Our group has recently isolated and identified novel yellow compounds named dilysyl-dipyrrolones A (DPL A; 1) and B (DPL B; 2) in a heated aqueous solution containing xylose and lysine under weakly acidic conditions. In this study, we isolated and identified a novel DPL derivative (DPL C; 3), which has the same structure as DPL B, except for containing a hydroxymethyl group. To estimate the formation mechanism of DPL derivatives, (13)C-labeled DPLs were prepared and analyzed with LC/MS and NMR. (13)C-labeling experiments using [1-(13)C] ribose showed that the formation pathway of DPL C was different from those of DPLs A and B. In addition, (13)C-labeling experiments using [u-(13)C5] ribose and [1-(13)C] lysine showed that C-6 of a methine moiety in DPL C was derived from C-5 of ribose or acetic acid in buffer. Based on these results, we postulated the formation mechanism of DPLs. We then showed that DPLs A and B had potent antioxidant activities.
A novel
Maillard pigment having partial structures of furan and
cyclopentathiazine, named furpenthiazinate, was isolated and identified.
Although this pigment was found in an acid hydrolysate of a Maillard
reaction solution between soy protein and xylose, the same pigment
was also formed by the Maillard reaction under strongly acidic conditions
between soy protein and xylose and cysteine and furfural. The structure
of its reduced form by NaBH4 was determined by MS, NMR,
and X-ray analysis and identified as 7-(2-furanyl)-2,3,4,4a,5,6-hexahydrocyclopenta[b][1,4]thiazin-4-ium-3-carboxylate, indicating that the
chemical structure of furpenthiazinate is 7-(2-furanyl)-2,3,5,6-tetrahydrocyclopenta[b][1,4]thiazine-3-carboxylic acid. Furpenthiazinate showed
an absorption maximum at 400 nm and strong yellow color under acidic
and neutral conditions. The color contribution of furpenthiazinate
was estimated to be more than 60% in a reaction solution prepared
from cysteine and furfural.
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