Axial OH Groups Research Articles

Exploring the versatility of carbohydrate-capped green silver nanoparticles as multi-dimensional reagents for targeted applications

The present work describes the impact of the structural diversity of different carbohydrate molecules on the morphology and sensing ability of silver nanoparticles synthesized by a green protocol using Ocimum sanctum leaf extract, used as the reducing agent. Seven different monosaccharides were utilized as capping and stabilizing agents during the green synthesis of silver nanoparticles. The carbohydrate-capped nanoparticles were more stable in solution than the uncapped counterpart for at least three months. The formation and stability of the silver nanoparticles were analyzed using UV-visible, fluorescence spectroscopy, and X-ray diffraction studies, whereas the morphological characteristics were studied using scanning electron microscopic analysis. The different types of interaction of sugar molecules with silver atoms were studied using DFT calculation. Silver nanoparticles synthesized using d-galactose and l-arabinose, having an axial-OH group at the C-4 position, were found to have the smallest size with maximum efficacy as chemosensors for toxic mercury (II) ions in aqueous medium. The green silver nanoparticles were successfully employed as antimicrobial agents against different gram-positive and gram-negative bacteria. The catalytic activity of silver nanoparticles in the degradation of organic dyes was also evaluated for Congo red, methyl orange and reactive blue in the presence of sodium borohydride. The nanoparticles showed excellent dye degradation activity with a minimum time duration.

Coordination Engineering Induced d-Band Center Shift on Single-Atom Fe Electrocatalysts for Enhanced Oxygen Reduction.

Part of the performance of single-atom catalysts (SACs) is greatly influenced by the microenvironment around a single metal site, of which the oxygen reduction reaction (ORR) is counted as one of them. However, an in-depth understanding of the catalytic activity regulation by the coordination environment is still lacking. In this study, a single Fe active center with axial fifth hydroxyl (OH) and asymmetric N,S coordination embedded in a hierarchically porous carbon material (Fe-SNC) are prepared. Compared to Pt/C and most of the reported SACs, as-prepared Fe-SNC has certain advantages in terms of ORR activity and maintains sufficient stability. Furthermore, the assembled rechargeable Zn-air battery exhibits impressive performance. The combination of multiple findings revealed that the introduction of S atoms not only facilitates the formation of porous structures but also facilitates the desorption and adsorption of oxygen intermediates. On the other hand, with the introduction of axial OH groups, the bonding strength of the ORR intermediate is reduced, and even the central position of the Fe d-band is optimized. The developed catalyst is expected to lead to further research on the multiscale design of the electrocatalyst microenvironment.

Intramolecular O-H⋯O and C-H⋯O hydrogen bond cooperativity in D-glucopyranose and D-galactopyranose-A DFT/GIAO, QTAIM/IQA, and NCI approach.

Density functional theory calculations are used to compute proton nuclear magnetic resonance (NMR) chemical shifts, interatomic distances, atom-atom interaction energies, and atomic charges for partial structures and conformers of α-D-glucopyranose, β-D-glucopyranose, and α-D-galactopyranose built up by introducing OH groups into 2-methyltetrahydropyran stepwisely. For the counterclockwise conformers, the most marked effects on the NMR shift and the charge on the OH1 proton are produced by OH2, those of OH3 and OH4 being somewhat smaller. This argues for a diminishing cooperative effect. The effect of OH6 depends on the configuration of the hydroxymethyl group and the position, axial or equatorial, of OH4, which controls hydrogen bonding in the 1,3-diol motif. Variations in the interaction energies reveal that a "new" hydrogen bond is sometimes formed at the expense of a preexisting one, probably due to geometrical constraints. Whereas previous work showed that complexing a conformer with pyridine affects only the nearest neighbour, successive OH groups increase the interaction energy of the N⋯H1 hydrogen bond and reduce its length. Analogous results are obtained for the clockwise conformers. The interaction energies for C-H⋯OH hydrogen bonding between axial CH protons and OH groups in certain conformers are much smaller than for O-H⋯OH bonds but they are largely covalent, whereas those of the latter are predominantly coulombic. These interactions are modified by complexation with pyridine in the same way as O-H⋯OH interactions: the computed NMR shifts of the CH protons increase, the atom-atom distances are shorter, and interaction energies are enhanced.

Two‐Electron Oxidation of Water Through One‐Photon Excitation of Aluminium Porphyrins: Molecular Mechanism and Detection of Key Intermediates

AbstractThe reaction mechanism of photochemical water oxidation catalyzed by AlIII porphyrins (AlTIIIMPyP(OH)2) covalently adsorbed through the coordination of one axial OH group on the surface of TiO2 particles was investigated by time‐resolved nanosecond laser flash photolysis (NLFP). Three types of transient species with completely different absorption and decay characteristics were detected by means of NFLP, demonstrating that the photochemical production of H2O2 from water is initiated only by one‐photon excitation of the catalyst followed by stepwise two‐electron conversion processes, providing a promising model for photochemical water oxidation. Laser flash excitation of AlIIITMPyP adsorbed on a transparent TiO2 film leads to the formation of the AlTMPyP(OH) radical cation through one‐electron injection from the excited singlet state of the catalyst to the conduction band of TiO2. The radical cation is subsequently deprotonated into AlTMPyP(O.), which further reacts with OH−/H2O to form a key intermediate, the radical anion of AlTMPyP(O‐OH), having a characteristic O−O bond at the axial position within a delay time of 2–3 μs. Thereafter the radical anion efficiently transfers a second electron to TiO2 leading to the formation of AlTIIIMPyP(O‐OH) (≈67 μs), which reacts further with OH−/H2O to liberate free hydrogen peroxide as an end‐product with regeneration of the starting catalyst, AlTIIIMPyP(OH), (≈500 μs).

Octacationic and axially di-substituted silicon (IV) phthalocyanines for photodynamic inactivation of bacteria

The photodynamic inactivation of both Gram-positive and Gram-negative bacteria by polycationic silicon (IV) phthalocyanine photosensitizers bearing eight N-methylated pyridyloxy substituents in their periphery and different silyl ether moieties (OSi(iBu), OSi(nBu)3 and OSiPh3) at their axial positions has been evaluated. To this end, the influence of the nature of the axial and peripheral substituents on the spectroscopic, photochemical, photophysical and biological properties of the series of silicon (IV) phthalocyanines has been assessed. Specifically, this series of novel silicon phthalocyanines showed good fluorescence, singlet oxygen yields in both DMF and physiological media and no aggregation in aqueous media. Therefore, all the compounds were tested in Gram-positive and Gram-negative bacteria, such as Staphylococcus aureus (S. aureus), Methicillin Resistant Staphylococcus aureus (MRSA), and Escherichia coli (E. coli) obtaining the best results for the compound bearing axial OH groups. This fact, in combination with its strong absorption within the “phototherapeutic window” (650–850 nm) and its negligible dark cytotoxicity, yields the previous compound as a promising candidate for future studies.

Self-association of C3 and C6 epimers of hyodeoxycholate anions in aqueous medium: Hydrophobicity, critical micelle concentration and aggregation number

Hyodeoxycholic acids anion (HD) and it's 3- and 6-epimers (3e-HD and 6e-HD) are suitable for biopharmaceutical application since they possess α equatorial i.e. β axial OH groups so their membranotoxicity is reduced. Self-association of stereoisomers 3e-HD and 6e-HD has not been studied so far. According to the capacity factor (k, RPHPLC), hydrophobicity decreases in the following series: HD>3e-HD>6e-HD. However, critical micelle concentration (CMC) is the lowest for the 6e-HD (10mM, 25°C) and the highest for 3e-HD (38mM, 25°C), which is contrary to expectations according to their hydrophobicity. Aggregation number (n) has the highest value for 6e-HD (8, 25°C) and the lowest value for the stereoisomer 3e-HD (4, 25°C). The change in demicellisation capacity factor (∆Cp) decreases in the following series: 6e-HD (316Jmol−1K−1)>HD (260Jmol−1K−1)>3e-HD (181Jmol−1K−1). In the plane of ln k – ln CMC epimer 6e-HD is outliner related to the hydrophobic congeneric group. This suggests that, in formation of micelles of HD and 3e-HD, besides hydrophobic effect other interactions are neglected. However, in formation of 6e-HD micelle, beside hydrophobic effect significant interaction exists different from hydrophobic interaction (formation of hydrogen bonds between building units of the micelle).

Does the “C3 effect” offset the Δ2 effect, as regards the solution flexibility of aldoses?

The solution flexibility of carbohydrates influences a variety of molecular recognition and regulatory processes. For aldoses and other monosaccharides, this flexibility is dictated by the orientations of the various hydroxyl (OH) groups present, which influences conformer and anomer ratios, interactions among these OH groups, and interactions between them and the surrounding solvent. Depending on the number and position of axial OH groups, a variety of structures can coexist in solutions at equilibrium. In 1950, as part of his pioneering studies on the shape of pyranoside rings, Reeves described the Δ2 effect, the greater destabilization of the pyranose ring conformation when the OH group on carbon 2 (C2 ) is in the axial position. It was later proposed by Angyal that the Δ2 effect could be cancelled by the presence of an axial OH on C3 , termed here the "C3 effect." Employing size-exclusion chromatography, an entropically-controlled separation technique, we have investigated whether or not the C3 and Δ2 effects indeed do compensate for one another with respect to their influence on the solution flexibility of select aldohexoses and aldopentoses. As will be seen, while qualitatively and semiquantitatively this mutual cancellation of effects does occur in aldohexoses, it does not appear to do so in aldopentoses. An explanation for the latter appears to lie in the variety of anomers and conformers present in equilibrium solutions of those aldopentoses studied and in the relative entropic contribution of individual conformers or anomers to the total solution flexibility.

Glycolaldehyde dimer in the stable crystal phase has axial OH groups: Raman, infrared and X-ray data analysis

Although a small molecule and a simple 1,4-dioxane derivative, 2,5-dihydroxy-1,4-dioxane or glycolaldehyde dimer has a crystal structure that has eluded researchers so far, chief reason lying presumably in substance polymorphism. Here we report for the first time on the stable crystal structure of the glycolaldehyde dimer characterized by the X-ray powder diffraction method at room temperature. It crystallizes in the monoclinic system, space group P21/c, with unit cell parameters a=5.9473(1)Å, b=8.3997(1)Å, c=5.61953(8)Å and β=114.8814(9)°. The glycolaldehyde dimer molecules are the trans-isomers with the electronegative hydroxyl groups in axial positions and molecules are arranged in hydrogen bonded layers parallel to (100). Each layer is stabilized by intermolecular medium strong O–H⋯O hydrogen bonds. The 1,4-dioxane ring of the molecule adopts a chair conformation.Raman and infrared spectra were re-examined and assigned with the help of ab initio calculation followed by a normal modes analysis modes most sensitive to crystal packing were identified as the C–OH deformations which were assigned to the IR and Raman bands at 1239 and 1237cm−1, respectively; the two O–C–O deformation modes, both associated with the Raman and IR bands at 561 and 535cm−1 and, the two OH torsion modes assigned to the IR bands at 630 and 604cm−1.

An uncommon highly oxidized multiple bonded Re28+ species

A long-sought metal-metal bonded species with an M(2)(8+) core has been structurally characterized. The diamagnetic compound {[Re(2)(hpp)(3)(OH)(O(3)SCF(3))](2)(μ-O)(2)}(O(3)SCF(3))(2), 2, has two σ(2)π(4) electron-poor triple-bonded Re(2)(8+) units embraced by three bicyclic guanidinate ligands (hpp) and axial OH and triflate groups; the two dimetal units are held together by bridging oxo-groups.

Spontaneous Reduction of Mixed 2,2′‐Bipyridine/Methylamine/Chloro Complexes of PtIV in Water in the Presence of Light Is Accompanied by Complex Isomerization, Loss of Methylamine, and Formation of a Strong Oxidant, Presumably HOCl

Three 2,2'-bipyridine (2,2'-bpy) complexes of Pt(IV) have been synthesized, characterized by X-ray crystallography, and their solution behavior in D(2)O studied by (1)H NMR spectroscopic analysis: mer-[PtCl(3)(2,2'-bpy)(MeNH(2))]ClH(2)O (4), trans-[PtCl(2)(2,2'-bpy)(MeNH(2))(2)]Cl(2) (5), and trans-[Pt (2,2'-bpy)(MeNH(2))(2)(OH)(2)]Cl(2) (6; MeNH(2)=methylamine). Complexes 4 and 5 undergo hydrolysis of the Cl(-) ions, both in the dark and daylight, as evident from a drop in the pH value. Two solvolysis products were detected in the case of 4, which is indicative of species with equatorial and axial OH(-) groups. The hydrolysis reaction of 5 implies that an axial Cl(-) group is replaced by an OH(-) moiety; in contrast, 6 remains virtually unaffected. Ordinary daylight, in particular irradiation with a 50-W halogen lamp, initially causes ligand-isomerization processes, which are followed by the reduction of 4 and 5 to Pt(II) species. This reduction of 4 and 5 is accompanied by the formation of hypochlorous acid, as demonstrated qualitatively in the decoloration test of indigo, and loss of MeNH(2), which is particularly pronounced in the case of 5. The formation of Pt(II) compounds is established on the basis of the J coupling constants of (195)Pt with selected (1)H NMR resonances. The results obtained herein are possibly also relevant to the chemistry of Cl-containing Pt(IV) antitumor agents and their reactions with DNA.

5,7-Substituted thiazolo[2,3- a]pyrimidines: Synthesis, stereochemistry and crystal structure

Reaction of 2-aminothiazoline ( 1) with α,β-unsaturated carbonyl compounds 2 under mild conditions (acetone, room temperature) gives two diastereomers 5-R-7-hydroxy-5 H-tetrahydrothiazolo[2,3- a]pyrimidines 3. According to 1H NMR, major isomers of 3 have axial OH-groups whereas minor isomers have equatorial OH-groups. The X-ray investigation of compound 3i reveals only A type diastereomer in the crystal phase. The asymmetric unit contains two forms (A1 and A2) with slightly different geometrical parameters. Each of them consists of a pair of enantiomers E1 and E2. As a result, the asymmetric unit contains the centrosymmetric dimers (A1E1 … A1E2 and A2E2 … A2E1), due to the intermolecular hydrogen bonds. 5,7-Diaryl-5 H-2,3-dihydrothiazolo[2,3- a]pyrimidines 4 were obtained via reaction of 1 with 2 under stronger conditions (DMF or chloroform, heating). Structure of product 4p was confirmed by X-ray structural analysis.

Electronic and infrared spectroscopy of jet-cooled (±)-cis-1-amino-indan-2-ol hydrates

The role of conformational isomerism in molecular interaction has been studied using the example of jet-cooled complexes of (+/-)-cis-1-amino-indan-2-ol with water. The two formerly evidenced conformers of (+/-)-cis-1-amino-indan-2-ol easily form hydrates and dihydrates, which have been studied by means of laser-induced fluorescence and IR/UV double resonance spectroscopy, as well as ab initio calculations. All the 1 : 1 and 1 : 2 complexes with water evidenced in this work involve "ring" structures, in which the water monomer or dimer acts as an acceptor from the NH(2) and a donor to the OH groups of (+/-)-cis-1-amino-indan-2-ol. However, the water lies externally to the indan frame in the hydrates of conformer I of (+/-)-cis-1-amino-indan-2-ol, which possesses axial NH(2) and equatorial OH groups, and above it for the hydrates with the less stable conformer II, with equatorial NH(2) and axial OH groups. Consequently, the different steric constraints which exist in the two conformers result in different hydrogen bond topologies, with an additional OH[dot dot dot]pi interaction for the hydrates of conformer II.

Stereoelectronic Substituent Effects

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Anti-2-Hydroxy-2-methyl-1-tetralone oxime: X-ray and density functional theory study

Crystals of the title racemic compound, C 11 H 13 NO 2 , consist of two types of molecules (conformers); one molecule has an exocyclic OH group in an equatorial position and the other has this group in an axial position. Consequently, the hydrogen-bond schemes for the two molecules are different. The molecules with equatorial OH groups create infinite parallel chains (formed by the same enantiomer), connected by centrosymmetric dimers of molecules (of mixed enantiomers), both with axial OH groups. Possible interconversion of the conformers and the flexibility of the molecule were studied by means of different MP2 and density functional theory (DFT) methods. The optimization of the structure by the DFT method confirmed the types of the hydrogen bonds.

Stereoelectronic Substituent Effects

An investigation was carried out on the influence of the stereochemistry of substituents, particularly hydroxyl groups, on their electronic effects in piperidines, carbohydrates (pyranosides), and related compounds. Polar groups, such as OH, OR, and F, were found in the 3 and 4 position to be much more electron-withdrawing when positioned equatorially rather than axially. In contrast, little difference in electronic effects was observed from apolar groups as a result of epimerization. These observations were believed to be caused by differences in charge-dipole interactions and were used to explain why stereoisomeric glycosides hydrolyze with different rates. The conformational changes of hydroxylated piperidines and related compounds as a function of pH were likewise explained from the different substituent effects of axial and equatorial OH groups.

Conformational effects on glycoside reactivity: study of the high reactive conformer of glucose.

The effect of conformation on glycoside reactivity was investigated by studying the hydrolysis of a selection of 3,6-anhydroglucosides as models for glucose in the highly reactive (1)C(4) conformation. Methyl 3,6-anhydro-beta-D-glucopyranoside was found to hydrolyze 200-400 times faster than methyl glucosides in the (4)C(1) conformation, while methyl 3,6-anhydro-beta-D-galactopyranoside, which is in the B(1,4) conformation, was less reactive than methyl beta-D-galactopyranoside. Methyl (3,6-anhydro-beta-D-glucopyranosyl)-(1 --> 6)-alpha-D-glucopyranoside, methyl (3,6-anhydro-alpha-D-glucopyranosyl)-(1 --> 6)-alpha-D-glucopyranosyl-(1 --> 6)-alpha-D-glucopyranoside, and methyl (3,6-anhydro-beta-D-glucopyranosyl)-(1 --> 6)-alpha-D-glucopyranosyl-(1 --> 6)-alpha-D-glucopyranoside were prepared and found to react selectively at the anhydro residue. The finding that (1)C(4) conformers of glucosides are highly reactive species is in accordance with and supports previous results showing that axial OH groups are less electron withdrawing than equatorial OH groups.

Hydrogen-bonding cooperativity: using an intramolecular hydrogen bond to design a carbohydrate derivative with a cooperative hydrogen-bond donor centre.

Neighbouring groups can be strategically located to polarise HO.OH intramolecular hydrogen bonds in an intended direction. A group with a unique hydrogen-bond donor or acceptor character, located at hydrogen-bonding distance to a particular OH group, has been used to initiate the hydrogen-bond network and to polarise a HO.OH hydrogen bond in a predicted direction. This enhanced the donor character of a particular OH group and made it a cooperative hydrogen-bond centre. We have proved that a five-membered-ring intramolecular hydrogen bond established between an amide NH group and a hydroxy group (1,2-e,a), which is additionally located in a 1,3-cis-diaxial relationship to a second hydroxy group, can be used to select a unique direction on the six-membered-ring intramolecular hydrogen bond between the two axial OH groups, so that one of them behaves as an efficient cooperative donor. Talose derivative 3 was designed and synthesised to prove this hydrogen-bonding network by NMR spectroscopy, and the mannopyranoside derivatives 1 and 2 were used as models to demonstrate the presence in solution of the 1,2-(e,a)/five-membered-ring intramolecular hydrogen bond. Once a well-defined hydrogen-bond is formed between the OH and the amido groups of a pyranose ring, these hydrogen-bonding groups no longer act as independent hydrogen-bonding centres, but as hydrogen-bonding arrays. This introduces a new perspective on the properties of carbohydrate OH groups and it is important for the de novo design of molecular recognition processes, at least in nonpolar media. Carbohydrates 1-3 have shown to be efficient phosphate binders in nonpolar solvents owing to the presence of cooperative hydroxy centres in the molecule.

Stereoselective Intercalation of Hexose for Layered Double Hydroxide by Calcination–Rehydration Reaction

Abstract The intercalation of four kinds of hexose for Mg–Al layered double hydroxide (LDH) by the calcination–rehydration reaction using the Mg–Al oxide precursor has been investigated. In spite of non-ionized organic guest, hexose was intercalated for the LDH, and the intercalation behavior of hexose was greatly influenced by the number of axial OH group. The intercalation driving force was thought to be based on hydrogen bond between axial OH group of the intercalated hexose and hydroxide basal layer.

Synthesis of 2-Amino-2-deoxy-β-D-galactopyranosyl-(1→4)-2-amino-2-deoxy-β-D-galactopyranosides: Using Various 2-Deoxy-2-phthalimido-D-galactopyranosyl Donors and Acceptors

A systematic study is presented of the efficiency of the most common glycosylation methods using standard 2-deoxy-2-phthalimidogalactopyranosyl donors ethyl 4-O-acetyl-3,6-di-O- benzyl-2-deoxy-2-phthalimido-1-thio-β-D-galactopyranoside (3a), 4-O-Acetyl-3,6-di-O-benzyl- 2-deoxy-2-phthalimido-β-D-galactopyranosyl bromide (4), 4-O-acetyl-3,6-di-O-benzyl-2-deoxy-2-phthalimido-β-D-galactopyranosyl fluoride (5b), O-(4-O-acetyl-3,6-di-O-benzyl-2-deoxy-2-phthalimido-β-D-galactopyranosyl) trichloroacetimidate (7) and ethyl 3,6-di-O-benzyl-2-deoxy-2-phthalimido-1-thio-β-D-galactopyranoside (8), pent-4-enyl 3,6-di-O-benzyl- and 3-O-allyl-6-O-benzyl-2-deoxy-2-phthalimido-β-D-galactopyranoside (10a) and (10b) and pent-4-enyl 3,6-di-O-benzyl-2-deoxy-2-phthalimido-4-O-(trimethylsilyl)-β-D-galactopyranoside (11) as glycosyl acceptors in the synthesis of 2-amino-2-deoxy-β-D-galactopyranosyl-(1→4)-2-amino-2-deoxy-β-D-galactopyranosides 12, 16a and 17a. It was found that due to a low reactivity of the axial OH(4) group of glycosyl acceptors, disaccharides 16b and 17b with α(1→4) bond were also formed. The unexpected intermolecular migration of ethylsufanyl group from the reducing end of glycosyl acceptor 8 the reducing end of the activated form of glycosyl donor 4 in the glycosylation step to give ethylsulfanyl derivative 3a was proved. For preparation of the glycosyl donors and glycosyl acceptors with galacto configuration an approach based on epimerization of 4-O-mesyl derivatives of appropriate synthons with gluco configuration 2a and 2b was employed.

Conformational effects on lipase-mediated acylations of 2-substituted cyclohexanols

Lipase-mediated acetylations of trans- and cis-2-substituted cyclohexanols gave the corresponding (1 R)-cyclohexyl acetates and (1 S)-cyclohexanols in high yields and ee, but c-4- tert-butyl- c-2-ethenyl- r-1-cyclohexanol was unreactive owing to the steric interaction between the axial OH group and the axial H atoms at the 3- and 5-positions. In the cis-isomer the OH group occupies an equatorial position to bind to the lipase, and less bulky axial alkenyl and alkynyl groups might not so much prevent acetylations than an alkyl group.