Characterization of the Chemical Structure of Novel Colored Maillard Reaction Products from Furan-2-carboxaldehyde and Amino Acids

Abstract

Colored compounds formed by Maillard-type reactions from furan-2-carboxaldehyde and primary and secondary amino acids including l-alanine and l-proline, respectively, have been identified. When furan-2-carboxaldehyde was heated with l-proline in aqueous solution at pH 7.0, an intensely yellow compound was generated, which was identified as 5(S)-(2-carboxy-1-pyrrolidinyl)-2-hydroxy-(E,E)-2,4-pentadienal-(S)-(2-carboxypyrrolidine)imine (1) by application of several one- and two-dimensional NMR experiments and, in addition, by MS, UV, and IR spectroscopy. Further thermal treatment of compound 1 resulted, upon a ring closure reaction, in the formation of (E)-4,5-bis[(S)-2-carboxy-1-pyrrolidinyl]-2-cyclopenten-1-one (5), which has been, to our knowledge, as yet not reported in the literature. To confirm the proposed structures, l-proline was substituted by pyrrolidine and piperidine, leading to analogous N-cyanines (2 and 3) and cyclopentenones (4 and 6). On the other hand, thermal treatment of an aqueous solution of furan-2-carboxaldehyde and l-alanine led to the formation of the novel red compounds 8a and 8b, which were identified as (S)-4-[(E)-1-formyl-2-(2-furyl)ethenyl]-5-(2-furyl)-2-[(E)-(2-furyl)methylidene]-2,3-dihydo-α-amino-3-oxo-1H-pyrrole-6-acetic acid and the corresponding 2-(Z)-(2-furyl)methylidene isomer. This is the first time that chromophoric compounds comprising four linked rings with an amino acid moiety incorporated were identified in a Maillard reaction system. Keywords: Nonenzymatic browning; Maillard reaction; furan-2-carboxaldehyde; alanine; proline; (S)-4-[(E)-1-formyl-2-(2-furyl)ethenyl]-5-(2-furyl)-2-[(E)-(2-furyl)methylidene]-2,3-dihydro-α-amino-3-oxo-1H-pyrrole-1-acetic acid