.05

Figure 2. Effect of (α)-tocopherol on inhibition of linoleic acid oxidation in Fe²⁺-vitamin C induced linoleic acid oxidation model. Data are the mean ± standard deviation of triplicate tests

Figure 3. Effect of milk oligopeptides and milk oligopeptides with optimal (α)-tocopherol concentration on inhibition of linoleic acid oxidation in Fe²⁺-vitamin C-induced linoleic acid oxidation model. (a) Malondialdehyde, (b) Peroxide value and (c) Conjugated diene and triene. Data are the mean ± standard deviation of triplicate tests. # represents significant difference between milk oligopeptides, (α)-tocopherol, milk oligopeptide-(α)-tocopherol and the control on the means at a significant level of 0.05. * means significant difference between milk oligopeptides, milk oligopeptide-(α)-tocopherol and (α)-tocopherol on the means at a significant level of 0.05

Figure 4. Effect of (α)-tocopherol on alteration of maximal absorption wavelength of milk oligopeptides in UV-Vis spectra. a→ e represents the concentration of (α)-tocopherol. The concentrations are: a = 0.0 mmol/L, b = 0.05 mmol/L, c = 0.10 mmol/L, d = 0.20 mmol/L, and e = 0.30 mmol/L

Table 1. Fluorescent quenching constant (K_sv) and quenching rate constant (K_q) of milk oligopeptide-(α)-tocopherol complexes

Complex	KSV (× 10^11 L mol^−1)	Kq(× 10^19 L mol^−1 s^−1)
PYYAK-(α)tocopherol	8.56 ± 0.22^b	8.56 ± 0.22^b
NQFLPYPYYAK-(α)tocopherol	7.47 ± 0.44^c	7.47 ± 0.44^c
IAKYI -(α)tocopherol	8.84 ± 0.52^b	8.84 ± 0.52^b
IPIQY -(α)tocopherol	12.90 ± 0.55^a	12.90 ± 0.55^a
IPIQYV-(α)tocopherol	7.03 ± 0.54^c	7.03 ± 0.54^c

Data are mean ± standard deviation of triplicate test. Different letters in each column represent significant difference in the means at a significant level of 0.05.

Figure 5. Effect of (α)-tocopherol on reduction of fluorescent intensity of milk oligopeptides. a→ f represents the concentration of (α)-tocopherol. The concentrations are: a = 0.0 × 10⁻¹⁰ mol/L, b = 0.05 × 10⁻¹⁰ mol/L, c = 0.10 × 10⁻¹⁰ mol/L, d = 0.15 × 10⁻¹⁰ mol/L, e = 0,20 × 10⁻¹⁰ mol/L and f = 0.25 × 10⁻¹⁰ mol/L

Figure 6. Stern-Volmer fitting curve using milk oligopeptides and (α)-tocopherol. Stern-Volmer formula: ${\mathrm{F}}_0/\mathrm{F}=1+{\mathrm{K}}_{\mathrm{q}}{\tau }_0\left[\mathrm{Q}\right]=$ $1+{\mathrm{K}}_{\mathrm{S}\mathrm{V}}\left[\mathrm{Q}\right]$, where F₀ and F represent fluorescent intensity of milk oligopeptide before and after the addition of (α)-tocopherol. K_q and K_sv are quenching rate constant and quenching, respectively. ${\tau }_0$represents the average life span of fluorescent molecule without addition of (α)-tocopherol

Figure 7. Fourier transform infrared spectrum of milk oligopeptides and milk oligopeptide-(α)-tocopherol complexes

Figure 8. Second derivative and curve-fitting analysis of secondary structure of milk oligopeptide and milk oligopeptide-(α)-tocopherol complex using PeakFit 4.12 software

Table 2. Secondary structure percentage in milk oligopeptides and milk oligopeptide-(α)-tocopherol complexes

Secondary Structure	α-helix(%)	β-sheet(%)	β-turn(%)	random coil (%)	β-Turn/β-sheet
PYYAK	29.8	29.0	41.2	–	1.42
PYYAK-(α) tocopherol	29.8	28.3	41.9	–	1.48
NQFLPYPYYAK	41.0	–	29.0	30.0
NQFLPYPYYAK-(α) tocopherol	29.1	29.2	41.7	–	1.43
IPIQYV	30.9	27.4	41.7	–	1.52
IPIQYV-(α) tocopherol	29.3	28.5	42.2	–	1.48
IPIQY	30.9	27.4	41.7	–	1.52
IPIQY-(α) tocopherol	30.1	27.9	42.0	–	1.51
IAKYI	–	28.9	41.7	29.4	1.44
IAKYI-(α) tocopherol	26.2	30.8	43.0	–	1.40