Pyridoxamine, a scavenger agent of carbohydrates

Theoretical Study of the Iron Complexes with Aminoguanidine: Investigating Secondary Antioxidant Activity

A thorough analysis of the thermodynamic stability of various complexes of aminoguanidine (AG) with Fe(III) at a physiological pH is presented. Moreover, the secondary antioxidant activity of AG is studied with respect to its kinetic role in the Fe(III) reduction to Fe(II) when reacting with the superoxide radical anion or ascorbate. Calculations are performed at the M05(SMD)/6-311+G(d,p) level of theory. Solvent effects (water) are taken into account in both geometry optimizations and frequency calculations employing the SMD solvation method. Even though the results of this study show that AG can form an extensive number of stable complexes with Fe(III), none of these can reduce the rate constant of the initial step of the Haber-Weiss cycle when the reducing agent is O2•-. However, when the reductant is the ascorbate anion, AG is capable of reducing the rate constant of this reaction significantly, to the point of inhibiting the production of •OH radicals. In fact, the most stable complex of Fe(III) with AG, having a ∆Gf° of -37.9 kcal/mol, can reduce the rate constant of this reaction by 7.9 × 105 times. Thus, AG possesses secondary antioxidant activity relative to the Fe(III)/Fe(II) reduction with ascorbate, but not with O2•-. Similar results have also been found for AG relative to the Cu(II)/Cu(I) reduction, in agreement with experimental results.

Theoretical Study of the Copper Complexes with Aminoguanidine: Investigating Secondary Antioxidant Activity

A systematic study of the thermodynamic stability of various Cu(II) complexes with aminoguanidine (AG) is performed, together with the study of its secondary antioxidant activity. Calculations have been carried out at the M05(SMD)/6-311+G(d,p) level of theory using water as the solvent. The results obtained indicate that AG is capable of forming a wide array of stable coordination compounds with Cu(II) under physiological pH conditions, and it possesses some degree of secondary antioxidant activity when coordinating to copper. The most thermodynamically stable complex can slow down 2.8 times the first step of the Haber-Weiss cycle (from 7.71 × 109 to 2.80 × 109 M-1 s-1) and slightly reduce the potential damage that the formation of •OH radicals can cause. The results of this research add to previous knowledge on this molecule, which could be used as a potential glycation inhibitor.

Isolation, identification, and proposed formation mechanism of a novel hydrophilic compound formed by Maillard reaction between pyridoxamine and pentose

Pyridoxamine (PM) could competitively protect amino groups in proteins from glycating agents. Although PM is expected to react with saccharides, available data therein are limited. In this study, a novel hydrophilic compound from a model reaction solution containing PM and xylose was isolated and identified as (6aR,9aR)-1,8,9-trihydroxy-2,6a-dimethyl-6a,9a-dihydrocyclopenta[5,6]pyrano[3,4-c]pyridin-7(5H)-one with a tricyclic structure. This compound appeared to be specifically formed from pentose via 1-deoxypentosone, and its formation was facilitated over a pH range of 7.0-8.0. After heating at 90 °C for 5 h in a reaction mixture containing 30 mM PM and pentose at pH 7.4, this compound was obtained at a yield of 6.95-8.53 mM.

How Does Pyridoxamine Inhibit the Formation of Advanced Glycation End Products? The Role of Its Primary Antioxidant Activity

Pyridoxamine, one of the natural forms of vitamin B6, is known to be an effective inhibitor of the formation of advanced glycation end products (AGEs), which are closely related to various human diseases. Pyridoxamine forms stable complexes with metal ions that catalyze the oxidative reactions taking place in the advanced stages of the protein glycation cascade. It also reacts with reactive carbonyl compounds generated as byproducts of protein glycation, thereby preventing further protein damage. We applied Density Functional Theory to study the primary antioxidant activity of pyridoxamine towards three oxygen-centered radicals (•OOH, •OOCH3 and •OCH3) to find out whether this activity may also play a crucial role in the context of protein glycation inhibition. Our results show that, at physiological pH, pyridoxamine can trap the •OCH3 radical, in both aqueous and lipidic media, with rate constants in the diffusion limit (>1.0 × 108 M - 1 s - 1 ). The quickest pathways involve the transfer of the hydrogen atoms from the protonated pyridine nitrogen, the protonated amino group or the phenolic group. Its reactivity towards •OOH and •OOCH3 is smaller, but pyridoxamine can still scavenge them with moderate rate constants in aqueous media. Since reactive oxygen species are also involved in the formation of AGEs, these results highlight that the antioxidant capacity of pyridoxamine is also relevant to explain its inhibitory role on the glycation process.

Design, synthesis, and herbicidal activity of novel 2-(arylamino)-5-methyl-4-methylene-7-(methylthio)-4H-pyrido[4,3-d][1,3]oxazine-8-carbonitrile derivatives

A series of 5-methyl-4-methylene-7-methylthio-2-arylmino-4 H-pyrido[4,3- d][1,3]oxazine-8-carbonitrile derivatives were synthesized via tandem aza-Wittig and annulation reactions with { N-[3-acetyl-5-cyano-2-methyl-6-(methylthio)pyridin-4-yl]imino}triphenylphosphorane and aryl isocyanate in dry dichloromethane. Their structures were clearly confirmed by infrared, 1H NMR, 13C NMR, high-resolution mass spectrometry, and X-ray single-crystal diffraction. All newly synthesized compounds were screened for herbicidal activities against monocotyledonous and dicotyledonous plants. The results indicated that the target compound 2-[(4-methoxyphenyl)amino]-5-methyl-4-methylene-7-(methylthio)-4 H-pyrido[4,3- d][1,3]oxazine-8-carbonitrile showed 100% inhibition rate to grain sorghum (monocotyledonous) at the concentration of 100 mg L−1.

A conceptual DFT study of the molecular properties of glycating carbonyl compounds

Several glycating carbonyl compounds have been studied by resorting to the latest Minnesota family of density functional with the objective of determinating their molecular properties. In particular, the chemical reactivity descriptors that arise from conceptual density functional theory and chemical reactivity theory have been calculated through a [Formula: see text]SCF protocol. The validity of the KID (Koopmans' in DFT) procedure has been checked by comparing the reactivity descriptors obtained from the values of the HOMO and LUMO with those calculated through vertical energy values. The reactivity sites have been determined by means of the calculation of the Fukui function indices, the condensed dual descriptor [Formula: see text] and the electrophilic and nucleophilic Parr functions. The glycating power of the studied compounds have been compared with the same property for simple carbohydrates.Graphical abstractSeveral glycating carbonyl compounds have been studied by resorting to the latest Minnesota family of density functional with the objective of determinating their molecular properties, the chemical reactivity descriptors and the validity of the KID (Koopmans' in DFT) procedure.

DFT study of the mechanism of the reaction of aminoguanidine with methylglyoxal

We have studied the mechanism of the reaction between aminoguanidine (AG) and methylglyoxal (MG) by carrying out Dmol3/DFT calculations, obtaining intermediates, transition-state structures, and free-energy profiles for all of the elementary steps of the reaction. Designed models included explicit water solvent, which forms hydrogen-bond networks around the reactants and intermediate molecules, facilitating intramolecular proton transfer in some steps of the reaction mechanism. The reaction take place in four steps, namely: (1) formation of a guanylhydrazone-acetylcarbinol adduct by condensation of AG and MG; (2) dehydration of the adduct; (3) formation of an 1,2,4-triazine derivative by ring closure; and (4) dehydration with the formation of 5-methyl 3-amino-1,2,4-triazine as the final product. From a microkinetic point of view, the first dehydration step was found to be the rate-determining step for the reaction, with the reaction having an apparent activation energy of 12.65 kcal mol⁻¹. Additionally, some analogous structures of intermediates and transition states for the reaction between AG and 2,3-dicarbonyl-phosphatidylethanolamine, a possible intermediate in Amadori-glycated phosphatidylethanolamine (Amadori-PE) autooxidation, were obtained to evaluate the reaction above a phosphatidylethanolamine (PE) surface. Our results are in agreement with experimental results obtaining by other authors, showing that AG is efficient at trapping dicarbonyl compounds such as methylglyoxal, and by extension these compounds joined to biomolecules such as PE in environments such as surfaces and their aqueous surroundings.

Experimental and computational study on molecular structure and vibrational analysis of 4,5-Bis(hydroxymethyl)-2-methylpyridin-3-ol by normal coordinate treatment

The experimental and theoretical vibrational spectra of 4,5-Bis(hydroxymethyl)-2-methylpyridin-3-ol were investigated. The experimental FT-IR (450-4000cm(-1)) and FT-Raman spectra (100-3600cm(-1)) of the molecule in the solid phase were recorded. Theoretical vibrational frequencies and geometric parameters (bond lengths and bond angles) were calculated using ab initio Hartree Fock (HF) and density functional B3LYP method with 6-31G(d), 6-311++G(d,p) basis sets by Gaussian program, for the first time. The complete assignments were performed on the basis of the potential energy distribution (PED) of the vibrational modes, calculated with scaled quantum mechanical (SQM) method. The formation of the hydrogen bond was investigated using NBO calculations. The calculated HOMO and LUMO energies show that charge transfer occur within the molecule.

The hydrophobic substituent in aminophospholipids affects the formation kinetics of their Schiff bases

Schiff bases (SBs) are the initial products of non-enzymatic glycation reactions, which are associated to some diabetes-related diseases. In this work, we used physiological pH and temperature conditions to study the formation kinetics of the SBs of 1,2-dihexanoyl-sn-glycero-3-phosphoethanolamine (DPHE) and 1,2-dihexanoyl-sn-glycero-3-phospho-l-serine (DHPS) with various glycating compounds and with pyridoxal 5'-phosphate (an effective glycation inhibitor). Based on the obtained results, the hydrophobic environment simultaneously decreases the nucleophilic character of the amino group (k1) and increases its pKa, thereby increasing the formation rate of SB (kobs). Therefore, the presence of hydrophobic chains in aminophospholipids facilitates the formation and stabilization of SBs, and also, in a biological environment, their glycation. Additionally, the results confirm the inhibitory action of B6 vitamers on aminophospholipid glycation.

Carbonylation of myofibrillar proteins through the maillard pathway: effect of reducing sugars and reaction temperature

Carbonylation is recognized as one of the most remarkable chemical modifications in oxidized proteins and is generally ascribed to the direct attack of free radicals to basic amino acid residues. The purpose of this work was to investigate the formation of specific carbonyls, α-aminoadipic and γ-glutamic semialdehydes (AAS and GGS, respectively), in myofibrillar proteins (MP) through a Maillard-type pathway in the presence of reducing sugars. The present study confirmed the concurrent formation of protein carbonyls and advanced glycation end-products (AGEs) during incubation (80 °C/48 h) of MP (4 mg/mL) in the presence of reducing sugars (0.5 M). Copper irons (10 μM) were found to promote the formation of protein carbonyls, and a specific inhibitor of the Maillard reaction (0.02 M pyridoxamine) blocked the carbonylation process which emphasize the occurrence of a Maillard-type pathway. The Maillard-mediated carbonylation occurred in a range of reducing sugars (0.02-0.5 M) and reaction temperatures (4-110 °C) compatible with food systems. Upcoming studies on this topic may contribute further to shed light on the complex interactions between protein oxidation and the Maillard reaction and the impact of the protein damage on food quality and human health.

Role of pyridoxamine in the formation of the Amadori/Heyns compounds and aggregates during the glycation of beta-lactoglobulin with galactose and tagatose

The effect of pyridoxamine on the Maillard reaction during the formation of conjugates of beta-lactoglobulin with galactose and tagatose under controlled conditions (pH 7, 0.44 aw, 40 and 50 degrees C, for 6 days) has been studied, for the first time, by means of the changes in reducing carbohydrates, formation of Amadori or Heyns compounds, and aggregates and browning development. The results showed the formation of interaction products between pyridoxamine and galactose or tagatose either in the presence or in the absence of beta-lactoglobulin, indicating that pyridoxamine competes with the free amino groups of beta-lactoglobulin for the carbonyl group of both carbohydrates. Thus, a small inhibitory effect of pyridoxamine on the initial stages of the Maillard reaction was pointed out. Furthermore, much lower aggregation and color formation rates were observed in the conjugates of beta-lactoglobulin galactose/tagatose with pyridoxamine than without this compound, supporting its potent inhibitory effect on the advanced and final stages of the Maillard reaction. These findings reveal the usefulness of food-grade inhibitors of the advanced stages of the Maillard reaction, such as pyridoxamine, that, in combination with mild storage conditions, could lead to the formation of safer neoglycoconjugates without impairing their nutritional quality.