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Hydrogen-bond Interactions (hydrogen-bond + interaction)
Selected AbstractsPoly[[hexaaquabis[,4 -2-hydroxy-5-(4-sulfonatophenyldiazenyl)benzoato]dibarium(II)] 4,4,-bipyridine solvate]ACTA CRYSTALLOGRAPHICA SECTION C, Issue 9 2007Hong Liu The title compound, {[Ba2(C13H8N2O6S)2(H2O)6]·C10H8N2}n, possesses a novel two-dimensional porous coordination network, in which each BaII ion is nine-coordinated by three carboxylate O atoms, two sulfonate O atoms and four water molecules in an irregular coordination environment. Hydrogen-bond interactions between coordinated water molecules and sulfonate/hydroxyl groups hold the network layers together and produce a three-dimensional supramolecular architecture. [source] Towards Selective Recognition of Sialic Acid Through Simultaneous Binding to Its cis -Diol and Carboxylate FunctionsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 17 2010Martín Regueiro-Figueroa Abstract A series of receptors containing phenylboronic acid and urea or thiourea units have been designed for simultaneous recognition of the cis -diol and carboxylate functions of sialic acids, which are known to be overexpressed on the surfaces of tumor cells. The interaction of the receptors with 5-acetylneuraminic acid (Neu5Ac) and 2-,- O -methyl Neu5Ac (MeNeu5Ac) in DMSO solution has been investigated bymeans of spectrophotometric titrations and 1H, 13C, and 11B NMR spectroscopy. Additionally, we have also investigated the binding of these receptors with competing monosaccharides such as D -(+)-glucose, D -fructose, methyl ,- D -galactoside, and methyl ,- D -mannoside. Our results show that 2-{[3-(4-nitrophenyl)thioureido]methyl}phenylboronic acid (3a) recognizes both Neu5Ac and MeNeu5Ac with good selectivity with regard to the remaining monosaccharides investigated. DFT calculations performed at the B3LYP/6-31G(d) level show that this selectivity is due to a cooperative two-site binding of Neu5Ac through 1) ester formation by interaction at the phenylboronic acid function of the receptor and 2) hydrogen-bond interaction between the thiourea moiety and the carboxylate group of Neu5Ac. Compound 3a can therefore be considered a promising synthon for the design of contrast agents for magnetic resonance imaging of tumors. In contrast, the analogue of 3a containing a urea moiety , compound 3b , displays strong binding to all monosaccharides investigated, due to two-site binding through interaction on the phenylboronic acid function of the receptor and a hydrogen-bond interaction between the urea moiety and the sugar hydroxy groups. [source] Strong asymmetric hydrogen bonding in 2-(oxamoylamino)ethylammonium oxamate,oxamic acid (1/1)ACTA CRYSTALLOGRAPHICA SECTION C, Issue 3 2010Marta Martinez Belmonte The title compound, C4H10N3O2+·C2H2NO3,·C2H3NO3, contains at least 11 distinct hydrogen-bond interactions showing a great variety of bond strengths. The shortest and strongest hydrogen bond [O...O = 2.5004,(12),Å] is found between the uncharged oxamic acid molecule and the oxamate monoanion. The grouping formed by such a strong hydrogen bond can thus be considered as a hydrogen bis(oxamate) monoanion. It lacks crystallographic symmetry and the two oxamate groups have different conformations, showing an asymmetric hydrogen-bond interaction. Significantly, the asymmetry allows us to draw a direct comparison of site basicity for the two inequivalent carboxylate O atoms in the planar oxamate anion. The constituent molecular ions of (I) form ribbons, where all amide and carboxylate groups are coplanar. Graph-set analysis of the hydrogen-bonded networks reveals the R22(10) and R22(9) homodromic nets as important structure-directing motifs, which appear to be a common feature of many oxamate-containing compounds. [source] Structural basis of the histidine-mediated vitamin D receptor agonistic and antagonistic mechanisms of (23S)-25-dehydro-1,-hydroxyvitamin D3 -26,23-lactoneACTA CRYSTALLOGRAPHICA SECTION D, Issue 8 2010Shinji Kakuda TEI-9647 antagonizes vitamin D receptor (VDR) mediated genomic actions of 1,,25(OH)2D3 in human cells but is agonistic in rodent cells. The presence of Cys403, Cys410 or of both residues in the C-terminal region of human VDR (hVDR) results in antagonistic action of this compound. In the complexes of TEI-9647 with wild-type hVDR (hVDRwt) and H397F hVDR, TEI-9647 functions as an antagonist and forms a covalent adduct with hVDR according to MALDI,TOF MS. The crystal structures of complexes of TEI-9647 with rat VDR (rVDR), H305F hVDR and H305F/H397F hVDR showed that the agonistic activity of TEI-9647 is caused by a hydrogen-bond interaction with His397 or Phe397 located in helix 11. Both biological activity assays and the crystal structure of H305F hVDR complexed with TEI-9647 showed that the interaction between His305 and TEI-9647 is crucial for antagonist activity. This study indicates the following stepwise mechanism for TEI-9647 antagonism. Firstly, TEI-9647 forms hydrogen bonds to His305, which promote conformational changes in hVDR and draw Cys403 or Cys410 towards the ligand. This is followed by the formation of a 1,4-Michael addition adduct between the thiol (,SH) group of Cys403 or Cys410 and the exo -methylene group of TEI-9647. [source] Towards a Molecular Understanding of Cation,Anion Interactions,Probing the Electronic Structure of Imidazolium Ionic Liquids by NMR Spectroscopy, X-ray Photoelectron Spectroscopy and Theoretical CalculationsCHEMISTRY - A EUROPEAN JOURNAL, Issue 30 2010Till Cremer Abstract Ten [C8C1Im]+ (1-methyl-3-octylimidazolium)-based ionic liquids with anions Cl,, Br,, I,, [NO3],, [BF4],, [TfO],, [PF6],, [Tf2N],, [Pf2N],, and [FAP], (TfO=trifluoromethylsulfonate, Tf2N=bis(trifluoromethylsulfonyl)imide, Pf2N=bis(pentafluoroethylsulfonyl)imide, FAP=tris(pentafluoroethyl)trifluorophosphate) and two [C8C1C1Im]+ (1,2-dimethyl-3-octylimidazolium)-based ionic liquids with anions Br, and [Tf2N], were investigated by using X-ray photoelectron spectroscopy (XPS), NMR spectroscopy and theoretical calculations. While 1H,NMR spectroscopy is found to probe very specifically the strongest hydrogen-bond interaction between the hydrogen attached to the C2 position and the anion, a comparative XPS study provides first direct experimental evidence for cation,anion charge-transfer phenomena in ionic liquids as a function of the ionic liquid's anion. These charge-transfer effects are found to be surprisingly similar for [C8C1Im]+ and [C8C1C1Im]+ salts of the same anion, which in combination with theoretical calculations leads to the conclusion that hydrogen bonding and charge transfer occur independently from each other, but are both more pronounced for small and more strongly coordinating anions, and are greatly reduced in the case of large and weakly coordinating anions. [source] A Dual-Response [2]Rotaxane Based on a 1,2,3-Triazole Ring as a Novel Recognition StationCHEMISTRY - A EUROPEAN JOURNAL, Issue 47 2009Haiyan Zheng Abstract Two novel multilevel switchable [2]rotaxanes containing an ammonium and a triazole station have been constructed by a CuI -catalyzed azide,alkyne cycloaddition reaction. The macrocycle of [2]rotaxane containing a C6-chain bridge between the two hydrogen bonding stations exhibits high selectivity for the ammonium cation in the protonated form. Interestingly, the macrocycle is able to interact with the two recognition stations when the bridge between them is shortened. Upon deprotonation of both [2]rotaxanes, the macrocycle moves towards the triazole recognition site due to the hydrogen-bond interaction between the triazole nitrogen atoms and the amide groups in the macrocycle. Upon addition of chloride anion, the conformation of [2]rotaxane is changed because of the cooperative recognition of the chloride anion by a favorable hydrogen-bond donor from both the macrocycle isophthalamide and thread triazole CH proton. [source] Hierarchical Structures in Tin(II) OxalatesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 9 2008Padmini Ramaswamy Abstract Six new SnII oxalates exhibiting a hierarchy of structures have been prepared employing hydrothermal methods. The compounds I [C10N2H10][Sn(C2O4)2], II [C10N2H10][Sn2(C2O4)3], and III [C8N4H26][Sn(C2O4)2]2·2H2O possess zero-dimensional molecular structures; IV [C10N2H8]2[Sn(C2O4)]2 and V [C12N2H8][SnC2O4] have one-dimensional chain structures; and compound VI [C5N2H14]2[Sn4(C2O4)6]·7H2O has a two-dimensional layer structure. The SnII ions have 4- and 6-coordination with square-pyramidal or pentagonal-bipyramidal geometry, in which the lone pair of electrons also occupies one of the vertices. Weak intermolecular forces such as hydrogen-bond interactions, ,···, interactions, and lone-pair,, interactions have been observed and appear to lendstructural stability. Theoretical studies indicate that the ,···, interaction energy between the bound 1,10-phenanthroline molecules is of the order of 5,6 kcal,mol,1 in V. Natural bond orbital (NBO) analysis on two model compounds, II and IV, indicates reasonable lone-pair,, interactions. The close structural relationship between all the compounds indicates that a building-up process from the zero-dimensional monomer can be considered. The present structures provide opportunities for evaluating the structure-directing role of the lone pair of electrons of SnII.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Highly Efficient and Practical Pyrrolidine,Camphor-Derived Organocatalysts for the Direct ,-Amination of AldehydesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 29 2010Pang-Min Liu Abstract A series of pyrrolidine,camphor-derived organocatalysts (1,4) were designed and synthesised. These organocatalysts were used for direct ,-amination of aldehydes with dialkyl azodicarboxylates to give the desired ,-aminated products in high chemical yields (up to 92,%) and with high to excellent levels of stereoselectivity (up to >99,% ee). The reactions proceeded rapidly (within 5 min) with low catalyst loading (5 mol-%) at ambient temperature. Enantioselective aminations of asymmetric ,,,-disubstituted aldehydes in the catalytic system were studied, with reasonable to high stereoselectivities (up to 75,% ee) being obtained. The utility of this methodology was demonstrated with the synthesis of derivatives of ,-amino-,-butyrolactone and a tetrasubstitutedcyclohexane-derived amino alcohol with high stereoselectivities. Transition models were proposed for the asymmetric ,-amination reactions; they involve hydrogen-bond interactions between the nucleophilic enamine formed in situ and the nitrogen source. [source] Ratiometric Fluorescence Sensing of Fluoride Ions by an Asymmetric Bidentate Receptor Containing a Boronic Acid and Imidazolium GroupEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 18 2009Zhaochao Xu Abstract The synthesis of the first examples of anion receptors that utilize boron,fluoride interactions and (C,H)+···F, -type ionic hydrogen-bond interactions in the binding of F ions is reported herein. o -, m -, and p -Phenylboronic acids were linked to naphthoimidazolium through a methylene group. On the basis of fluorescence and 19F NMR studies, we have confirmed that the addition of fluoride to a boron center occurs prior to the formation of (C,H)+···F, -type ionic hydrogen bond with the imidazolium moiety. More importantly, these investigations have demonstrated that only the receptor bearing the ortho -directed boron and imidazolium exhibits enhanced fluoride binding. The increased binding ability of the asymmetric bidentate receptor of ortho -boronic acid and imidazolium towards F, enables it to sense fluoride ions in a 95:5 CH3CN/HEPES aqueous solution. The fluorescence responses to different anions were also explored; the ortho -boron-imidazolium receptor displayed ratiometric fluorescence changes and a high selectivity towards fluoride ions over other anions (Cl,, Br,, CH3CO2,, HSO4,, and H2PO4,).(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Synthesis and structural characteristics of lithocholate triads: steroid-type channels occupied by spacer fragmentsACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2008Urszula Rychlewska Reported in this paper are the syntheses and X-ray investigations of C2 symmetrical molecular A,B,A triads consisting of two steroid units (lithocholic acid or its methyl ester) joined together by linkers derived from bifunctional molecules such as terephthalic acid or N,N,-dicarboxypiperazine. Unlike their monomeric analogues, some of these compounds form inclusion complexes. All steroidal triads form crystals that are highly pseudo-centrosymmetric, in which the constituting molecules are held together either exclusively by van der Waals forces or form lattice inclusion complexes, with guest molecules hydrogen bonded to the host. The presence of carboxyl groups promotes the inclusion of pyridine molecules and the formation of the well known carboxylic acid...pyridine hydrogen bonds. Combined with pairwise face-to-face ,-stacking between pyridine rings, these hydrogen-bond interactions lead to the formation of extended supramolecular tapes, analogous to polymers. The co-crystals of pyridine and a lithocholic acid triad undergo a symmetry-lowering phase transition from a P1 cell with Z = 1 to a P1 cell with Z = 2. The two structures are virtually the same, the two independent molecules in the larger cell being related by pseudo-translation. Changes in the type of spacer between two methyl lithocholate units from planar aromatic (terephthalic acid) to highly puckered aliphatic six-membered ring (N,N,-dicarboxypiperazine) bring about inclusion properties and changes in side-chain conformation in a crystal. Although the efficient packing of these highly elongated molecules is hindered, as indicated by low values of crystal density, ranging from 1.16 to 1.19,g,cm,3, several very short C...O and H...H contacts are present in the crystals. [source] Interplay between dipolar, stacking and hydrogen-bond interactions in the crystal structures of unsymmetrically substituted esters, amides and nitriles of (R,R)- O,O,-dibenzoyltartaric acidACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2001Urszula Rychlewska The compounds analysed are the O,O,-dibenzoyl derivatives of (R,R)-tartaric acid, asymmetrically substituted by ester, amide and nitrile groups. Benzoylation does not introduce drastic changes to the molecular conformation. All investigated molecules adopt the planar T conformation of the four-carbon chain with noticeably smaller departures from the ideal conformation than observed in the nonbenzoylated analogs. Primary and secondary amides always orient the C=O bond antiperiplanar (a) with respect to the nearest C*,O bond, while methylester groups adjust their conformation to that adopted by the amide substituent situated at the other end of the molecule. Tertiary amides and carboxyl groups place their carbonyls at the same side as the nearest C*,O bond (the s form), but often deviations from coplanarity of the two bonds are significant and higher than those observed in the nonbenzoylated series. The results presented demonstrate the importance of dipole/dipole interactions between CO and ,C*H groups in stabilizing the molecular conformation, and between carbonyl groups in stabilizing crystal packing of the molecules that lack classical hydrogen-bond donor groups. An illustration is provided as to how a small change in mutual orientation of molecules arranged in a close-packed fashion causes a change in the character of intermolecular interactions from van der Waals to sandwich stacking between the benzoyloxy phenyls, and to dipolar between the benzoyloxy carbonyls. Hydrogen-bonded molecules tend to orient in a head-to-tail mode; the head-to-head arrangement being limited to cases in which terminal carbonyl groups are situated at one side of the molecule. The orientation of the benzoyloxy substituents with respect to the carbon main chain is such that the (O=)C,O,C,H bond system often deviates significantly from planarity. [source] The diastereoisomers 2-[(S/R)-2-chloro-3-quinolyl]-2-[(R)-1-(4-methoxyphenyl)ethylamino]acetonitrile at 100,KACTA CRYSTALLOGRAPHICA SECTION C, Issue 9 2009Souheila Ladraa In the structures of the two enantiopure diastereoisomers of the title compound, C20H18ClN3O, which crystallize in different space groups, the molecules are very similar as far as bond distances and angles are concerned, but more substantial differences are observed in some torsion angles. The crystal structures of both molecules can be described as zigzag layers along the c axis. The packing is stabilized by hydrogen-bond interactions of N,H...O, C,H...Cl and C,H..., types for 2-[(R)-2-chloro-3-quinolyl]-2-[(R)-1-(4-methoxyphenyl)ethylamino]acetonitrile, and of N,H...N, C,H...O and C,H..., types for 2-[(S)-2-chloro-3-quinolyl]-2-[(R)-1-(4-methoxyphenyl)ethylamino]acetonitrile, resulting in the formation of two- and three-dimensional networks. [source] A new three-dimensional cobalt(II) coordination polymer with a 4-connected CdSO4 -like topologyACTA CRYSTALLOGRAPHICA SECTION C, Issue 8 2009Zhan-Lin Xu The title cobalt(II) coordination polymer, poly[[diaquacobalt(II)]-,4 -3,3,-(p -phenylene)diacrylato], [Co(C12H8O4)(H2O)2]n, was obtained by reaction of Co(NO3)2·6H2O and 3,3,-(p -phenylene)diacrylic acid (H2L) under hydrothermal conditions. Each CoII cation sits on a centre of inversion and is hexacoordinated by six O-atom donors in an octahedral geometry. The CoII centres are connected by four centrosymmetric L2, anions, resulting in a three-dimensional framework structure. The coordinated water molecules and carboxylate O atoms form hydrogen-bond interactions, stabilizing the structure of the three-dimensional framework. Topologically, the framework represents a rare example of the three-dimensional 4-connected CdSO4 network type. The metal cations and the organic ligand both show in-plane coordination with respect to the extended structure. [source] Structures of 1-hydrophenanthroimidazoles: building blocks in the synthesis of expanded-ring bis(imidazoles)ACTA CRYSTALLOGRAPHICA SECTION C, Issue 8 2009Robert T. Stibrany The structures of 1H -phenanthro[9,10- d]imidazole, C15H10N2, (I), and 3,6-dibromo-1H -phenanthro[9,10- d]imidazole hemihydrate, C15H8Br2N2·0.5H2O, (II), contain hydrogen-bonded polymeric chains linked by columns of ,,, stacked essentially planar phenanthroimidazole monomers. In the structure of (I), the asymmetric unit consists of two independent molecules, denoted (Ia) and (Ib), of 1H -phenanthro[9,10- d]imidazole. Alternating molecules of (Ia) and (Ib), canted by 79.07,(3)°, form hydrogen-bonded zigzag polymer chains along the a -cell direction. The chains are linked by ,,, stacking of molecules of (Ia) and (Ib) along the b -cell direction. In the structure of (II), the asymmetric unit consists of two independent molecules of 3,6-dibromo-1H -phenanthro[9,10- d]imidazole, denoted (IIa) and (IIb), along with a molecule of water. Alternating molecules of (IIa), (IIb) and water form hydrogen-bonded polymer chains along the [110] direction. The donor,acceptor distances in these N(imine)...H,O(water)...H,N(amine) hydrogen bonds are the shortest thus far reported for imidazole amine and imine hydrogen-bond interactions with water. Centrosymmetrically related molecules of (IIa) and (IIb) alternate in columns along the a -cell direction and are canted by 48.27,(3)°. The present study provides the first examples of structurally characterized 1H -phenanthroimidazoles. [source] Poly[[tetraaquatris(,3 -2,2-dimethylmalonato)dilanthanum(III)] monohydrate]ACTA CRYSTALLOGRAPHICA SECTION C, Issue 5 2009Ming-Lin Guo In the title complex, {[La2(C5H6O4)3(H2O)4]·H2O}n, the La atoms are connected by bridging O atoms from carboxylate groups to build, through centres of inversion, two-dimensional layers parallel to the ac plane containing decanuclear 20-membered rings. The coordinated water molecules are involved in intralayer hydrogen-bond interactions. Adjacent layers are linked via hydrogen bonding to the solvent water molecules. This work represents the first example of a new substituted malonate,lanthanide complex. [source] Bis(5-aminotetrazole-1-acetato-,O)tetraaquacobalt(II) and catena -poly[[cadmium(II)]-bis(,-5-aminotetrazole-1-acetato-,3N4:O,O,)]ACTA CRYSTALLOGRAPHICA SECTION C, Issue 1 2008Qiao-Yun Li The CoII atom in bis(5-aminotetrazole-1-acetato)tetraaquacobalt(II), [Co(C3H4N5O2)2(H2O)4], (I), is octahedrally coordinated by six O atoms from two 5-aminotetrazole-1-acetate (atza) ligands and four water molecules. The molecule has a crystallographic centre of symmetry located at the CoII atom. The molecules of (I) are interlinked by hydrogen-bond interactions, forming a two-dimensional supramolecular network structure in the ac plane. The CdII atom in catena -poly[[cadmium(II)]-bis(,-5-aminotetrazole-1-acetato], [Cd(C3H4N5O2)2]n, (II), lies on a twofold axis and is coordinated by two N atoms and four O atoms from four atza ligands to form a distorted octahedral coordination environment. The CdII centres are connected through tridentate atza bridging ligands to form a two-dimensional layered structure extending along the ab plane, which is further linked into a three-dimensional structure through hydrogen-bond interactions. [source] 4-[N,N -Bis(2-cyanoethyl)amino]pyridineACTA CRYSTALLOGRAPHICA SECTION C, Issue 8 2003Jun Ni The title compound, 3,3,-(4-pyridylimino)dipropanenitrile, C11H12N4, has a twofold axis and consists of a pyridine ring head and two cyanoethyl tails, the three groups being linked by an N atom. The planar geometry around the amino N atom suggests conjugation with the ,-system of the pyridine ring. The molecules are stacked in a layer structure via relatively weak to very weak intermolecular C,H,, and C,H,N hydrogen-bond interactions. [source] Acetone-4-methylthiosemicarbazone at 220,KACTA CRYSTALLOGRAPHICA SECTION C, Issue 2 2000Simon Parsons In the title compound, C5H11N3S, the trans conformation is stabilized by a weak intramolecular N,H,N hydrogen bond. Unusually, one N,H bond is not involved in any hydrogen-bond interactions and instead the molecules form a one-dimensional polymer via N,H,S intermolecular hydrogen bonds. [source] The crystallographic structure of the aldose reductase,IDD552 complex shows direct proton donation from tyrosine 48ACTA CRYSTALLOGRAPHICA SECTION D, Issue 8 2004Federico Ruiz The X-ray crystal structure of human aldose reductase (ALR2) in complex with the inhibitor IDD552 was determined using crystals obtained from two crystallization conditions with different pH values (pH 5 and 8). In both structures the charged carboxylic head of the inhibitor binds to the active site, making hydrogen-bond interactions with His110 and Tyr48 and electrostatic interactions with NADP+. There is an important difference between the two structures: the observation of a double conformation of the carboxylic acid moiety of the inhibitor at pH 8, with one water molecule interacting with the main configuration. This is the first time that a water molecule has been observed deep inside the ALR2 active site. Furthermore, in the configuration with the lower occupancy factor the difference electron-density map shows a clear peak (2.5,) for the H atom in the hydrogen bond between the inhibitor's carboxylic acid and the Tyr48 side-chain O atom. The position of this peak implies that this H atom is shared between both O atoms, indicating possible direct proton transfer from this residue to the inhibitor. This fact agrees with the model of the catalytic mechanism, in which the proton is donated by the Tyr48 hydroxyl to the substrate. These observations are useful both in drug design and in understanding the ALR2 mechanism. [source] Noncovalent Modulation of pH-Dependent Reactivity of a Mn,Salen Cofactor in Myoglobin with Hydrogen PeroxideCHEMISTRY - A EUROPEAN JOURNAL, Issue 30 2009Jun-Long Zhang Dr. Abstract To demonstrate protein modulation of metal-cofactor reactivity through noncovalent interactions, pH-dependent sulfoxidation and 2,2,-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid) (ABTS) oxidation reactivity of a designed myoglobin (Mb) containing non-native Mn,salen complex (1) was investigated using H2O2 as the oxidant. Incorporation of 1 inside the Mb resulted in an increase in the turnover numbers through exclusion of water from the metal complex and prevention of Mn,salen dimer formation. Interestingly, the presence of protein in itself is not enough to confer the increase activity as mutation of the distal His64 in Mb to Phe to remove hydrogen-bonding interactions resulted in no increase in the turnover numbers, while mutation His64 to Arg, another residue with ability to hydrogen-bond interactions, resulted in an increase in reactivity. These results strongly suggest that the distal ligand His64, through its hydrogen-bonding interaction, plays important roles in enhancing and fine-tuning reactivity of the Mn,salen complex. Nonlinear least-squares fitting of rate versus pH plots demonstrates that 1,Mb(H64X) (X=H, R and F) and the control Mn,salen 1 exhibit pKa values varying from pH,6.4 to 8.3, and that the lower pKa of the distal ligand in 1,Mb(H64X), the higher the reactivity it achieves. Moreover, in addition to the pKa at high pH, 1,Mb displays another pKa at low pH, with pKa of 5.0±0.08. A comparison of the effect of different pH on sulfoxidation and ABTS oxidation indicates that, while the intermediate produced at low pH conditions could only perform sulfoxidation, the intermediate at high pH could oxidize both sulfoxides and ABTS. Such a fine-control of reactivity through hydrogen-bonding interactions by the distal ligand to bind, orient and activate H2O2 is very important for designing artificial enzymes with dramatic different and tunable reactivity from catalysts without protein scaffolds. [source] Anion Receptors Containing -NH Binding Sites: Hydrogen-Bond Formation or Neat Proton Transfer?CHEMISTRY - A EUROPEAN JOURNAL, Issue 1 2005Valeria Amendola Dr. Abstract When the amide-containing receptor 1+ is in a solution of dimethyl sulfoxide (DMSO) in the presence of basic anions (CH3COO,, F,, H2PO4,), it undergoes deprotonation of the -NH fragment to give the corresponding zwitterion, which can be isolated as a crystalline solid. In the presence of less basic anions (Cl,, Br,, NO3,), 1+ establishes true hydrogen-bond interactions of decreasing intensity. The less acidic receptor 2+ undergoes neat proton transfer with only the more basic anions CH3COO, and F,, and establishes hydrogen-bond interactions with H2PO4,. An empirical criterion for discerning neutralisation and hydrogen bonding, based on UV/Vis and 1H NMR spectra, is proposed. [source] | ||||||||||