Tuesday, December 09, 2008

J Phys Chem B - October to December

Solvation of Carbohydrates in N,N′-Dialkylimidazolium Ionic Liquids: A Multinuclear NMR Spectroscopy Study
Richard C. Remsing
, Gonzalo Hernandez§, Richard P. Swatloski, Walter W. Massefski, Robin D. Rogers* and Guillermo Moyna*
J. Phys. Chem. B, 2008, 112 (35), pp 11071–11078

Abstract: The solvation of carbohydrates in N,N′-dialkylimidazolium ionic liquids (ILs) was investigated by means of 13C and 35/37Cl NMR relaxation and 1H pulsed field gradient stimulated echo (PFG-STE) diffusion measurements. Solutions of model sugars in 1-n-butyl-3-methylimidazolium chloride ([C4mim]Cl), 1-allyl-3-methylimidazolium chloride ([CC2mim]Cl), and 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]) were studied to evaluate the effects of cation and anion structure on the solvation mechanism. In all cases, the changes in the relaxation times of carbon nuclei of the IL cations as a function of carbohydrate concentration are small and consistent with the variation in solution viscosities. Conversely, the 35/37Cl and 13C relaxation rates of chloride ions and acetate ion carbons, respectively, have a strong dependency on sugar content. For [C2mim][OAc], the correlation times estimated from 13C relaxation data for both ions reveal that, as the carbohydrate concentration increases, the reorientation rate of the anion decreases faster than that of the cation. Although not as marked as the variations observed in the relaxation data, similar trends were obtained from the analysis of cation and, in the case of [C2mim][OAc], anion self-diffusion coefficients of the sugar/IL systems. Our results show that the interactions between the IL cation and the solutes are nonspecific, confirm that the process is governed by the interactions between the IL anion and the carbohydrate, and, more importantly, indicate no change in the solvation mechanism regardless of the structure of the anion.

Effects of Salt and Nanoparticles on the Segmental Motion of Poly(ethylene oxide) in Its Crystalline and Amorphous Phases: 2H and 7Li NMR Studies
M. Vogel
*, C. Herbers and B. Koch
J. Phys. Chem. B, 2008, 112 (36), pp 11217–11226

Abstract: We use 2H NMR to investigate the segmental motion of poly(ethylene oxide) (PEO) in neat and nanocomposite materials that do and do not contain salt. Specifically, in addition to a neat low-molecular-weight PEO, we study mixtures of this polymer with TiO2 nanoparticles and LiClO4. To characterize the polymer dynamics over a wide range of time scales, we combine 2H NMR spin−lattice relaxation, line-shape, and stimulated-echo analyses. The results consistently show that the presence of nanoparticles hardly affects the behavior of the polymer, while addition of salt leads to substantial changes; e.g., it reduces the crystallinity. For neat PEO and a PEO−TiO2 mixture, stimulated-echo spectroscopy enables measurement of rotational correlation functions for the crystalline phase. Analysis of the decays allows us to determine correlation times, to demonstrate the existence of a nonexponential relaxation, which implies a high complexity of the polymer dynamics in the crystal, and to show that the reorientation can be described as a large-angle jump. For a PEO−TiO2−LiClO4 mixture, we use 2H and 7Li NMR to study the polymer and the lithium dynamics, respectively. Analysis of the 7Li spin−lattice relaxation reveals a high lithium ionic mobility in this nanocomposite polymer electrolyte. The 7Li stimulated-echo decay is well described by a stretched exponential extending over about 6 orders of magnitude, indicating that a broad and continuous distribution of correlation times characterizes the fluctuations of the local lithium ionic environments.

Nuclear Magnetic Shielding of the 113Cd(II) Ion in Aqua Solution: A Combined Molecular Dynamics/Density Functional Theory Study
Xin Li
*, Zilvinas Rinkevicius, Yaoquan Tu§, He Tian and Hans Ågren
J. Phys. Chem. B, 2008, 112 (36), pp 11347–11352

Abstract: We present a combined molecular dynamics simulation and density functional theory investigation of the nuclear magnetic shielding constant of the 113Cd(II) ion solvated in aqueous solution. Molecular dynamics simulations are carried out for the cadmium−water system in order to produce instantaneous geometries for subsequent determination of the nuclear magnetic shielding constant at the density functional theory level. The nuclear magnetic shielding constant is computed using a perturbation theory formalism, which includes nonrelativistic and leading order relativistic contributions to the nuclear magnetic shielding tensor. Although the NMR shielding constant varies significantly with respect to simulation time, the value averaged over increasing number of snapshots remains almost constant. The paramagnetic nonrelativistic contribution is found to be most sensitive to dynamical changes in the system and is mainly responsible for the thermal and solvent effects in solution. The relativistic correction features very little sensitivity to the chemical environment, and can be disregarded in theoretical calculations when a Cd complex is used as reference compound in 113Cd NMR experiments, due to the mutual cancelation between individual relativistic corrections.

Pore Structure, Thinning Effect, and Lateral Diffusive Dynamics of Oriented Lipid Membranes Interacting with Antimicrobial Peptide Protegrin-1: 31P and 2H Solid-State NMR Study
Sungsool Wi
* and Chul Kim
Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061
J. Phys. Chem. B, 2008, 112 (36), pp 11402–11414
Abstract: Membrane pores that are induced in oriented membranes by an antimicrobial peptide (AMP), protegrin-1 (PG-1), are investigated by 31P and 2H solid state NMR spectroscopy. We incorporated a well-studied peptide, protegrin-1 (PG-1), a β-sheet AMP, to investigate AMP-induced dynamic supramolecular lipid assemblies at different peptide concentrations and membrane compositions. Anisotropic NMR line shapes specifying toroidal pores and thinned membranes, which are formed in membrane bilayers by the binding of AMPs, have been analyzed for the first time. Theoretical NMR line shapes of lipids distributed on the surface of toroidal pores and thinned membranes reproduce reasonably well the line shape characteristics of our experimentally measured 31P and 2H solid-state NMR spectra of oriented lipids binding with PG-1. The lateral diffusions of lipids are also analyzed from the motionally averaged one- and two-dimensional 31P and 2H solid-state NMR spectra of oriented lipids that are binding with AMPs.

Oxygen-17 Nuclear Magnetic Resonance Study of the Structure of Mixed Cation Calcium−Sodium Silicate Glasses at High Pressure: Implications for Molecular Link to Element Partitioning between Silicate Liquids and Crystals
Sung Keun Lee
*, George D. Cody, Yingwei Fei and Bjorn O. Mysen
J. Phys. Chem. B, 2008, 112 (37), pp 11756–11761

Abstract: The structure of silicate glasses and the corresponding liquids at high pressure and their structure−property relations remain difficult questions in modern physical chemistry, geochemistry, and condensed matter physics. Here we report high- resolution solid-state O-17 3QMAS NMR spectra for mixed cation Ca−Na silicate glasses quenched from melts at high pressure up to 8 GPa. The spectra provide the experimental evidence for the varying pressure-dependence in two different types of nonbridging oxygen (NBO) environments (i.e., Na−O−Si and mixed {Ca,Na}−O−Si) in the single glass composition. The percentage of NBO drops significantly with increasing pressure and is a complex function of melt composition, including cation field strength of network modifying cations. A decrease in NBO fraction with pressure is negatively correlated with the element partitioning coefficient between crystals and liquids at high pressure.

High-Resolution Characterization of Liquid-Crystalline [60]Fullerenes Using Solid-State Nuclear Magnetic Resonance Spectroscopy
Sergey V. Dvinskikh
, Kazutoshi Yamamoto, David Scanu§, Robert Deschenaux§ and Ayyalusamy Ramamoorthy*

J. Phys. Chem. B, 2008, 112 (39), pp 12347–12353
Abstract: Liquid-crystalline materials containing fullerenes are valuable in the development of supramolecular switches and in solar cell technology. In this study, we characterize the liquid-crystalline and dynamic properties of fullerene-containing thermotropic compounds using solid-state natural abundance 13C NMR experiments under stationary and magic angle spinning sample conditions. Chemical shifts spectra were measured in isotropic, liquid-crystalline nematic and smectic A and crystalline phases using one-dimensional 13C experiments, while two-dimensional separated local-field experiments were used to measure the 1H−13C dipolar couplings in mesophases. Chemical shift and dipolar coupling parameters were used to characterize the structure and dynamics of the liquid-crystalline dyads. NMR data of fullerene-containing thermotropic liquid crystals are compared to that of basic mesogenic unit and mesomorphic promoter compounds. Our NMR results suggest that the fullerene−ferrocene dyads form highly dynamic liquid-crystalline phases in which molecules rotate fast around the symmetry axis on the characteristic NMR time scale of ∼10−4 s.

An Unexpected Phase Transition during the [2 + 2] Photocycloaddition Reaction of Cinnamic Acid to Truxillic Acid: Changes in Polymorphism Monitored by Solid-State NMR
Ryan C. Nieuwendaal, Marko Bertmer
and Sophia E. Hayes*
Department of Chemistry and Center for Materials Innovation, Washington University, 1 Brookings Drive, Saint Louis, Missouri 63108
J. Phys. Chem. B, 2008, 112 (41), pp 12920–12926

Abstract: We have detected a phase transition during the progress of the solid-state [2 + 2] photocycloaddition reaction of α-trans-cinnamic acid. The reaction was monitored using 13C CPMAS experiments as a function of irradiation time of the parent α-trans-cinnamic acid, which forms the product dimer, α-truxillic acid. UV light centered at 350 nm was used for photoirradiation, which is in the “tail” of the absorption band of cinnamic acid. Two different crystal polymorphs of α-truxillic acid are observed (P21/n and C2/c) at different stages of conversion of the parent crystal, assigned through 13C NMR and powder X-ray diffraction. The two polymorphs showed clear, distinguishable patterns in the 13C NMR spectra: a 2-peak versus 3-peak pattern corresponding to sites on the 4-membered sp3 hybridized ring in the photoproduct. A phase transition is observed midway through the reaction, which we have assigned to the conversion of the P21/n polymorph to the C2/c polymorph of α-truxillic acid. The packing energy of the resultant mixed crystal of cinnamic acid and truxillic acid changes during the course of the photoreaction, which allows for the C2/c polymorph of truxillic acid to appear. Both phases have been confirmed via X-ray powder diffraction. Two techniquesdifferential scanning calorimetry and solid-state CPMAS NMR using increasingly fast rotational frequenciesdemonstrate that the P21/n phase is metastable.

Hydrogen NMR of H2−TDF−D2O Clathrate
Lasitha Senadheera
and Mark S. Conradi*
Departments of Physics, and Chemistry, Washington University, One Brookings Drive, Saint Louis, Missouri 63130
J. Phys. Chem. B, 2008, 112 (44), pp 13695–13700

Abstract: The three-component clathrate H2−TDF−D2O offers hydrogen storage at lower pressure, but with reduced weight fraction of H2, compared to H2−H2O clathrate. In H2−TDF−D2O, H2 resides exclusively and singly in the small cages of structure II, allowing the rotational behavior of H2 in this nominally uniform environment to be probed. Here we report NMR measurements of the H2 line shape and relaxation times T1, T2, and T1ρ. The principal differences in the results, compared to the H2−D2O binary system, are the dips in T2 and T1ρ near 28 K due to thermally activated reorientation of TDF molecules, line-narrowing and decreases in T2 and T1ρ near 175 K due to D2O reorientations and diffusion, and the apparent absence of H2 diffusion between small cages.

Furanose Dynamics in the HhaI Methyltransferase Target DNA Studied by Solution and Solid-State NMR Relaxation
Dorothy Echodu
, Gil Goobes§, Zahra Shajani, Kari Pederson, Gary Meints, Gabriele Varani and Gary Drobny*
J. Phys. Chem. B, 2008, 112 (44), pp 13934–13944
Abstract: Both solid-state and solution NMR relaxation measurements are routinely used to quantify the internal dynamics of biomolecules, but in very few cases have these two techniques been applied to the same system, and even fewer attempts have been made so far to describe the results obtained through these two methods through a common theoretical framework. We have previously collected both solution 13C and solid-state 2H relaxation measurements for multiple nuclei within the furanose rings of several nucleotides of the DNA sequence recognized by HhaI methyltransferase. The data demonstrated that the furanose rings within the GCGC recognition sequence are very flexible, with the furanose rings of the cytidine, which is the methylation target, experiencing the most extensive motions. To interpret these experimental results quantitatively, we have developed a dynamic model of furanose rings based on the analysis of solid-state 2H line shapes. The motions are modeled by treating bond reorientations as Brownian excursions within a restoring potential. By applying this model, we are able to reproduce the rates of 2H spin−lattice relaxation in the solid and 13C spin−lattice relaxation in solution using comparable restoring force constants and internal diffusion coefficients. As expected, the 13C relaxation rates in solution are less sensitive to motions that are slower than overall molecular tumbling than to the details of global molecular reorientation, but are somewhat more sensitive to motions in the immediate region of the Larmor frequency. Thus, we conclude that the local internal motions of this DNA oligomer in solution and in the hydrated solid state are virtually the same, and we validate an approach to the conjoint analysis of solution and solid-state NMR relaxation and line shapes data, with wide applicability to many biophysical problems.

Carboxylic Acid-Doped SBA-15 Silica as a Host for Metallo-supramolecular Coordination Polymers
D. Akcakayiran, D. Mauder§, C. Hess, T. K. Sievers#, D. G. Kurth#, I. Shenderovich§, H.-H. Limbach§ and G. H. Findenegg*
J. Phys. Chem. B, 2008, 112 (46), pp 14637–14647

Abstract: The adsorption of a metallo-supramolecular coordination polymer (Fe−MEPE) in the cylindrical pores of SBA-15 silica with pure and carboxylic acid (CA) carrying pore walls has been studied. Fe−MEPE is an intrinsically stiff polycation formed by complexation of Fe(II)−acetate with an uncharged ditopic bis-terpyridine ligand. The adsorption affinity and kinetics of the Fe−MEPE chains is strongly enhanced when the pore walls are doped with CA, and when the pH of the aqueous medium or temperature is increased. The initial fast uptake is connected with a decrease of pH of the aqueous solution, indicating an ion-exchange mechanism. It is followed by a slower (presumably diffusion-controlled) further uptake. The maximum adsorbed amount of Fe−MEPE in the CA-doped material corresponds to a monolayer of Fe−MEPE chains disposed side-by-side along the pore walls. The stoichiometry of Fe−MEPE in the pores (determined by XPS) was found to be independent of the loading and similar to that of the starting material. The mean chain length of Fe−MEPE before and after embedding in the CA-doped matrix was studied by solid-state 15N NMR using partially 15N-labeled Fe−MEPE. It is shown that the average chain length of Fe−MEPE is reduced when the complex is incorporated in the pores.

Order and Dynamics of a Liquid Crystalline Dendrimer by Means of 2H NMR Spectroscopy
Valentina Domenici
*, Mario Cifelli, Carlo Alberto Veracini, Natalia I. Boiko, Elena V. Agina and Valery P. Shibaev
J. Phys. Chem. B, 2008, 112 (47), pp 14718–14728

Abstract: A complete Deuterium NMR study performed on partially deuterated liquid crystalline carbosilane dendrimer is here reported. The dendrimer under investigation shows a SmA phase in a large temperature range from 381 to 293 K, and its mesophasic properties have been previously determined. However, in this work the occurrence of a biphasic region between the isotropic and SmA phases has been put in evidence. The orientational order of the dendrimer, labeled on its lateral mesogenic units, is here evaluated in the whole temperature range by means of 2H NMR, revealing a peculiar trend at low temperatures (T < 326 K). This aspect has been further investigated by a detailed analysis of the 2H NMR spectral features, such as the quadrupolar splitting, the line shape, and the line-width, as a function of temperature. In the context of a detailed NMR analysis, relaxation times (T1 and T2) have also been measured, pointing out a slowing down of the dynamics by decreasing the temperature, which determines from one side the spectral changes observed in the NMR spectra, on the other the observation of a minimum in the T1.

A Joined Theoretical−Experimental Investigation on the 1H and 13C NMR Signatures of Defects in Poly(vinyl chloride)
Philippe d’Antuono, Edith Botek and Benoît Champagne
Joris Wieme, Marie-Françoise Reyniers and Guy B. Marin
Peter J. Adriaensens and Jan M. Gelan
J. Phys. Chem. B, 2008, 112 (47), pp 14804–14818

Abstract: 1H and 13C chemical shifts of PVC chains have been evaluated using quantum chemistry methods in order to evidence and interpret the NMR signatures of chains bearing unsaturated and branched defects. The geometrical structures of the stable conformers have been determined using molecular mechanics and the OPLS force field and then density functional theory with the B3LYP functional and the 6-311G(d) basis set. The nuclear shielding tensor has been calculated at the coupled-perturbed Kohn−Sham level (B3LYP exchange-correlation functional) using the 6-311+G(2d,p) basis set. The computational scheme accounts for the large number of stable conformers of the PVC chains, and average chemical shifts are evaluated using the Maxwell−Boltzmann distribution. Moreover, the chemical shifts are corrected for the inherent and rather systematic errors of the method of calculation by employing linear regression equations, which have been deduced from comparing experimental and theoretical results on small alkane model compounds containing Cl atoms and/or unsaturations. For each type of defect, PVC segments presenting different tacticities have been considered because it is known from linear PVC chains that the racemic (meso) dyads are characterized by larger (smaller) chemical shifts. NMR signatures of unsaturations in PVC chains have been highlighted for the internal CHCH and CHCCl units as well as for terminal unsaturations like the chloroallylic CHCHCH2Cl group. In particular, the 13C chemical shifts of the two sp2 C atoms are very close for the chloroallylic end group. The CH2 and CHCl units surrounding an unsaturation present also specific 13C chemical shifts, which allow distinguishing them from the others. In the case of the proton, the CH2 unit of the CHClCH2CClCH segment presents a larger chemical shift (2.6−2.7 ppm), while some CHCl units close to the CHCH unsaturations appear at rather small chemical shifts (3.7 ppm). The CH2Cl and CHClCH2Cl branches also display specific signatures, which result in large part from modifications of the equilibrium conformations and their reduced number owing to the increased steric interactions. These branches lead to the appearance of 13C peaks at lower field associated either to the CH unit linking the CH2Cl and CHClCH2Cl branches (50 ppm) or to the CHCl unit of the ethyl branches (60 ppm). The corresponding protons resonate also at specific frequencies: 3.5−4.0 ppm for the CH2Cl branch or 3.8−4.2 ppm for the terminal unit of the CHClCH2Cl branch. Several of these signatures have been detected in the experimental 1H and 13C NMR spectra and are consistent with the reaction mechanisms.

Acid−Base Interactions and Secondary Structures of Poly-l-Lysine Probed by 15N and 13C Solid State NMR and Ab initio Model Calculations
Alexandra Dos
, Volkmar Schimming, Sergio Tosoni§ and Hans-Heinrich Limbach*
J. Phys. Chem. B, 2008, 112 (49), pp 15604–15615

Abstract: The interactions of the 15N-labeled amino groups of dry solid poly-l-lysine (PLL) with various halogen and oxygen acids HX and the relation to the secondary structure have been studied using solid-state 15N and 13C CPMAS NMR spectroscopy (CP = cross polarization and MAS = magic angle spinning). For comparison, 15N NMR spectra of an aqueous solution of PLL were measured as a function of pH. In order to understand the effects of protonation and hydration on the 15N chemical shifts of the amino groups, DFT and chemical shielding calculations were performed on isolated methylamine−acid complexes and on periodic halide clusters of the type (CH3NH3+X−)n. The combined experimental and computational results reveal low-field shifts of the amino nitrogens upon interaction with the oxygen acids HX = HF, H2SO4, CH3COOH, (CH3)2POOH, H3PO4, HNO3, and internal carbamic acid formed by reaction of the amino groups with gaseous CO2. Evidence is obtained that only hydrogen-bonded species of the type (Lys−NH2···H−X)n are formed in the absence of water. 15N chemical shifts are maximum when H is located in the hydrogen bond center and then decrease again upon full protonation, as found for aqueous solution at low pH. By contrast, halogen acids interact in a different way. They form internal salts of the type (Lys−NH3+X−)n via the interaction of many acid−base pairs. This salt formation is possible only in the β-sheet conformation. By contrast, the formation of hydrogen-bonded complexes can occur both in β-sheet domains as well as in α-helical domains. The 15N chemical shifts of the protonated ammonium groups increase when the size of the interacting halogen anions is increased from chloride to iodide and when the number of the interacting anions is increased. Thus, the observed high-field 15N shift of ammonium groups upon hydration is the consequence of replacing interacting halogen atoms by oxygen atoms.

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