Studies on the reaction of trans-2-heptenal with peanut proteins

Structural characterization of peroxyl radical oxidative products of antioxidant peptides from hydrolyzed proteins

This work aimed to characterize oxidative products of five unique antioxidant peptides (P1: YFDEQNEQFR, P2: GQLLIVPQ, P3: SPFWNINAH, P4: NINAHSVVY, P5: RALPIDVL) from hydrolyzed oat proteins. Peptides were reacted with 2,2'-Azobis(2-amidinopropane) dihydrochloride, a common peroxyl radical generator. Chromatographic data showed that peptide P3 was the most oxidized (67 ± 4 %) while also displaying the most ability to scavenge radicals in the oxygen absorbance capacity assay (ORAC) with an activity of 2.16 ± 0.09 μM Trolox equivalents/μM peptide. Structural characterization using mass spectrometry showed the presence of four oxidative products of P3, three of which were mono-oxygenated and the fourth di-oxygenated. The identification of these oxidative products is new and provides an opportunity to investigate their biological function. A good correlation (r = 0.889) between the degree of oxidation and the ORAC data, demonstrates the usefulness of using oxidative peptide data to predict their radical scavenging activities.

9-Oxononanoic Acid and Its Lysine Schiff Base Adduct as a Novel Lipation Product in Peanuts

9-Oxononanoic acid (9-ONA) was quantitated in peanuts roasted at 170 °C by GC-MS (EI). After roasting peanuts for 40 min, 9-ONA decreased from 1010 μmol/kg protein in the unheated sample to 722 μmol/kg protein, most likely due to modifications of nucleophilic side chains of protein-bound amino acids (lipation). After heating N^α-acetyl-l-lysine and 9-ONA in model experiments, a Schiff base in its reduced form, namely, N^ε-carboxyoctyl-acetyl lysine, as well as two isomeric pyridinium derivatives, namely, dicarboxyhexylcarboxyheptylpyridinium-acetyl lysine 1 and 2, were tentatively identified by HPLC-ESI-MS/MS. Based on the identified lipation products of 9-ONA, it can be assumed that lipation reactions represent a mirror-image reaction. For quantitation of N^ε-carboxyoctyllysine (COL) in roasted peanuts by means of HPLC-ESI-MS/MS, samples were reduced with sodium borohydride and acid hydrolyzed. For the first time, COL was quantitated after reduction in roasted peanuts. Furthermore, after prolonged roasting of peanuts for 40 min, COL decreased from 139.8 to 22.5 μmol/kg protein, which provides initial evidence for lipation of nucleophilic side chains of protein-bound amino acids by glycerol-bound oxidized fatty acids (GOFAs, e.g., 9-ONA) with the formation of neo-lipoproteins.

Glycerol-bound oxidized fatty acids: formation and occurrence in peanuts

For peanuts, roasted at 170 °C, the formation of selected glycerol-bound oxidized fatty acids (GOFAs), namely 9-oxononanoic acid (9-ONA), azelaic acid (AZA) and octanoic acid, was observed by GC-MS (EI). The content of octanoic acid as well as AZA increased with continuous roasting time (from 59 mg/kg peanut oil to 101 mg/kg peanut oil and from not detectable to 8 mg/kg peanut oil, respectively), whereas the content of 9-ONA initially decreased from 25 mg/kg peanut oil to 8 mg/kg peanut oil (20 min) and increased again up to 37 mg/kg peanut oil following roasting for 40 min. Due to its aldehyde function, 9-ONA could contribute to amino acid side chain modifications as a result of lipation, which could directly influence the functional properties of peanut proteins. Both 9-ONA and octanoic acid are potential markers of thermal processes. Furthermore, in model experiments using methyl linoleate and methyl oleate, up to 18 oxidized fatty acids could be identified as methyl esters, 9-ONA as well as octanoic acid as major components and a faster formation of GOFAs under roasting conditions (170 °C, 20 min). In addition, 9-ONA contributes to the formation of AZA and octanoic acid in both free and bound form as a result of oxidative subsequent reactions in presence of iron (III).

Effects of "nine steaming nine sun-drying" on proximate composition, protein structure and volatile compounds of black soybeans

Nine steaming nine sun-drying is a traditional processing technology for food or medicinal materials. The dynamic changes of the proximate composition, protein structure and volatile compounds during nine-time steaming and sun-drying of black soybeans (BS) were studied. The proximate composition results showed that the content of protein, carbohydrate and fat of BS decreased after processing, whereas the relative content of amino acids remained basically unchanged. Protein structure was evaluated using Fourier transform infrared spectroscopy (FT-IR), Ultraviolet absorption spectroscopy (UV) and Fluorescence spectroscopy. FT-IR result revealed that the relative contents of β-sheet and β-turn of the secondary structure of black soybean protein isolate (BSPI) decreased but the relative contents of α-helix and random coil increased after steaming and sun-drying. The results of UV and fluorescence spectroscopy confirmed changes in the protein conformation. In addition, SPME-GCMS analysis demonstrated that hydrocarbons, alcohols and aldehydes were the main volatile compounds. The relative contents of 1-octen-3-ol and hexanal, which are the main sources of beany flavor decreased significantly compared with raw BS. Principal component analysis (PCA) results showed that the volatile compounds of nine steamed and nine sun-dried BS could be well distinguished during the process. These findings may therefore provide a scientific basis for the application of nine-time steamed and sun-dried BS in food industry and contribute to the understanding of process-induced chemical transformations in this ancient processing technique.

Food Protein Sterylation: Chemical Reactions between Reactive Amino Acids and Sterol Oxidation Products under Food Processing Conditions

Sterols, especially cholesterol and phytosterols, are important components of food lipids. During food processing, such as heating, sterols, like unsaturated fatty acids, can be oxidized. Protein modification by secondary products of lipid peroxidation has recently been demonstrated in food through a process called lipation. Similarly, this study was performed to assess, for the first time, the possibility of reactions between food proteins and sterol oxidation products in conditions relevant for food processing. Therefore, reaction models consisting of oxysterol (cholesterol 5α,6α-epoxide) and reactive amino acids (arginine, lysine, and methionine) were incubated in various conditions of concentration (0–8 mM), time (0–120 min), and temperature (30–180 °C). The identification of lysine adducts through thin-layer chromatography (TLC), high-performance liquid chromatography (HPLC) with a diode array detector (DAD), and electrospray ionization (ESI) mass spectrometry (MS) evidenced a reaction with lysine. Moreover, the HPLC-ESI with tandem mass spectrometry (MS/MS) analyses allowed observation of the compound, whose mass to charge ratio m/z 710.5 and fragmentation patterns corresponded to the reaction product [M + H]⁺ between cholesterol-5α,6α-epoxide and the ε-amino-group of N_α-benzoylglycyl-l-lysine. Moreover, kinetic studies between N_α-benzoylglycyl-l-lysine as a model for protein-bound lysine and cholesterol 5α,6α-epoxide were performed, showing that the formation of lysine adducts strongly increases with time, temperature, and oxysterol level. This preliminary study suggests that in conditions commonly reached during food processing, sterol oxidation products could react covalently with protein-bound lysine, causing protein modifications.

Analysis of Protein Oxidation in Food and Feed Products

Food and feed proteins are subject to oxidation reactions during production, processing, and storage. Several individual oxidized amino acids have been described in model systems and food; however, protein oxidation in food is still mostly assessed by the analysis of protein carbonylation. In the present review, the chemistry of protein oxidation and its implications for protein functionality, food flavor, and nutritional physiology are briefly summarized. Limitations of generic methods targeting redox-relevant functional groups and properties of typical reaction products, such as the determination of protein carbonyls and fluorescence spectroscopy, are presented. Methods for the quantitation of individual oxidation products of susceptible amino acids, such as cysteine, methionine, phenylalanine, tyrosine, and tryptophan, are reported. Special regard is paid to limitations resulting from the required hydrolysis procedures and unintended formation of the analytes during sample pretreatment. If available, results from food analysis obtained by different methods are compared. Suggestions and requirements for future works on protein oxidation in food and nutrition are given.

A Dual Perspective of the Action of Lysine on Soybean Oil Oxidation Process Obtained by Combining 1H NMR and LC-MS: Antioxidant Effect and Generation of Lysine-Aldehyde Adducts

Little is still known about both the effect of amino acids on the oxidation course of edible oils and the modifications that the former may undergo during this process. Bearing this in mind, the objective of this work was to study the evolution of a system consisting of soybean oil with 2% of l-lysine under heating at 70 °C and stirring conditions, analyzing how the co-oxidation of the oil and of the amino acid affects their respective evolutions, and trying to obtain information about the action mechanism of lysine on soybean oil oxidation. The study of the oil progress by 1H Nuclear Magnetic Resonance (1H NMR) showed that the presence of lysine noticeably delays oil degradation and oxidation products generation in comparison with a reference oil without lysine. Regarding lysine evolution, the analysis by 1H NMR and Liquid Chromatography-Mass Spectrometry of a series of aqueous extracts obtained from the oil containing lysine over time revealed the formation of lysine adducts, most of them at the position, with n-alkanals, malondialdehyde, (E)-2-alkenals, and toxic oxygenated α β-unsaturated aldehydes. However, this latter finding does not seem enough to explain the antioxidant action of lysine.

Identification of the Key Aroma Compounds in Gluten-Free Rice Bread

Rice flour is commonly used as a raw material in the preparation of gluten-free bread. However, compared to wheat bread, its aroma is different and often not accepted by consumers. Aroma profile analyses of the crumb and crust of freshly baked rice bread indicated a strong rice-like flavor but lower intensities of the typical roasty wheat bread odor qualities. By application of aroma extract dilution analyses (AEDA), the most odor active compounds in both rice and wheat bread samples were characterized. In addition, two batches of rice flour were used. In particular, 2-aminoacetophenone and 4-vinylphenol reached high flavor dilution (FD) factors in the rice bread samples but were not detected in wheat bread. In the rice bread crust 1-octene-3-one, ( E)-2-nonenal, and 4-methylquinazoline reached significantly higher FD factors than in the wheat bread crust, while those of maltol and 2-methoxy-4-vinylphenol were significantly lower. In a stored rice flour and in the corresponding rice bread, lipid degradation products such as hexanal were more intense compared to fresh rice flour. Finally, five aroma-active compounds, namely, 2-butyl-2-heptenal, 2-propyl-2-octenal, 4-hydroxy-2-octenoic acid lactone, 4-hydroxy-2-nonenoic acid lactone, and 4-methylquinazoline, were identified for the first time in rice-containing products.

Identification and Quantitation of the Lipation Product 2-Amino-6-(3-methylpyridin-1-ium-1-yl)hexanoic Acid (MP-Lysine) in Peanuts

The lipid peroxidation product acrolein was semiquantitated by GC-MS (EI) in unheated and heated peanut oil, respectively, representing a model system for peanut roasting. Depending on the heating time, acrolein levels significantly increased from 0.2 to 10.7 mg/kg oil. As a result of heating N(α)-acetyl-l-lysine and acrolein, the pyridinium derivative 2-acetamido-6-(3-methylpyridin-1-ium-1-yl)hexanoic acid (MP-acetyl lysine) was identified. In addition, the lysine derivative 2-amino-6-[5-(hydroxymethyl)-3,6-dihydro-2H-pyridin-1-yl]hexanoic acid was identified after reduction and hydrolysis. After preparation of 2-amino-6-(3-methylpyridin-1-ium-1-yl)hexanoic acid (MP-lysine) as reference material, its amounts were quantitated in acrolein-modified peanut proteins by HPLC-ESI-MS/MS after acid hydrolysis, showing that at low acrolein concentrations, the modification of lysine could be entirely explained by the formation of MP-lysine. Furthermore, for the first time, MP-lysine was quantitated in peanut samples in amounts up to 10.2 mg/kg, showing an increase depending on the roasting time. Thus, MP-lysine might represent a marker to evaluate the extent of food protein lipation by acrolein.

Glycation Reactions of Casein Micelles

After suspensions of micellar casein or nonmicellar sodium caseinate had been heated, respectively, in the presence and absence of glucose for 0-4 h at 100 °C, glycation compounds were quantitated. The formation of Amadori products as indicators for the "early" Maillard reaction were in the same range for both micellar and nonmicellar caseins, indicating that reactive amino acid side chains within the micelles are accessible for glucose in a comparable way as in nonmicellar casein. Significant differences, however, were observed concerning the formation of the advanced glycation end products (AGEs), namely, N(ε)-carboxymethyllysine (CML), pyrraline, pentosidine, and glyoxal-lysine dimer (GOLD). CML could be observerd in higher amounts in nonmicellar casein, whereas in the micelles the pyrraline formation was increased. Pentosidine and GOLD were formed in comparable amounts. Furthermore, the extent of protein cross-linking was significantly higher in the glycated casein micelles than in the nonmicellar casein samples. Dynamic light scattering and scanning electron microscopy showed that glycation has no influence on the size of the casein micelles, indicating that cross-linking occurs only in the interior of the micelles, but altered the surface morphology. Studies on glycation and nonenzymatic cross-linking can contribute to the understanding of the structure of casein micelles.

4-Hydroxy-2-nonenal (4-HNE) and Its Lipation Product 2-Pentylpyrrole Lysine (2-PPL) in Peanuts

After synthesis of a deuterated 4-hydroxy-2-nonenal (4-HNE) standard, the formation of 4-HNE during heating of peanut oil and whole peanuts, respectively, was measured by GC-MS. Whereas a significant increase in 4-HNE levels was observed for peanut oil, the amount of 4-HNE decreased when whole peanuts were roasted due to lipation reactions with amino acid side chains of the proteins. The ε-amino group of lysine was identified as the favored reaction partner of 4-HNE. After heating N(α)-acetyl-l-lysine and 4-HNE, a Schiff base, a novel pyridinium derivative, a 2-pentylpyrrol derivative and, following reduction and hydrolysis, a reduced, cyclized Michael adduct were identified. 2-Amino-6-(2-pentyl-1H-pyrrol-1-yl)hexanoic acid (2-PPL) was synthesized and quantitated in peanut proteins, which had been incubated with various amounts of 4-HNE by HPLC-ESI-MS/MS after enzymatic hydrolysis. At low 4-HNE concentrations the modification of lysine could be entirely explained by the formation of 2-PPL. Additionally, 2-PPL was quantified for the first time in peanut samples, and an increase depending on the roasting time was observed. 2-PPL represents a suitable marker to evaluate the extent of food protein lipation by 4-HNE.