N-Substituted pyrroles (1), 2-furaldehyde (2), and N-substituted 2-formylpyrroles (3), formed in pentose (hexose) Maillard systems, were identified as components of extraordinary polycondensation activity. The polycondensation was studied in model experiments with 2), and N-(2-methoxycarbonylethyl)pyrrole (1b)/2-furaldehyde (2), respectively. MALDI-TOF-MS spectra indicated regular oligomers of up to 15-30 methine-bridged N-methyl(or N-2-methoxycarbonylethyl)pyrroles. With participation of aldehyde 2, furan rings instead of pyrrole rings were incorporated. The oligomers 5-11 were isolated and identified by MS and NMR techniques. A complementary experiment with N-methyl-2-[ 13 C]formylpyrrole ([ 13 CHO]-3a)/N-methylpyrrole (1a) was performed. The relevance of the new (type II) melanoidin-like oligomers/polymers in Maillard reactions is discussed and, in conclusion, a corresponding structure for native melanoidins is proposed. The oligomers 5, 6, 8, and 9 were tested for antioxidative activity in an iron(III) thiocyanate assay.
Keywords: Model compounds for melanoidins; pyrroles from pentoses and hexoses; β-dicarbonyl pathway of the Maillard reaction; polycondensation of N-methylpyrrole with N-methyl-2-formylpyrrole (N-methyl-2-[ 13 C]formylpyrrole) or 2-formylfuran; polycondensation of N-(2-methoxycarbonylethyl)pyrrole with N-(2-methoxycarbonylethyl)-2-formylpyrrole or 2-formylfuran; MALDI-TOF-MS analysis of melanoidin-like oligomers/polymers; antioxidative activity of oligomeric model compounds
In the 2-deoxy-D-ribose/methyl 4-aminobutyrate Maillard system a trapped N-substituted 2-(hydroxymethyl)pyrrole is one of the major products. However, nontrapped representatives of this type of compound were hitherto not found in other Maillard model systems, indicating their extraordinary reactivity. Model experiments with 2-deoxy-D-ribose/methylamine enabled the detection of N-methyl-2-(hydroxymethyl)pyrrole (1) and some derived linear oligomers (2, 3) as minor components. Consequently, 1 was synthesized and its oligomerization studied under very mild acidic conditions. The deformylated dimeric bis(N-methyl-2-pyrrolyl)methane (2) and trimeric N-methyl-2,5-bis(N-methyl-2-pyrrolylmethyl)pyrrole (3) were characterized by GC/MS and NMR. Higher regular oligomers up to 6 N-methyl-2-pyrrolylmethyl units as well as corresponding dehydro-oligomers up to 12 units were identified by MALDI-TOF-MS. A complementary experiment starting with N-methyl-2-hydroxy[(13)C]methylpyrrole ([(13)CH(2)OH]-1) confirmed the structure and the oligomerization pathway. The possible significance of this type of model oligomer for the melanoidin formation in Maillard reactions is discussed. The antioxidative activity of the isolated dimer and trimer was tested in Fe(III)-thiocyanate and DPPH assays.
In a series of model experiments L-proline was heated with monosaccharides for 1.5 h at 150 °C as well as pyrrolidine with D-glucose at 100 °C. Nineteen pyrrolidines and four piperidines were characterized and quantified by capillary GC-MS and nitrogen selective detector. Individual components were isolated by preparative GC and investigated by MS, IR, and NMR spectroscopy. 3-(l-Pyrrolidinyl)-2-butanone, l-furfurylpyrrolidine, 2-acetyl-3-(l-pyrrolidinyl)furan, 3-(l-pyrrolidinyl)tetrahydro-2-furanone, 2hydroxy-1-(1-pyrrolidinyl)-l-buten-3-one, 2-acetyl-, 2-propionyl-, 2-(2-furyl)-, and 2-(5-methyl-2-furyl)piperidine were identified for the first time as proline specific Maillard products.
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