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Density functional theory method and B3LYP/6‐311++G(d,p) level of theory were used to determine the acidity of α‐carbon in the hydroxyl derivatives of beta diketones in the gas phase. An investigation of acidity strength in the gas phase indicates that α‐carbon of hydroxyl derivatives of beta diketones become stronger acids than the α‐carbon of beta diketone itself as their conjugate bases gain more stability via both enolate and hydrogen bond formation. Natural bond orbital and quantum theory of atoms in molecules analyses also confirm the role of hydrogen bond interactions on increasing the acidity of α‐carbon of hydroxyl derivatives of beta diketones. [ABSTRACT FROM AUTHOR]

The unusual monosaccharaides such as deoxy‐hexose sugars, including methyl‐pentose and aldo‐pentose, are promising and important sugars in life science. However, little research on H‐bond interactions in these systems has been reported. The aldo‐pentose has a proton instead of the CH2OH group on C5; conversely, methyl‐pentose has a CH3 group on C5. The aim of the present study is to investigate the role and nature of intramolecular H‐bonds on acidity of CH3‐pentose sugars (L‐fucose and L‐rhamnose) and aldo‐pentose sugars (D‐xylose, L‐lyxose, D‐ribose, and L‐arabinose) using B3LYP/6‐311++G (d, p) level. The calculated acidity values (ΔHacid) of these Dexoy‐hexose were found to be from 343 to 369 kcal.mol−1, indicating they are stronger acid than ethanol and 2‐propanol with the acidity values of 378.3 and 375.1 kcal.mol−1, respectively. This is related to the stabilization of the conjugate bases of these sugar through intramolecular H‐bonds, which were analyzed in this study using atoms in molecules (AIM) and natural bonding orbital (NBO) methods. AIM and NBO analyses indicate the presence of one bifurcated intramolecular H‐bond in the conjugate bases of L‐lyxose and L‐arabinose and two bifurcated H‐bonds in conjugate base of D‐ribose, whereas the conjugate bases of L‐fucose, L‐rhamnose, and D‐xylose present one normal intramolecular H‐bond. According to the topological parameters and charge transfer data, existence of normal and bifurcated intramolecular H‐bonds could greatly increase acidity of deoxy‐hexose sugars. The H‐bond strength in the conjugate bases of aldo‐pentose sugars is higher than that in the conjugate bases of methyl‐pentose sugars including CH3 group on C5, making aldo‐pentose sugars stronger acid than methyl‐pentose sugars. [ABSTRACT FROM AUTHOR]

• Acidity enhancement of bioorganic compounds. • Hydrogen bonding effects on acidity of bioorganic molecules. • Stability of the conjugate base via H-bond network. Density functional theory method and B3LYP/6-31+G(d) level of theory were used to predict the acidity of barbiturates and their corresponding metabolites in the gas and solution phase (H 2 O). Polarized continuum model was applied to calculate p K a values of barbiturates and metabolites. A comparison between acidity of barbiturates and metabolites in the gas and solution phase indicates that the acidity strength of barbiturates enhances with the increase of intramolecular hydrogen bonds in metabolites. This acidity can increase to 9.47 kcal/mol in gas phase and 2.73 p K a units in solution phase for a typical metabolite of barbiturate due to the effect of intramolecular hydrogen bonds. Also, linear correlation between gas phase acidity (ΔG acid) and solution phase (p K a) was obtained and can be used for p K a prediction of metabolites in solution phase. [ABSTRACT FROM AUTHOR]

The derivatives of glucose such as glucosamine (β-D-GlcN) and N-acetyl-D-β-glucosamine (GlcNAc) are significant in several biological systems. D-GlcN has been used widely to treat osteoarthritis in humans and animal models as well as GlcNAc has been proposed as a treatment for autoimmune diseases. The DFT/B3LYP/6–311++G (d,p) method as well as QTAIM and NBO analyses were used to the acidity values of D-GlcN and GlcNAc sugars and their complexes with alkali ions in the gas phase. The Li+, Na+ and K+ prefer bi-dentate chelate in these complexes. The computed results indicate that metal ion affinity (MIA) in GlcNAc is higher than that in D-GlcN. There are direct correlations between the MIA values of D-GlcN and GlcNAc sugars and the atomic numbers of Li, Na, and K. The calculated acidity values for GlcNAc at C 2 –NH and C 6 –HO 6 sites are 331 and 333 kcal mol−1, respectively. Whereas the calculated acidity values for D-GlcN at C 2 –NH and C 6 –HO 6 sites are 365 and 372 kcal mol−1, respectively. The AIM and NBO analyses indicate the presence of intramolecular H-bonds in GlcNAc sugar in both its neutral form and conjugate base; whereas D-GlcN indicates intramolecular H-bonds in only its conjugate base. Image 1 • Hydrogen bonding effect on Acidity of d -Glucosamine and N-Acetyl- d -Glucosamine. • Metal Ion Affinity of d -Glucosamine and N-Acetyl- d -Glucosamine. • D-Glucosamine and N-acetyl-D-glucosamine as ideal cation exchangers models. [ABSTRACT FROM AUTHOR]

In this study, the effects of the remote intramolecular hydrogen bonding on the acidity of hydroxy‐1,4‐benzoquinone derivatives have been investigated ab initio by employing density functional theory (DFT) methods. The computational studies were performed for both gas and solution (H2O, DMSO, and MeCN solutions) phases. Our results indicated that remote hydrogen bonding could play an important role in increasing the acidity of hydroxy‐1,4‐benzoquinone. Noncovalent interaction reduced density gradient (NCI‐RDG) methods were used to visualize the attractive and repulsive interactions in the studied acids and their conjugate bases. Natural bond orbital (NBO) analysis was performed to confirm intramolecular hydrogen bonding effect on acidity. Quantum theory of atoms in molecules (QTAIM) was also used to study the nature of hydrogen bonds. QTAIM results showed that the intramolecular hydrogen bonds in these structures are electrostatic (closed‐shell) interactions in nature. In this study, the effects of the remote intramolecular hydrogen bonding on the acidity of hydroxy‐1,4‐benzoquinone derivatives are explored by using ab initio and density functional theory (DFT) calculations. It was found that remote hydrogen bonding could play a surprisingly role in increasing the acidity of hydroxy‐1,4‐benzoquinone. [ABSTRACT FROM AUTHOR]