Novel Maillard Pigment, Furpenthiazinate, Having Furan and Cyclopentathiazine Rings Formed by Acid Hydrolysis of Protein in the Presence of Xylose or by Reaction between Cysteine and Furfural under Strongly Acidic Conditions

Abstract

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.

Conditions and Mechanism of Formation of the Maillard Reaction Pigment, Furpenthiazinate, in a Model System and in Some Acid Hydrolyzates of Foods and its Biological Properties

“…The structure of furpenthiazinate suggests that it is formed from one molecule of cysteine, one molecule of furfural, and one molecule of a C4 fragment that appears to be derived from furfural (Figure 1). 20 However, the exact origin of the furfural C4 fragment remains unclear. To examine this, furpenthiazinate was prepared from two types of D-ribose labeled with one 13 C (1-13 C, the 1-position of the carbon atom was labeled; 5-13 C, the 5-position of the carbon atom was labeled) and analyzed using LC-MS/MS and NMR to determine which degradation product of pentose forms furpenthiazinate.…”

“…The furpenthiazinate standard was prepared as described previously. 20 Preparation of Cysteine-Furfural Solutions to Examine the Effect of Reaction Conditions on the Formation of Furpenthiazinate. For the effect of cysteine to furfural molar ratio, 2 M of HCl (5 mL) containing L-cysteine (0, 4, 8, 16, 32, 64, or 128 mM) and furfural (128 mM) was placed in a test tube with a cap and heated at 110 °C for 1 h. For the effect of the concentrations of cysteine and furfural, 2 M HCl (5 mL) containing L-cysteine (0, 16, 32, 64, or 128 mM) and furfural at 4 times as much concentration (0, 64, 128, 256, and 512 mM) as that of cysteine was placed in a test tube with a cap and heated at 110 °C for 1 h. For the effect of incubation time, a solution (5 mL) containing 64 mM L-cysteine, 256 mM furfural, and 2 M HCl was placed in a test tube with a cap and heated at 110 °C for 0, 0.5, 1, 2, 4, 8, and 24 h. For the effect of HCl concentrations, HCl (0, 0.001, 0.01, 0.1, 1, 2, or 6 M, 5 mL) containing 64 mM L-cysteine and 256 mM furfural were placed in a test tube with a cap and heated at 110 °C for 24 h. For the effect of oxygen, a reaction solution (5 mL) containing 64 mM L-cysteine, 256 mM furfural, and 2 M HCl was placed in a test tube with a cap, which was flushed with nitrogen before heating at 110 °C for 1 or 24 h.…”

“…Instead, a novel yellow pigment, furpenthiazinate, was isolated and identified (Figure 1). 20 Furpenthiazinate is also formed by the reaction between cysteine and furfural, and its structure contains a cyclopentathiazine ring and a furan ring. Cyclopentathiazines can be synthesized from α-keto spiro-thiazolidine 21 or by the reaction between cysteine and cyclopentanediones.…”

“…Figure1. Schematic depicting the formation of furpenthiazinate obtained from a previous study with some modifications 20. Notes: Furpenthiazinate was formed during the acid hydrolysis of soy protein in the presence of xylose via the Maillard reaction between cysteine and furfural.…”

What are melanoidins, polymers in food?

“…and by the reaction between two amino groups of arginine residues and furfural (b) [59]. furpenthiazinate (11 in Figure 5), was identified [71]. Although the relationship between these low-molecular-weight pigments and the formation of melanoidins is not examined yet, these compounds have a carbonyl group or a part of the structure which may cause an aldol reaction.…”