Formation Of Hydrogen Bridges Research Articles

Benzocrown derivatives as ionophores for alkali cations, II. Synthesis and comparison of urethane‐, ether‐, and ester‐linked bis‐crown compounds and several „crown‐ring free”︁ analogues

AbstractNitro derivatives of bis‐crown ethers with urethane (2), ether (10), and ester (9, 16) linkage not being able to form hydrogen bridges were synthesized, and their K+ selectivities were determined. A remarkable effect of the NO2 group on the selectivity was observed when a NO2…NH hydrogen bond could be formed while the others showed only slight improvement in general electroanalytical parameters. Several analogues without the crown ether ring (25, 26) were also prepared, but these compounds lost their alkaline ionophore properties.

Benzoatokomplexe des Rutheniums, 1. Mitt. Synthese, Eigenschaften und Kristallstruktur von Ru2(C6H5COO)4(C6H5COOH)2 und Ru2(C6H5COO)5·C6H5COOH / Benzoato Complexes of Ruthenium , Part 1 Synthesis, Properties and Crystal Structure of Ru2(C6H5COO)4(C6H5COOH)2 and Ru2(C6H5COO)5·C6H5COOH

Ru2(C6H5COO)4(C6H5COOH)2 (1) is formed as main product besides small amounts of Ru2(C6H5COO)5·C6H5COOH (2) by the reaction of Ruthenium (III) acetylacetonate in molten benzoic acid. 1 forms monoclinic black crystals with the space group P21/n and the lattice constants a = 1091.0, b = 1164.1, c = 1526.8 pm, β = 90.53°, Z = 2.2 is obtained as triclinic red needles with the space group P1̄ and the lattice constants a - 1178.4, b = 1605.3, c = 1043.9 pm, a = 99.07°, β = 95.45°, y - 83.22°, Z = 2. Both com pounds exhibit carboxylate bridged dimers [Ru2(C6H5COO)4]n+ (n=0, 1). In case of the isolated Ru(II) dimer 1 two benzoic acid m olecules occupy axial positions and form hydrogen bridges to carboxylate O-atoms. 1 is paramagnetic with two unpaired electrons per dimeric unit corresponding to an electron configuration σ2π4δ2π*3δ* and a bond order of 2 (Ru-Ru = 226.3 pm ). It decomposes at 230 °C to Ru2(C6H5COO)4 and at 350 °C to Ru metal. 2 is polymeric. A benzoate anion functions as bridging ligand between [Ru2(C6H5COO)4]+ dimers. The benzoic acid molecule is weakly associated to the polymer by an H-bridge. A Ru-Ru bond order of 2.5 and a Ru-Ru distance of 227.7 pm are observed.

Die Chloro-rhodate(III) [RhCl6]3- und [RhCl5(H2O)]2- · Kristallstruktur von (NH4)3RhCl6 · H2O / The Chloro Rhodates(III) [RhCl6]3- and [RhCl5(H2O)]2- · Crystal Sructure of (NH4)3RhCl6 · H2O

(NH4)3RhCl6 · H2O is obtained in form of deep red needles by slow evaporation of an aqueous solution of RhCI3 and excess NH4Cl. It crystallizes in the orthorhombic space group Pnma with lattice constants a = 1219.3. b = 700.7, c = 1416.7 and Z = 4. The structure is built up by octahedral RhCl6 3- anions and NH4 + cations. The water molecules form hydrogen bridges with NH4 + cations. The reaction of (NH4)3RhCl6 with Ag+ yields Ag2[RhCl5(H2O)] and AgCl.

Interaction of amides with VOAO4.nH2O (A=P, As)

Abstract The laminar compounds VOPO4.2H2O and VOAsO4.3H2O interact with primary and tertiary amides to produce either dehydration or intercalation reactions, or sometimes both. The tetragonal structure of the starting solids was preserved, and in some cases, mixtures of phases with different degrees of swelling were seen. The interaction between the tertiary amides and the substrates is by the C=O and V=O groups, either directly, or indirectly, through the coordinated water. When the compounds are hydrated, the primary amides also interact through the NH2 groups to form Hydrogen bridges. In the anhydrous compounds resulting from the reaction of formamide, when refluxed with VOPO4.2H2O and VOAsO4.3H2O, the amide coordinates directly to the Vanadium through the NH2 groups.

Calculation of vibrational spectra of H 5O 2+ and H 3 O 2− ions in anharmonic approximation

The potential surfaces of two-dimensional O⋯H⋯O bridges were calculated by the CNDO method for H 5O 2 + and H 3O 2 − ions with a modified potential of the atomic frameworks. Approximating the surfaces by a fourth power Taylor series, the force parameters up to the fourth order were computed for the bonds forming hydrogen bridges. The force constants and the electrical anharmonicity coefficients of the angles adjacent to hydrogen bonds were estimated. Vibrational absorption spectra calculated for isolated ions with due regard to the electrical and mechanical anharmonicity are in good agreement with the experimental data.

On the nature of Romanowsky-Giemsa staining and the Romanowsky-Giemsa effect. I. Model experiments on the specificity of Azures B-Eosin Y stain as compared with other thiazine dye-Eosin Y combinations

After incorporation into a polyacrylamide matrix, the biopolymers DNA, RNA, heparin, hyaluronic acid, collagen and the synthetic polymers poly(U) and poly(A, U) were stained with the pure thiazine dyes, Methylene Blue, the Azures and Thionin alone and combined with Eosin Y. Satisfactory spectrophotometric agreement was obtained between the staining reactions of the biopolymers in the artificial matrix and those in their natural surroundings. This was especially true with respect to the specificity of the Azure B-Eosin Y dye-pair, which is based on the generation, on suitable substrates, of a purple colour, the Romanowsky-Giemsa effect (RGE), with an absorbance maximum near 550 nm. In the model experiments, DNA, heparin, hyaluronic acid and collagen were found to be RGE-positive and poly(U), poly(A, U) and RNA RGE-negative. A theory of RGE is proposed which complies with the new and earlier observations: after saturation of available anionic binding sites and aggregate formation by Azure B, electron donor acceptor complexes are formed between Eosin Y and Azure B via hydrogen-bridge formation of the aminosubstituent proton of Azure B and between Eosin Y and the biopolymer surface. Charge-transfer complex formation may also account for the qualitative identity of Azure B-Eosin Y and Azure A-Eosin Y spectra of substrates, which are coloured purple. Quantitatively, Azure A-Eosin Y is less efficient in giving RGE. The generation of RGE is time-dependent. Equilibrium staining is attained after about 120 h. The implications of the results for the biological application of Romanowsky-Giemsa staining are discussed briefly.

Kinetics of the Reaction of Solvated Electrons with Water in Liquid Ammonia

AbstractSolvated electrons (< 10−6 M) produced by flash photolysis in ammonia react with water according to the rate law d[e−]/dt = k · [e−] · [H2O] with the rate constant k = 0.16 ± 0.03 M−1 sec−1 at – 40°C. The rate law is compatible with the reaction e− + H2O → H + OH−, but the activation energy Ea (e−) = 6 ± 1 kJ/mol is too small for this endothermic process with ΔH0 = 52 kJ/mol. Therefore we conclude that in the rate determining step an unknown intermediate is formed which in turn recombines to molecular hydrogen. Evidence for this intermediate was given already 15 years ago in the pure aqueous system in which solvated electrons a few msec after their reaction could be regenerated by a photo flash too soft to photo‐generate them directly. – The reaction of very low concentrated metal ammonia solutions with water follows the same kinetics. But at higher concentrations (0.1 M) the relative reaction rate is very low. With sodium the reaction is slower than with cesium, in deutero‐ammonia it is slower than in normal ammonia. It is almost of zero order with respect to the metal and of one ha1f order with respect to water and has an activation energy Ea (Na) = 32 ± 3 kJ/mol (normal sodium‐ammonia solution). The observed kinetics suggest that the solvated electrons are still the only reactive species in these solutions; but their concentration is lower than the metal concentration by many orders of magnitude due to the well known spin pairing equilibrium, which must include a metal cation and a solvent molecule. The stability of the spin paired species increases with decreasing cation size and with increasing tendency of the solvent to form hydrogen bridges. The high experimental activation energy is partly due to the temperature dependence of the spin pairing equilibrium.

Nuclear-magnetic-resonance investigation of 15N-labeled flavins, free and bound to Megasphaera elsdenii apoflavodoxin.

Flavin derivatives, enriched with 15N (approximately equal to 95%) at the four nitrogen atoms of the isoalloxazine ring, have been investigated in the oxidized and the two-electron reduced state by the 15N nuclear magnetic resonance technique. The measurements were conducted with aqueous and chloroform solutions of flavin. A comparison of the chemical shifts of the N(1) and N(5) atoms of oxidized flavin in the two solvents revealed that these atoms are sensitive indicators for possible hydrogen-bridge formation to these atoms. The N(5) atom of oxidized flavin resonates at low field and shifts about 300 ppm upfield upon reduction. A pKa of 6.8 was determined from pH-dependent 15N NMR measurements of the two-electron reduced flavin molecule. In addition it is also shown that reduced flavin in aqueous solution possesses a more coplanar structure than in chloroform solution. The 15N chemical shifts of flavin bound to Megasphaera elsdenii apoflavodoxin indicate that various hydrogen bridges are formed between the prosthetic group and the apoprotein. Especially the N(1) atom of the prosthetic group in the oxidized state seems to form a strong hydrogen bond with the apoprotein. In the reduced state the prosthetic group is bound in the anionic form and possesses an almost coplanar structure. These results are in agreement with published crystallographic data on the related flavodoxin from Clostridium MP. Where possible 15N-1H, 15N-15N and 13C-15N coupling constants were determined. Some of the coupling constants are useful parameters for the elucidation of the planarity of free and protein-bound flavin and for the evaluation of the interaction between flavin and apoprotein. Spin-lattice relaxation measurements show that the relaxation of the 15N(3)H group of flavin is predominantly determined by dipole-dipole interaction. The calculated rotational correlation times of flavin in two different solvents were determined and are in good agreement with published results.

Structural requirements of amino acids and related compounds for stimulation of receptors in crayfish walking leg

Previous recordings from single afferent neurons in the walking legs of the crayfish demonstrated the presence of amino acid sensitivity; several characteristics of this extracellularly recorded impulse discharge, such as responses to mixtures of effective amino acids and cross adaptation experiments, indicate that all such units represent a single type of receptor. Here the effectiveness of more than 100 systematically varying amino acids and analog substances were tested in eliciting a response of the cell. Molecules lacking the amino or carboxyl group were found to be ineffective. The amino group must be unsubstituted and bear an ionic charge; modifications of the carboxyl group, including removal of charge, are tolerated with some loss of effectiveness. Replacement of theα-hydrogen by a methyl group reduced effectiveness by about 1 decade. Amino acids not in the L configuration, or the amino group of which was not in theα position, were still less effective. The affinity of the amino acids for the receptor was related to the number of C atoms in the side chain, with a maximum at 3. Molecules with voluminous, hydrophobic or negatively charged side chains had little effect. For high effectiveness, the side chain should be short and bear a hydrophilic group, but there is relatively little restriction on its chemical properties. These findings suggest the presence of three subunits in the receptive area. One of the proposed subunits would be involved in ionic binding to the amino group and the two others, in the formation of hydrogen bridges to the carboxyl group and the side chain. Their orientation with respect to one another would be expected to meet certain steric requirements.

Crystal structure of the cyclotetraphosphate Ca2[P4O12] · 4 H2O

AbstractCa2[P4O12] · 4 H2O crystallizes in the monoclinic space group P21/n, a = 7.668, b = 12.895, c = 7.144 Å, β 107.00°, Dx = 2.28 g · cm−3, Z = 2.In the structure there are ringlike anions, which are composed of 4 PO4 tetrahedra connected by oxygen bridges. The Ca2+ are surrounded by 7 oxygen atoms. Each two cation polyhedra are connected by a common edge to pairs which are isolated from one another. The water molecules form hydrogen bridges with one another and with the anion rings.

Antidiabetic Behavior of Biguanides

The existence of active electron pairs on some nitrogen atoms in phenformin hydrochloride is inferred from the presence of a hydrogen catalytic polarographic wave. This finding emphasizes the ability of biguanides to form hydrogen bridges with other molecular species such as amino acids and proteins, as well as to form coordination complexes with zinc and other metallic cations by means of these electron pairs. The antidiabetic action of phenformin and other related biguanides can be explained in terms of competition between these molecules and insulin to coordinate cationic oligoelements together with their ability to form hydrogen bonds between the biguanide moiety and insulin itself.

Significance of the goblet-cell mucin layer, the outermost luminal barrier to passage through the gut wall

The decisive luminal barrier to the passage of a compound through the gut wall would appear to be the mucin secreted by goblet-cells. The absorption of compounds of different chemical structures, measured in animal experiments, is directly proportional to diffusion through isolated goblet-cell mucin and inversely proportional to retention in the mucin and, with reservation also to molecular weight. The dependence of diffusion through and retention by the mucin on pH and ionic strength are discussed with reference to changes in the electrical charge and structure of the mucus and its ability to form hydrogen bridges.

Crystal and Molecular Structure of Bis(N,N,N′,N′ -tetramethyl-1,4-diaminobenzeniumyl) [Wurster s Blue Cation]-tetracyanoplatinate(II)-mono-hydrate

Abstract The title compound crystallizes in the monoclinic space group A 2/m (C2h 3) with a = 10.61(3) Å, b = 9.511(5) Å and c = 16.47(5) Å, β = 124.2(1)° and two formula units in the unit cell. The structure was solved by Patterson and Fourier methods and refined by least squares to R = 0.071 based on 1251 observed intensities. The organic cations are slightly distorted and are arranged in dimers along the crystallographic a-axis. There are “mixed stacks” of the planar [Pt(CN)4]2− units which are separated from each other by the WB2 2+ dimers along b. There is IR-evidence for the formation of hydrogen bridges between [Pt(CN)4]2− units and the crystal water during the modification change of the compound at room temperature.

Structural investigations of nickel(II) complexes. Part I. Crystal structure of bisaquotetrakisquinolinedithiocyanatenickel(II) complex

The green crystals of Ni(NCS)2(qn)4(H2O)2 are of triclinic symmetry with the space group P1 and the dimensions of the unit cell: a = 12.502(8), b = 8.966(8), c = 7.558(4) Å, α = 95.69(6), β = 92.58(5), γ = 92.18(7)° and V = 841(1) Å3, the number of formula units being Z = 1. The structure was refined by the method of full matrix least squares, the final value of R was 0.073. The nickel atoms lie in the centre of the symmetry and are pseudooctahedrally coordinated by four N atoms (two of the NCS groups, NiN = 2.04 Å, and two from the quinoline molecules, NiN = 2.18 Å) and two O atoms (from the water molecules, NiO = 2.13 Å). The bond angles in the coordination polyhedra are near to 90°. Two other quinoline molecules form hydrogen bridges with the coordinated water molecules. The thiocyanate group is a monodentate ligand and is nearly linear with the interatomic distances of NC = 1.16 and CS = 1.62 Å. The quinoline molecules are planar and exhibit the interatomic distances of CN of 1.32 to 1.37 Å and CC of 1.32 to 1.47 Å.

Dielectric loss of N‐substituted nylon 6.6 and of polyester 6.6

In this report the results are summarized of measurements concerning the influence of intermolecular hydrogen bridge formation on the mechanisms responsible for the so-called p- and y-relaxations in the dielectric loss spectrum of nylon 6.6’) (see McCrum et al.’, p. 478497). To that end the loss spectra and the IR carbonyl absorptions in the range from 1 500 to 1 700 cm - were studied of polyester 6.6**)a nd of nylon 6.6, modified by partly substitution of amide-hydrogen by methyl or ethyl groups. This work is intended to give a contribution to the elucidation of the still existing controversy2- s, concerning the mechanisms mentioned above. Up till now, the idea of Woodward et aL6) of N-substitution as a tool in the interpretation of these secondary transitions in the dielectric loss spectrum of nylon 6.6, was not pursued further.

Study of hydrogen bridge formation in free radical solutions via dynamic nuclear polarization

Study of hydrogen bridge formation in free radical solutions via dynamic nuclear polarization

Enolization and Hydrogen Bridge Formation in Cyclopentane β‐Tricarbonyl Systems

AbstractProtons situated next to the “ene”‐side of enolized β‐diketone systems are less deshielded than when they are situated next to the “one” side in the isomeric enol system. As published before (1) the ethers and esters of the same system show the opposite behaviour. This allows discussion regarding the structure of several cyclopentane β‐tricarbonyl derivatives, in which the different enol forms can be differentiated by I.R. and P.M.R. spectroscopy.

The crystal structure of 4,6-dimethyl-2-hydroxypyrimidine. I

Crystals of 4, 6-dimethyl-2-hydroxypyrimidine dihydrate, C6H8ON2.2H2O, are monoclinic with a = 6.80 A., b = 13.45 A., c = 9.18 A., β = 101°. The cell contains four molecules. The space group is P21/n, but is strongly pseudo-A2/a. Coordinates for the atoms are derived and refined by two-dimensional Fourier syntheses and the method of steepest descents. The pyrimidine molecule is found to be planar within the limits of present accuracy and the bond lengths are C—C: 1.52, 1.56 A.; C N: 1.34, 1.35, 1.38, 1.39 A.; C OH: 1.25 A.; C CH: 1.37, 1.39 A.; (all ± 0.04 A.). Two water molecules form hydrogen bridges to the hydroxyl group, and each water is also linked to a nitrogen atom in the next molecule and to another water molecule; all hydrogen bridges are of moderate strength, the lengths lying between 2.7 and 3.0 A.