Release Of Water Molecules Research Articles

Thermodesorption study of barium and strontium cerates

The method of thermodesorption spectroscopy has been used for study of kinetic of hydrogen, oxygen and water molecule release from doped and undoped barium and strontium cerates. It was found that oxygen and water molecules are released from doped barium cerate samples whereas only water molecule desorption took place from doped strontium cerate samples. Shapes of thermodesorption spectra for pure barium cerate indicate that phase transformations take place in low and high temperature regions. It was shown that oxygen molecule desorption from doped barium cerates can be described by a second-order kinetic equation and an activation energies of reaction were determined.

Complexation of Bisphenol A with Calix[6]arene‐Polymer Conjugates

AbstractSodium calix[6]arenehexasulfonic acid (SCX6) was introduced into polymer molecules by the cross‐linking of SCX6 with epichlorohydrin (PolySCX6) or by the covalent binding of carboxymethylated SCX6 to an amino group‐carrying vinyl polymer (Poly(SCX6HMA‐co‐AAm)). The complexation of bisphenol A (BPA) with the SCX6 group in the polymers was examined fluorimetrically using acridine red (AR) as a probe. From the inhibitory effect of BPA on the complexation of AR with the SCX6 group in the polymers, the association constant (Kassoc) of BPA with the SCX6 group was evaluated and the value for BPA with the SCX6 group in Poly(SCX6HMA‐co‐AAm) was much larger than that with the non‐conjugated SCX6, probably because of the support of hydrophobic interactions between the guest and the hydrocarbon moieties in the polymer (polymer main chain and hexamethylene groups introduced for the conjugation of SCX6). The Kassoc value for bisphenol B (BPB) with a non‐conjugated SCX6 was larger than those for bisphenol A and bisphenol F, because of the higher hydrophobicity of BPB among the bisphenols examined. Thermodynamic parameters indicated that the inclusion of BPA into the cavity of sodium propyloxycalix[6]arenehexasulfonic acid (SPCX6) was enthalpy driven, and the entropy change was largely negative (−92 J · mol−1 · K−1). This is probably because of a tight fit of guest molecules in the SPCX6 cavity, resulting in the loss of freedom of both the SPCX6 and the guest molecules. The entropy change for the complexation of BPA with the SCX6 group in the polymer was less negative, probably because of a partial release of water molecules around the SCX6 group upon the complexation. The effect of the structure of guest and host molecules on the complexation was also examined.Chemical structure of various calix[6]arenes: R = H, sodium calix[6]arenehexasulfonic acid; R = C3H7, sodium propyloxycalix[6]arenehexasulfonic acid; R = C4H9, sodium butyloxycalix[6]arenehexasulfonic acid; R = C6H13, sodium hexyloxycalix[6]arenehexasulfonic acid.magnified imageChemical structure of various calix[6]arenes: R = H, sodium calix[6]arenehexasulfonic acid; R = C3H7, sodium propyloxycalix[6]arenehexasulfonic acid; R = C4H9, sodium butyloxycalix[6]arenehexasulfonic acid; R = C6H13, sodium hexyloxycalix[6]arenehexasulfonic acid.

In situ time resolved synchrotron powder diffraction study of mordenite

The step by step thermal dehydration process of the zeolites mordenite from Pashan (Poona, India) (Na 3.51 K 0.14 Ca 1.89 Mg 0.09 Sr 0.01 )[Fe 3+ 0.03 Al 7.40 Si 40.53 O 96 ] · 27.26H 2 O, has been studied in situ by synchrotron powder diffraction. The time-resolved experiment was performed using a translating imaging plate system. The structure refinements by full profile Rietveld analysis were performed in the Cmcm space group in the temperature range from 25 to 830°C. The results of structure refinements indicate that the slight cell-volume contraction (∼ 1.9%) is related to the release of water molecules from the channels: above 375°C, the water loss lead to an enlargement of the 8-membered ring parallel to [010], which occurred by a flattening of T3-O9-T3 and T4-O4-T3 bridges. The dehydration process is reflected not only in the content of water molecules in the zeolite channel, but also in the temperature behaviour of the unit cell parameters. The cell parameters b and c decreased regularly as the temperature rose, whereas a decreased up to 400°C then increased up to 630°C, and finally decreased until the end of experiment. The removal of water molecules was accompanied by a spreading of the initial Ca sites into many positions bonded to the framework oxygens. The increased interaction with the framework oxygens of Ca sites was intimately related to the distortion of the 12-ring which is in turn related to the lengthening of the a cell parameter.

Water status and carbohydrate pools in tulip bulbs during dormancy release

Summary • Changes in the physical state of cellular water and its interrelations with carbohydrate metabolism were studied during preplanting storage of tulip bulbs (Tulipa gesneriana‘Apeldoorn’). • Magnetic resonance imaging, light and scanning electron microscopy and high-performance anion exchange chromatography with pulsed amperometric detection were used to follow time-dependent changes during bulb storage at 17 or 20°C (nonchilled) or 4°C (chilled). • No visible differences in scale structure and central bud development were observed microscopically between chilled and nonchilled bulbs. However, the scales of the chilled bulbs exhibited higher water content, faster starch degradation and increased concentrations of sucrose and ethanol-soluble fructan. Quantitative measurements of magnetization transfer (MT) indicated a smaller fraction of a solid or a restricted-mobility proton pool in the scales of the chilled bulbs. By contrast, the MT effect was significantly higher in the central bud of the chilled than in the nonchilled bulbs. • Degradation of storage polysaccharides to low-molecular-weight sugar molecules during release from dormancy could be accompanied by local release of water molecules tightly bound to the polysaccharide granules into the bulk water, or by an influx of free water molecules due to increased osmotic potential caused by the raised sugar concentration, or by a combination of both effects.

Thermal and spectral properties of halogenosalicylato-Cu(II) complexes

This paper reports an investigation of thermal and spectral properties of the complexes Cu2(3,5Cl2sal)4⋅H2 O( I), Cu(3,5-Br2sal)2⋅2H2 O( II), Cu(3,5-I2sal)2⋅2H2 O( III) and Cu(5-Isal)2⋅2H2 O( IV), (where sal=salicylate). TG, DTG, DTA, IR, EPR and electronic spectra have been applied to investigate thermal and spectral properties of these complexes. The chemical composition of the complexes has been identified by means of elemental analysis and complexometric titration. Schemes of destruction of these complexes are suggested. Heating of these compounds first resulted in a release of water molecules. The thermal stability of these complexes can be ordered in the sequence: IV=I<II<III. The final product of the thermal decomposition was CuO in all cases. IR data suggested a unidentate coordination of carboxylates to Cu(II) in complexes I–IV.

The formation of amino acid and dipeptide complexes with α-cyclodextrin and cucurbit[6]uril in aqueous solutions studied by titration calorimetry

The complex stabilities and the thermodynamic data for the complexation of α-cyclodextrin and cucurbit[6]uril with some amino acids (glycine, l-alanine, l-valine, l-phenylalanine, 6-amino hexanoic acid, 8-amino octanoic acid, 11-amino undecanoic acid) and dipeptides (glycyl–glycine, glycyl– l-valine, glycyl– l-leucine and glycyl– l-phenylalanine) have been determined in aqueous solution by calorimetric titrations. The complex formation with α-cyclodextrin is mainly favoured by entropic contributions due to the release of water molecules from the cavity of the ligand. The values of the reaction enthalpies are small with the exception of 11-amino undecanoic acid. In case of the ligand cucurbit[6]uril, ion–dipole interactions between the protonated amino groups of the amino acids and the carbonyl groups take place. By steric reasons these interactions are lowered for native amino acids because the polar carboxylic groups are always located outside the hydrophobic cavity of cucurbit[6]uril. The complexes of both ligands with dipeptides in water are characterised by hydrophobic interactions and in case of cucurbit[6]uril by additional ion–dipole interactions.

Free-energy contributions to complex formation between botulinum neurotoxin type B and synaptobrevin fragment.

Free-energy terms that contribute to complex formation between the catalytic domain of botulinum neurotoxin type B (BoNT/B-L(C)) and a 36-residue synaptobrevin fragment were estimated by using a combination of microscopic simulations and continuum methods. The complex for a non-hydrolyzed substrate was calculated by optimizing an energy function applied to the X-ray co-crystal structure of BoNT/B-L(C) bound with reaction products from a cleaved synaptobrevin peptide, refined to high crystallographic thermal factors. The estimated absolute binding affinity of the simulation structure is in good qualitative agreement with the experimental free energy of Michaelis complex formation, given the approximations of the model calculations. The simulation structure revealed significant complex stabilization from the hydrophobic effect, while the electrostatic cost of releasing water molecules from the interface determined to be highly unfavorable. By partitioning the total electrostatic and hydrophobic terms into residue free-energy contributions, a binding-affinity 'signature' for synaptobrevin was developed from the optimized conformation. The results demonstrate the effect of substrate length on complex formation and identify a peripheral high-affinity binding site near the N-terminal region that might initiate cooperative activation responsible for the large minimal substrate length requirement. The so-called SNARE motif is observed to contribute negligible free energy of binding.

Butyrylcholinesterase-catalyzed hydrolysis of N-methylindoxyl acetate: analysis of volume changes upon reaction and hysteretic behavior

Hydrolysis of the neutral substrate N-methylindoxyl acetate (NMIA) by wild-type human butyrylcholinesterase (BuChE) and peripheral site mutants (D70G, Y332A, D70G/Y332A) was found to follow the Michaelis–Menten kinetics. K m was 0.14 mM for wild-type, and 0.07–0.16 mM for D70G, Y332A and D70G/Y332A, indicating that the peripheral site is not involved in NMIA binding. The values of k cat were of the same order for all enzymes: 12,000–18,000 min −1. Volume changes upon substrate binding (−Δ V K m ) and the activation volumes (Δ V k cat ‡) associated with hydrolysis of NMIA were calculated from the pressure dependence of the catalytic constants. Values of −Δ V K m indicate that NMIA binds to an aromatic residue, presumed to be W82, the active site binding locus. Binding is accompanied by a release of water molecules from the gorge. Residue 70 controls the number of water molecules that are released upon substrate binding. The values of Δ V k cat ‡, which are positive for wild-type and faintly positive for D70G, clearly indicate that the catalytic steps are accompanied by re-entry of water into the gorge. Results support the premise that residue D70 is involved in the conformational stabilization of the active site gorge and in control of its hydration. A slow transient, preceding the steady state, was seen on a time scale of several minutes. The induction time rapidly increased with NMIA concentration to reach a limit at substrate saturation. Much shorter induction times (<1 min) were seen for hydrolysis of benzoylcholine (BzCh) by wild-type BuChE and for hydrolysis of butyrylthiocholine (BuSCh) by the active site mutants E197Q and E197Q/G117H. This slow transient was interpreted in terms of hysteresis without kinetic cooperativity. The hysteretic behavior of BuChE results from a slow conformational equilibrium between two enzyme states E and E′. NMIA binds only to the primed form E′. Kosmotropic salts and hydrostatic pressure were found to shift the equilibrium toward E′. The E→E′ transition is accompanied by a negative activation volume (Δ V 0 ‡=−45±10 ml/mol), and the E′ form is more compact than E. Hydration water in the gorge of E′ appears to be more structured than in the unprimed form.

The study of new zinc(II) aliphatic carboxylate complex compounds by methods of thermal analysis

Four new complex compounds were prepared by the reaction of zinc bromobutyrate and organic ligands. The general formula of the synthetized complex compounds are (2-Brbut)2Zn⋅L and (4-Brbut)2Zn⋅L2⋅nH2O (but=butyrate, L=theobromine (tbr), theophylline (tph), methyl-3-pyridyl carbamate (mpc), n=0-1). The compounds were characterized by chemical analysis and IR spectroscopy. The thermal behaviour of the zinc(II) complexes was studied by thermal analysis. Thermal decomposition in the case of hydrated compounds starts with the release of water molecules. Then molecules of organic ligands and the bromobutyrate anion are released and decomposed. CH3CH2CH=O, CO, CH2=CHCH=O, CH2O and ZnBr2 were found as gaseous products of thermal decomposition during heating up to 700°C. IR, mass spectroscopy, X-ray powder diffraction and chemical analysis were used for the determination of solid and gaseous intermediates and products of the thermal decomposition.

Structural Basis for the Function of Pyridoxine 5′-Phosphate Synthase

Background: Pyridoxal 5′-phosphate is the active form of vitamin B 6 that acts as an essential, ubiquitous coenzyme in amino acid metabolism. In Escherichia coli, the pathway of the de novo biosynthesis of vitamin B 6 results in the formation of pyridoxine 5′-phosphate (PNP), which can be regarded as the first synthesized B 6 vitamer. PNP synthase (commonly referred to as PdxJ) is a homooctameric enzyme that catalyzes the final step in this pathway, a complex intramolecular condensation reaction between 1-deoxy-D-xylulose-5′-phosphate and 1-amino-acetone-3-phosphate. Results: The crystal structure of E. coli PNP synthase was solved by single isomorphous replacement with anomalous scattering and refined at a resolution of 2.0 Å. The monomer of PNP synthase consists of one compact domain that adopts the abundant TIM barrel fold. Intersubunit contacts are mediated by three additional helices, respective to the classical TIM barrel helices, generating a tetramer of symmetric dimers with 422 symmetry. In the shared active sites of the active dimers, Arg20 is directly involved in substrate binding of the partner monomer. Furthermore, the structure of PNP synthase with its physiological products, PNP and P i, was determined at 2.3 Å resolution, which provides insight into the dynamic action of the enzyme and allows us to identify amino acids critical for enzymatic function. Conclusion: The high-resolution structures of the free enzyme and the enzyme–product complex of E. coli PNP synthase suggest essentials of the enzymatic mechanism. The main catalytic features are active site closure upon substrate binding by rearrangement of one C-terminal loop of the TIM barrel, charge–charge stabilization of the protonated Schiff-base intermediate, the presence of two phosphate binding sites, and a water channel that penetrates the β barrel and allows the release of water molecules in the closed state. All related PNP synthases are predicted to fold into a similar TIM barrel pattern and have comparable active site architecture. Thus, a common mechanism can be anticipated.

Thermoanalytical and IR-spectral Investigation of Mg(II) Complexes with Heterocyclic Ligands

Thermogravimetry (TG), differential thermal analysis (DTA) and other analytical methods have been applied to the investigation of the thermal behaviour and structure of the complexes Mg(pc)(na)3⋅3H2O (I), Mg(pc)(py)2⋅2H2O (II),Mg(pc)(pic)2⋅2H2O (III) and Mg(pc)(caf)2⋅4H2O (IV), where pc=2,6- pyridinedicarboxylate, na=nicotinamide,py=pyridine, pic=γ-picoline and caf=caffeine. The thermal decomposition of these compounds is multi-stage processes. The chemical composition of the complexes, the solid intermediates and the resultant products of thermolysis have been identified by means of elemental analysis and complexometric titration. Schemes of destruction of these complexes are suggested. Heating of these compounds first resulted in a release of water molecules. In complexes I, II and IV the loss of the molecular ligands (na, py and caf) occur (on the TG curves) in one step (-2na, -2py and -2caf) and in complex III in two steps (-pic, -pic). The final product of the thermal decomposition was MgO. The thermalstability of the complexes can be ordered in the sequence: IV<I<III<II. Nicotinamide, pyridine, γ-picoline and caffeine were co-ordinated to Mg(II) through the N atom of the respective heterocyclic ring. IR data suggested a unidentate co-ordination of carboxylates to Mg(II) in complexes I–IV.

THERMAL ANALYSIS OF MANGANESE(II) COMPLEXES WITH GLYCINE

The thermal decomposition behaviour of the manganese(II) complexes with glycine: Mn(gly)Cl2(H2O)2, Mn(gly)2Cl2, Mn(gly)Br2(H2O)2, Mn(gly)2Br2(H2O)2 was investigated by means of TG-DTG-DTA, Hi-Res-TA and DSC techniques. The evolved gas analysis was carried out by means of the coupled TG-FTIR system. Heating of the complexes results first in the release of water molecules. Next, the multi-stage decomposition process with degradation of glycine ligand occurs. Water, carbon dioxide and ammonia were detected in the gaseous products of the complexes decomposition. The temperature of NH3 evolution from the complexes is higher than from free glycine. The final residue in the air atmosphere is Mn2O3 which transforms into Mn3O4 at 930°C. In a nitrogen atmosphere, the complexes decompose into MnO.

In situ dehydration of yugawaralite

The structural response of the natural zeolite yugawaralite (CaAl 2 Si 6 O 16 ·4H 2 O) upon thermally induced dehydration has been studied by Rietveld analysis of temperature-resolved powder diffraction data collected in situ in the temperature range 315–791 K using synchrotron radiation. The room-temperature monoclinic structure [ Pc , a = 6.73200(9), b = 14.0157(2), c = 10.0607(1) A, β = 111.189(1) °, Z = 2, at 315 K] has the Ca cations in the channels coordinated to four framework O atoms and to four water molecules, with two of the water sites (OW1 and OW4) showing positional disorder progressively disappearing as the dehydration proceeds. The yugawaralite structure reacts to the release of water molecules with small changes in the Ca-O bond distances and minor distortions of the tetrahedral framework up to about 695 K. Above this temperature the Ca coordination falls below 7 (four framework O atoms and three water molecules) and a major rearrangement in the cation coordination takes place, causing a first order phase transition involving both a large decrease in the cell volume and the change in the space group symmetry. A satisfactory structure model for the high-temperature phase stable in the range 695–791 K could not be obtained because of the complexity of the structure. A model approximately describing the average structure [ Pn , a = 12.703(1), b = 13.067(1), c = 9.839(1) A, β = 110.894(9), Z = 4, at 749 K] has been used to follow the temperature evolution of the cell parameters in the measured temperature range. This model involves a sixfold coordination of the Ca cations (five framework O atoms and one water molecule). There is no indication of significant structure changes before collapse, likely occurring when the last water molecule is expelled from the structure and the cation coordination drops below 6, as observed in other Ca-rich zeolites (i.e., laumontite, scolecite, mesolite).

The thermodynamics of basic and amphoteric poly(amido-amine)s containing peptide nitrogens as potential binding sites for metal ions. Part 1

Five basic and amphoteric poly(amido-amine)s containing, besides carboxyl and tertiary amino groups, peptide nitrogens were synthesised by a Michael-type polyaddition reaction of a primary (N-methylamine) or a bis-secondary (N,N′-dimethyl-ethylenediamine) amine to flexible bis-acrylamides.Protonation and copper(II)-complex formation studies were performed in 0.1M NaCl at 25°C by potentiometry, calorimetry, viscometry and UV–Visible spectroscopy. Compared with the previously studied homologous PAAs, having the rigid bis-diacylpiperazine ring inserted in the monomer unit, the basic polymers showed lower basicity constants and increased polyelectrolyte behaviour. In fact, the n parameter of the modified Henderson–Hasselbalch equation was always higher than 1, suggesting reduced shielding among repeating units of the polymer. The presence of charges (positively ionised nitrogens or negative carboxylate groups) produced almost unitary n values because of enhanced shielding due to the larger hydration shell surrounding the ionised groups. In contrast, neutralization of ampholytes to zwitterions raised the n values owing to the release of water molecules, which increased the entropy contribution (ΔS°) and compacted the macromolecule.Copper(II) ions were easily complexed by amino nitrogens in a stable five-member ring showing three complex species of different stoichiometry [CuL, CuH(−1)L and CuH(−2)L] (L means the repeating unit of the polymer) on the whole range of pH investigated (3–11). Thermodynamic (logβ, −ΔH°, ΔS°) and UV–Visible data are in line with the proposed complex species that also involve deprotonation of one or two peptide nitrogens in different pH-ranges. Polyampholytes were able to bind copper(II) ions through peptide sites at low pHs (>3), while the basic PAAs needed to reach pH>5.

Time-resolved near-infrared and two-dimensional near-infrared correlation spectroscopic studies on polymerization of the silane coupling agent perfluorooctyltrimethoxysilane

The polymerization behavior of perfluorooctyltrimethoxysilane (PFOS) in ethanol, which is acid-catalyzed by 0.25 M HCl, has been examined using time-resolved near-IR and 2D near-IR correlation spectra. In the time-resolved near-IR spectra, the bands at 5164 and 4825 cm−1 have been assigned to the combination bands of water and ethanol OH groups, respectively. It has been found that the absorbance variation of the two near-IR bands occurs in a two-step process. The absorbance of the 5164 cm−1 band rapidly decreases in the initial step but increases exponentially in the second step, while that of the 4825 cm−1 band rapidly increases both in the initial step and, exponentially, in the second step. These results indicate that the time-dependent absorbance variation of the two near-IR bands reflects the polymerization process of PFOS, in which consumption and release of water molecules and release of methanol in the two-step process occur as a consequence of the acid-catalyzed hydrolysis of methoxy groups and the formation of silanols (SiOH) to form a siloxane bond. It has also been found that this polymerization process is distinctly reflected in the 2D near-IR correlation spectra.

Layered aluminophosphates III. Crystal structure and thermal properties of the novel layered aluminophosphate UiO-18

The synthesis and crystal structure of a novel layered aluminophosphate and its two high-temperature variants are described. Two of the structures were solved from powder X-ray diffraction data. The as-synthesised material (UiO-18-as) with a composition [Al 2(OH) 2(PO 4) 2(H 2O) 2] 2−[NH 3(CH 2) 3NH 3] 2+2H 2O crystallises in the orthorhombic space group Pnma with a=6.91809(8) Å, b=22.3000(4) Å, c=9.60666(12) Å, V=1482.053(36) Å 3 and Z=4. The aluminophosphate layers are composed of aluminium octahedra forming infinite chains along [1 0 0] that are cross-linked by phosphate tetrahedra. The layers are held together by hydrogen bonding interactions involving the interlamellar 1.3-propanediammonium ions and water molecules. A high-temperature variant exists around 100°C (UiO-18-100). This compound with the composition [Al 2(OH) 2(PO 4) 2] 2−[NH 3(CH 2) 3NH 3] 2+ crystallises in the monoclinic space group P2 1/ n with a=21.5516(12) Å, b=6.96437(22) Å, c=8.1635(4) Å, β=100.042(4)°, V=1206.51(11) Å 3 and Z=4. The transformation to UiO-18-100 involves removal of the interlamellar water molecules and water molecules coordinating aluminium. The inorganic sheets retain their structure with infinite chains of aluminium trigonal bipyramids cross-linked by phosphate tetrahedra. The conformation of the 1.3-propanediammonium ion changes from the straight one observed in UiO-18-as to a more staggered one. Another high-temperature variant exists around 200°C (UiO-18-200). 13C CP-MAS NMR data indicate that the 1.3-propanediammonium ion remains unaffected by the transformation to UiO-18-200, and the weight loss associated with it involves release of water molecules and probably formation of tetrahedral Al–O–Al bonding just as in UiO-15-225. Structure elucidation of this compound is, however, prevented by severe disorder.

Thermal and IR properties of Mg(II) complexes with heterocyclic ligands

Thermogravimetry (TG), differential thermal analysis (DTA) and other analytical methods have been applied to the investigation of the thermal behavior and structure of the compounds Mg(pc)(na) 2·2H 2O ( I), Mg(pc)(Et 2na) 2·3H 2O ( II), Mg(pc)(mpc) 2·H 2O ( III) and Mg(pc)(ron)·2H 2O ( IV), where pc=2,6-pyridindicarboxylate, na=nicotinamide, Et 2na= N, N-diethylnicotinamide, mpc=methyl-3-pyridyl carbamate and ron=3-pyridylcarbinol (ronicol). Thermal decomposition of these compounds are multi-stage processes. The composition of the complexes and the solid state intermediate and resultant products of thermolysis had been identified by means of elemental analysis and complexometric titration. The possible scheme of destruction of the complexes is suggested. Heating the compounds first results in a release of water molecules. In complexes I, II and IV the loss of the molecular ligands (na, Et 2na and ron) occur (on the TG curves) in one step (−2na, −2Et 2na and −ron) and in complex III in two steps (−mpc, −mpc). The final product of the thermal decomposition was MgO. The thermal stability of the complexes can be ordered in the sequence: II< III< I< IV. Et 2na, na, and ron were coordinated to Mg(II) through the nitrogen atom of the respective heterocyclic ring. IR data suggest to a unidentate coordination of carboxylates to Mg(II) in complexes I– IV.

Hydration effects of Ni(2+) binding to synthetic polynucleotides with regularly alternating purine-pyrimidine sequences.

High precision ultrasonic and densimetric techniques have been used to study the interaction of Ni(2+)ions with right-handed poly[d(G-C)].poly[d(G-C)], poly-[d(A-C)].poly[d(G-T)] and poly[d(A-T)].poly[d(A-T)] in 5 mM CsCl, 0.2 mM HEPES, pH 7.5 at 20 degrees C. From these measurements the changes in the apparent molar volume and the apparent molar adiabatic compressibility due to the interaction have been obtained. The volume effects of the binding, calculated per mole of Ni(2+)ions, range from 11.7 to 23.9 cm(3)mol(-1)and the compressibility effects range from 19.3 x 10(-4)to 43.1 x 10(-4)cm(3)mol(-1)bar(-1). These data are interpreted in terms of dehydration of the polynucleotides and Ni(2+)ions, i.e. the release of water molecules from the hydration shells of the molecules. An increase in G+C content gives an increase in volume and compressibility effects, indicating a rise in the extent of dehydration. The dehydration effects of Ni(2+)binding to poly[d(G-C)].poly[d(G-C)] are approximately twice those of poly[d(A-T)].poly[d(A-T)]. The volume and compressibility effects of Ni(2+)-EDTA complex formation have also been measured and used as a model system for quantitative estimation. These values revealed that Ni(2+)ions can coordinate two atomic groups of poly[d(G-C)]. poly[d(G-C)], while in the case of the Ni(2+)-poly[d(A-T)]. poly[d(A-T)] complex volume and compressibility effects correspond to one direct or two indirect (through water) contacts.

Micellization of decyldimethylbenzylammonium chloride at various temperatures studied by densitometry and conductivity

The critical micelle concentration (cmc), ionization degree of micellization, β, and standard free energy of micellization, Δ G 0 m, of decylbenzyldimethylammonium chloride in aqueous solution at 15, 25, 35, 45 and 55°C have been estimated from conductivity data. A shallow minimum of the cmc value appears at around 35°C. The apparent molar volumes of the monomeric and micellized surfactant were estimated from density data measured at 15, 25 and 35°C. It is suggested that release of water molecules from the hydration shell of the counter-ions may be an important contribution to the observed apparent expansion of the surfactant upon micellization.

Conversion of core oxos to water molecules by 4e-/4H+ reductive dehydration of the Mn4O2(6+) core in the manganese-oxo cubane complex Mn4O4(Ph2PO2)6: a partial model for photosynthetic water binding and activation.

Reaction of the Mn4O4(6+) "cubane" core complex, Mn4O4L6 (1) (L = diphenylphosphinate, Ph2PO2-), with a hydrogen atom donor, phenothiazine (pzH), forms the dehydrated cluster Mn4O2L6 (2), which has lost two mu-oxo bridges by reduction to water (H2O). The formation of 2 was established by electrospray mass spectrometry, whereas FTIR spectroscopy confirmed the release of water molecules into solution during the reduction of 1. UV-vis and EPR spectroscopies established the stoichiometry and chemical form of the pzH product by showing the production of 4 equiv of the neutral pz radical. By contrast, the irreversible decomposition of 1 to individual Mn(II) ions occurs if the reduction is performed using electrons provided by various proton-lacking reductants, such as cobaltocene or electrochemical reduction. Thus, cubane 1 undergoes coupled four-electron/four-proton reduction with the release of two water molecules, a reaction formally analogous to the reverse sequence of the steps that occur during photosynthetic water oxidation leading to O2 evolution. 1H NMR of solutions of 2 reveal that all six of the phosphinate ligands exhibit paramagnetic broadening, due to coordination to Mn ions, and are magnetically equivalent. A symmetrical core structure is thus indicated. We hypothesize that this structure is produced by the dynamic averaging of phosphinato ligand coordination or exchange of mu-oxos between vacant mu-oxo sites. The paramagnetic 1H NMR of water molecules in solution shows that they are able to freely exchange with water molecules that are bound to the Mn ion(s) in 2, and this exchange can be inhibited by the addition of coordinating anions, such as chloride. Thus, 2 possesses open or labile coordination sites for water and anions, in contrast to solutions of 1, which reveal no evidence for water coordination. Complex 2 exhibits greater paramagnetism than that of 1, as seen by 1H NMR, and it possesses a broad (440 G wide) EPR absorption, centered at g = 2, that follows a Curie-Weiss temperature dependence (10-40 K) and is visible only at low temperatures, compared to EPR-silent 1. Its comparison to a spin-integration standard reveals that 2 contains 2 equiv of Mn(II), which is in agreement with the formal oxidation state of 2Mn(II)2Mn(III) that was derived from the titration. The EPR and NMR data for 2 are consistent with a loss of two of the intermanganese spin-exchange coupling pathways, versus 1, which results in two "wingtip" Mn(II) S = 5/2 spins that are essentially magnetically uncoupled from the diamagnetic Mn2O2 base. Bond-enthalpy data, which show that O2 evolution via the reaction 1-->2 + O2, is strongly favored thermodynamically but is not observed in the ground state due to an activation barrier, are included. This activation barrier is hypothesized to arise, in part, from the constraining effect of the facially bridging phosphinate ligands.