Study On The Formation Law And Mechanism Of Harman And Norharman Participated In α-Dicarbonyl Compounds In Simulation System

Norharman and Harman are the two major β-carboline species in heterocyclic amines.They are not mutagenic,but they enhance the toxicity of other heterocyclic amines.Alpha-dicarbonyl compounds are important intermediates in Maillard and caramelization reactions and are widely present in food systems with high chemical activity.In this paper,a-dicarboxylic compounds were used as the cut-in point to investigate the role and mechanism of a-dicarboxylic compounds in Norharman and Harman formation.The density functional theory was used to calculate the reaction of a-dicarboxylic compounds with tryptophan to form Norharman and Harman.The reaction pathway was investigated by various wave function analysis methods.It has significant theoretical and practical significance for controlling the formation of heterocyclic amines in food heat processing and improving the quality,nutrition and safety of food.Glyoxal,pyruvaldehyde and diacetyl were selected as the representative of a-dicarboxylic compounds,and their simulation systems of co-heating with tryptophan were established.The results confirmed the influence of a-dicarboxylic compounds on the formation of Norharman and Harman.Glyoxal promoted the formation of Norharman and Harman most significantly,and increased with the increase of glyoxal concentration;pyruvic aldehyde promoted the formation of Harman at a ratio of 1:0.2 and 1:1,but with It decreased with increasing concentration;it inhibited the formation of Norharman at a ratio of 1:1 and 1:5.The effect of butanone on the formation of Harman is increased with the increase of the ratio of butanone.The amount of Norharman decreased slightly with the increase of the ratio of succinaldehyde,but it was not significant.Glyoxal has a significant effect on the formation of Norharman and Harman in the simulated system at 70℃,100℃,and 150℃,but the results are not the same.Glyoxal promoted the formation of Norharman and Harman during a period of heating at 70℃ for 26 h and heating at 100℃ for 20 h.The simulation system is completely different from 70 or 100℃ at 150℃.The presence of glyoxal inhibits the formation of Norharman and Harman,indicating that the higher temperature heating reaction of tryptophan alone has a more efficient reaction mechanism.The calculation was performed on the 6-311G(d,p)basis set level using the M062X method of density functional theory.The first step in the formation mechanism of Norharman and Harman is the Pictet-Spengler reaction of tryptophan with an a-dicarboxylic compound.Tryptophan can also be decarboxylated first,but the thermal decarboxylation energy barrier is higher,and the intermediate after decarboxylation reacts with glyoxal P-S.The decarboxylation reaction was carried out on both the Norharman and Harman generation pathways,and three reaction pathways were calculated based on water catalysis and no water.The reaction energy barrier of the catalytic decarboxylation in the Harman formation pathway is as high as 335.8 kJ/mol,which is similar to the energy barrier of thermal decarboxylation of tryptophan 337.7 kJ/mol.The transition state of the six-membered ring in the non-catalyzed decarboxylation reaction in the Norharman generation pathway reduced the energy barrier to 257.8 kJ/mol.The first step in the process of water-catalyzed decarboxylation in the Norharman generation pathway is the addition reaction of water molecules to the carboxyl group.Thereafter,H3C03+is removed,and the reaction energy barrier is 253.8 kJ/mol lower than the non-catalytic decarboxylation reaction.The Norharman generation mechanism process undergoes an intramolecular hydrogen transfer reaction and a ring-closing reaction to form an intermediate containing a three-membered ring with an energy barrier of 119.2 kJ/mol.Thereafter,the 1,2 rearrangement of the aldehyde group occurs in the absence of water and water catalysis,and is transferred to the nitrogen atom.Water molecules have a significant catalytic effect,reducing the energy barrier by 92.5 kJ/mol.The final step is the intramolecular hydrogen transfer reaction to form formaldehyde and Norharman.The first step in the subsequent part of the Harman generation mechanism is the intramolecular hydrogen transfer reaction to form a hydroxyl group.Thereafter,the hydrogenation reaction and the dehydration reaction are carried out,and further hydrogenation reaction is carried out to change the aldehyde group into a methyl group.Hydrogen transfer reactions also play an important role.The last three reactions are the departure of hydrogen atoms on the ring,and Harman is generated after the intermolecular hydrogen transfer reaction occurs.