Solvent relaxation study of pH-dependent hydration of poly(oxyethylene) shells in polystyrene-block-poly(2-vinylpyridine)-block-poly(oxyethylene) micelles in aqueous solutions

The hydration of the poly(oxyethylene) shell in polystyrene-block-poly(2-vinylpyridine)-block-poly(oxyethylene) micelles was investigated by monitoring the solvent relaxation response of a solvent-sensitive fluorophore (patman). It has been found that the relaxation occurs on the nanosecond time scale. Results for triblock copolymer micelles have been compared with those obtained for polystyrene-block-poly(2-vinylpyridine) micelles in order to evaluate the effect of the outer polyoxyethylene layer. Considerable pH-dependent changes in the hydration of poly(oxyethylene) units at the poly(2-vinylpyridine)/polyoxyethylene interface were observed. Additionally, the paper shows that the solvent relaxation technique is a suitable tool for studying polymeric nanoparticles and that the measurement of time-dependent half-width of the emission spectrum allows for estimation of the extent of relaxation process observed by a given experimental setup.     

Kinetics and mechanism of anation of hexaaquochromium(III) by 2-aminopyridine

The kinetics of substitution of aqua ligands from the hexaaquochromium(III) ion by 2-aminopyridine (2-ampyH⁺) in aqueous medium has been studied spectrophotometrically in the 40–55° C range. The rate law involving the outer sphere complex formation has been established at pH 2.7 as

Kinetics and mechanism of the interaction of hexaaquochromium(III) with octacyanomolybdate(IV) ions

The kinetics of the interaction of hexaaquochromium(III) ion with potassium octacyanomolybdate(IV) have been studied using conductance and spectrophotometric data. The mechanism of the reaction is discussed and the effect of H⁺ ion and the ionic strength on the rate of the reaction determined. The reaction is found to be pseudo-first order with respect to potassium octacyanomolybdate(IV) and inverse first order with [H₃O⁺]. The rate of the reaction increases with increase in ionic strength and temperature. Activation parameters have been calculated using the Arrhenius equation and have the values ΔE^* = 1.3 × 10² kJ mol^?1, ΔH^* = 129 kJ mol^?1, ΔS^* = ?315 e.u., ΔF^* = 2.3 × 10² kJ and A = 1.5 × 10^?3. The mechanism proposed is based on ion-pair formation and the rate equation obtained is given by: k_obs=$\text{[}{}_{\mathrm{kK}}{}_{\mathrm{<mtext>E</mtext>}}\text{[H}{}_3\text{O}{}^{\mathrm{+}}\text{]+}\text{k′K′k}{}_{\mathrm{E}}\text{k}{}_{\mathrm{h}}\text{][Mo(CN)}{}_8{}^{\mathrm{4?}}\text{]}\text{[H}{}_3\text{O}{}^{\mathrm{+}}\text{]+}\text{k}{}_{\mathrm{<mtext>h</mtext>}}\text{+[}\text{K}{}_{\mathrm{<mtext>E</mtext>}}\text{[H}{}_3\text{O}{}^{\mathrm{+}}\text{]+}\text{K′}{}_{\mathrm{E}}\text{k}{}_{\mathrm{h}}\text{][Mo(CN)}{}_8{}^{\mathrm{4?}}\text{]}$     

Molecular dynamics study of the hydration of lanthanum(III) and europium(III) including many-body effects

Lanthanides complexes are widely used as contrast agents in magnetic resonance imaging (MRI) and are involved in many fields such as organic synthesis, catalysis, and nuclear waste management. The complexation of the ion by the solvent or an organic ligand and the resulting properties (for example the relaxivity in MRI) are mainly governed by the structure and dynamics of the coordination shells. All of the MD approaches already carried out for the lanthanide(III) hydration failed due to the lack of accurate representation of many-body effects. We present the first molecular dynamics simulation including these effects that accounts for the experimental results from a structural and dynamic (water exchange rate) point of view.     

Theoretical study on electron transfer in biological systems (III)

Iritramolwular electron transfer of nlctal-containing spiro π-electron system was studied by AM1 method in the MOPAC-ET program developed by the present group. The results indicated that with the increasing of the outer electric field F, the activation energy of the reaction decreased. When F reaches a certain threshold value, the activation energy barrirr becomcs zero and the rate of reaction achieves the largest value. The results also indicated that electron transfer matrix elements V_AB and reorganization energy λ were not obviously affected by outer electric field while the exothermicity ΔE was directly proportional to it.     

Solvent effects on the redox potentials of tetraethylammonium hexacyanomanganate(III) and hexacyanoferrate(III)

Tetraethylammonium hexacyanomanganate(III) was studied in formamide, N-methylformamide, methanol, propylenecarbonate, N,N-dimethylthioformamide, N-methylthiopyrrolidinone(2), butyrolactone, acetonitrile, dimethylsulfoxide, N,N-dimethylformamide, N-methylpyrrolidinone(2), nitromethane and tetramethylenesulfone employing polarographic and voltammetric techniques. Reversible or nearly reversible behaviour for the reaction Mn(CN)₆^3?/Mn(CN)₆^2? was observed in most solvents on the stationary platinum electrode. The reaction Mn(CN)₆^3?/Mn(CN)₆^4? was studied on both the dropping mercury electrode and the stationary platinum electrode. Besides the reaction Mn(CN)₆^3?/Mn(CN)₆^4? several anodic waves due to successive reactions of mercury with the cyano-ligand of the complex occurred at the dropping mercury electrode. No redox reaction for (et₄N)₃Mn(CN)₆ was found in nitromethane. The polarographic behaviour of tetraethylammonium hexacyanoferrate(III) was studied in formamide, N-methylformamide, N-methylpyrrolidinone(2) and butyrolactone. The variation of E_1/2 and 1/2 (E_pa+E_pc) values versus bisphenylchromium(I)/bisbiphenylchromium(0) as reference redox system of the processes Mn(CN)₆^3?/Mn(CN)₆^2?, Mn(CN)₆^3?/Mn(CN)₆^4? and Fe(CN)₆^3?/Fe(CN)₆^4? with the nature of the solvent is discussed within the donor-acceptor concept. Correlations between the E_1/2 and 1/2(E_pa+E_pc) values and the acceptor properties of the solvent have been observed. The preparation of tetraethyl- and tetrabutylammonium hexacyanomanganate(III) is described.     

Solvent effects on electron transfer reactions. The system iron(III)-phenothiazine in water-alcohol mixtures

Summary The effect of solvent composition (aqueous mixtures of methanol, ethanol, propan-1-ol and propan-2-ol) on the rate constants and activation parameters of the electron transfer between iron(III) and phenothiazine has been investigated. The dependence of the kinetic parameters on the solvent composition is discussed with reference to previously investigated systems.     

Phosphinic derivative of DTPA conjugated to a G5 PAMAM dendrimer: an 17O and 1H relaxation study of its Gd(III) complex

A DTPA-based chelate containing one phosphinate group was conjugated to a generation 5 polyamidoamine (PAMAM) dendrimer via a benzylthiourea linkage. The Gd(III) complex of this novel conjugate has potential as a contrast agent for magnetic resonance imaging (MRI). The chelates bind Gd3+via three nitrogen atoms, four carboxylates and one phosphinate oxygen, and one water molecule completes the inner coordination sphere. The monomer Gd(III) chelates bearing nitrobenzyl and aminobenzyl groups ([Gd(DTTAP-bz-NO2)(H2O)]2- and [Gd(DTTAP-bz-NH2)(H2O)]2-) as well as the dendrimeric Gd(III) complex G5-(Gd(DTTAP))63) were studied by multiple-field, variable temperature 17O and 1H NMR. The rate of water exchange is faster than that of [Gd(DTPA)(H2O)]2- and very similar on the two monomeric complexes (8.9 and 8.3 x 10(6) s-1 for [Gd(DTTAP-bz-NO2)(H2O)]2- and [Gd(DTTAP-bz-NH2)(H2O)]2-, respectively), while it is decreased on the dendrimeric conjugate (5.0 x 10(6) s-1). The Gd(III) complex of the dendrimer conjugate has a relaxivity of 26.8 mM-1 s-1 at 37 degrees C and 0.47 T (corresponding to 1H Larmor frequency of 20 MHz). Given the contribution of the second sphere water molecules to the overall relaxivity, this value is slightly higher than those reported for similar size dendrimers. The experimental 17O and 1H NMR data were fitted to the Solomon-Bloembergen-Morgan equations extended with a contribution from second coordination sphere water molecules. The rotational dynamics of the dendrimeric conjugate was described in terms of global and local motions with the Lipari-Szabo approach.     

Mapping polypeptide self-recognition through (1)H off-resonance relaxation

1H NMR relaxation rates provide a readily available and sensitive probe ideally suited to investigate the weak (KD approximately micromolar to millimolar range) interactions that frequently mediate polypeptide oligomerization in the early steps of amyloid fibrillogenesis. However, the measurement of transverse and longitudinal 1H relaxation rates is experimentally challenging due to J-transfer and selectivity problems in CPMG and inversion-recovery experiments, respectively. We show here that these problems are effectively circumvented by measuring nonselective off-resonance relaxation rates using an effective field tilted by 35.5 degrees . When applied to the Halpha spins of the Abeta (12-28) peptide, the proposed experiment provides a residue-resolution self-recognition map which is fully consistent with previous independent mutational studies. The method is anticipated to be widely applicable not only to the fast growing family of amyloidogenic peptides but also to the screening and mapping of protein-ligand interactions in general.     

Solvent effect on (2,2,6,6-Tetramethylpiperidine-1-yl)oxyl (TEMPO): a RISM-SCF-SEDD study

The solvent dependence of several properties of (2,2,6,6-Tetramethylpiperidine-1-yl)oxyl (TEMPO) is investigated by the reference interaction site model self-consistent field (RISM-SCF) theory. Time-dependent density functional theory (TDDFT) coupled with RISM-SCF-SEDD (spatial electron density distribution) is used to evaluate the n ?? ??* transition energies and the results are compared to the reported experimental values.     

Solvent extraction of trivalent lanthanide Pr(III), Ho(III) and Er(III) with N-benzoyl-N-phenylhydroxylamine

Extraction of Pr(III), Ho(III) and Er(III) has been studied in the pH range of 1–10 with N-benzoyl-N-phenylhydroxylamine (BPHA) in benzene. The separation was found to be quantitative in borate media from pH 7 to 10, at an ionic strength of 0.1M (H⁺, BO₃ ^3–). The stoichiometric composition of the complexes under the optimal conditions of shaking time, pH and reagent concentration was formulated using slope analysis and found to be M(BPHA)₃, where M=Pr(III), Ho(III) and Er(III). The effect of various masking agents shows that citrate, ascorbate, EDTA, oxalate, fluoride and phosphate form stable complexes with these rare earths as compared to BPHA. The decontamination factors for different cations with respect to these rare earths under the optimum conditions have been evaluated.     

Solvent effect on the rate of homogeneous electron exchange of the Eu(III)-Eu(II) system in water-DMF mixtures

The kinetics of homogeneous electron exchange Eu³⁺/Eu²⁺ has been investigated in 1M HClO₄, water+dimethylformamide (DMF) mixed solvent by using labeled europium. Non-monotonic variations of the electron exchange rate constants as a function of the solvent composition was observed.     

Solvent Extraction Separation of Iridium(III) from Rhodium(III) by N-n-Octylaniline

The use ofN-n-octylaniline for the extraction of iridium(III) from malonate media is studied at pH 8.5. Iridium(III) extracted in the organic phase was stripped with 2.0 M hydrochloric acid and was determined spectrophotometrically by the stannous chloride–hydrobromic acid method at 385 nm. The extraction system is studied as a function of the equilibration time, diluent, reagent concentration and diverse ions. Experimental data have been analyzed graphically to determine the stoichiometry of the extracted species. It was found that the extraction of iridium(III) proceeds by an anion exchange mechanism and transforms into the extracted species [RR"NH₂ ⁺Ir(C₃H₂O₄)₂ ^–]_org. The method is simple, rapid, and selective and has been devised for the sequential separation of iridium(III) from rhodium(III), not only from each other, but also from other accompanying Platinum Group Metals (PGMs), Au(III), and base metals.     

Chiral 1D Dy(III) compound showing slow magnetic relaxation

Herein, we present one chiral Dy(III) compound, namely Dy(L)(3)(1, HL = 2-methylbenzoic acid), synthesized from the achiral HL ligand. Within 1, along a direction the seven-coordinated Dy(III) ion are bridged by double L carboxylate and single L oxygen to give rise to the 1D helical chain. The magnetic studies suggest small intra-chain ferromagnetic interactions. Alternating current (AC) magnetic measurements show a frequency dependence of magnetic susceptibilities in both in-phase, χ', and out-of-phase, χ'. However, no obvious peak can be found even under 1000 Oe static field, suggesting the prevalence of quantum tunnelling effect at low temperature.     

Photoacoustic spectroscopy study on intramolecular energy transfer and relaxation processes of Tb(III) complexes

The photoacoustic (PA) amplitude spectra and luminescence spectra of different Tb(III) complexes (Tb(AA)3.2H2O Na[Tb(AA)4], Tb(AA)3bpy and Tb(AA)3phen) have been measured, and the PA phase shifts of the different complexes calculated. Combined with the luminescence spectra, the PA amplitude spectra reflected the variation of the luminescence efficiency and the PA phase is directly relative to the relaxation processes. According to the variation of the luminescence efficiency and the phase shift, the intramolecular energy transfer and relaxation processes of different Tb(III) complexes were discussed.     

Longitudinal (1)H relaxation optimization in TROSY NMR spectroscopy

A general method to enhance the sensitivity of the multidimensional NMR experiments performed at high-polarizing magnetic field via the significant reduction of the longitudinal proton relaxation times is described. The method is based on the use of two vast pools of "thermal bath" 1H spins residing on hydrogens covalently attached to carbon and oxygen atoms in 13C,15N labeled and fully protonated or fractionally deuterated proteins to uniformly enhance longitudinal relaxation of the 1HN spins and concomitantly the sensitivity of multipulse NMR experiments. The proposed longitudinal relaxation optimization is implemented in the 2D [15N,1H]-LTROSY, 2D [15N,1H]-LHSQC and 3D LTROSY-HNCA experiments yielding the factor 2-2.5 increase of the maximal signal-to-noise ratio per unit time at 600 MHz. At 900 MHz, the predicted decrease of the 1HN longitudinal relaxation times can be as large as one order of magnitude, making the proposed method an important tool for protein NMR at high magnetic fields.     

Magnetic relaxation characteristics of Gd(III) in N,N-dimethylformamide according to 1H and 14N NMR data

The solvation of Gd³⁺ is studied in N,N-dimethylformamide by ¹H and ¹⁴N NMR in the temperature range of 298–373 K.