Characterization of gluten-free bread crumb baked at atmospheric and reduced pressures using TD-NMR

This research aimed to study the effects of using a partial vacuum for bread baking on macromolecules and water distribution in gluten-free bread. Bread baking under partial vacuum results in greater oven rise and a larger gas fraction in the crumb. Because water's boiling point decreases under reduced pressure, it was expected that its distribution within the dough and its interactions with the others dough's constituents (mainly starch) would differ from those in bread baked under atmospheric pressure. Time-domain nuclear magnetic resonance was used, as it has the rare capacity to quantify both gelatinization and retrogradation of starch. Complementary rheological measurements made it possible to show that crumb Young's modulus was mostly influenced by the gas fraction whereas there was little change in starch gelatinization and retrogradation when dough was baked under partial vacuum. When insufficiently hydrated (48%), the volume of breads was practically the same whatever the baking process. Meanwhile, the nuclear magnetic resonance results suggested that amylose short-term crystallization (on cooling) is dependent on water content. In addition, crumb Young's modulus during storage at room temperature correlated with an increase in free induction decay signal intensity.     

Solvent Effects in the Homogeneous Catalytic Reduction of Propionaldehyde with Aluminium Isopropoxide Catalyst: New Insights from PFG NMR and NMR Relaxation Studies

Solvent effects in homogeneous catalysis are known to affect catalytic activity. Whilst these effects are often described using qualitative features, such as Kamlet-Taft parameters, experimental tools able to quantify and reveal in more depth such effects have remained unexplored. In this work, PFG NMR diffusion and T₁ relaxation measurements have been carried out to probe solvent effects in the homogeneous catalytic reduction of propionaldehyde to 1-propanol in the presence of aluminium isopropoxide catalyst. Using data on diffusion coefficients it was possible to estimate trends in aggregation of different solvents. The results show that solvents with a high hydrogen-bond accepting ability, such as ethers, tend to form larger aggregates, which slow down the molecular dynamics of aldehyde molecules, as also suggested by T₁ measurements, and preventing their access to the catalytic sites, which results in the observed decrease of catalytic activity. Conversely, weakly interacting solvents, such as alkanes, do not lead to the formation of such aggregates, hence allowing easy access of the aldehyde molecules to the catalytic sites, resulting in higher catalytic activity. The work reported here is a clear example on how combining traditional catalyst screening in homogeneous catalysis with NMR diffusion and relaxation time measurements can lead to new physico-chemical insights into such systems by providing data able to quantify aggregation phenomena and molecular dynamics.     

Dynamic mechanical and solid-state NMR determination of molecular-scale heterogeneity in a raw elastomer,a microcrystalline polymer,and their blend

Dynamic mechanical and solid-state ¹³C nuclear magnetic resonance (NMR) analyses have been used to assess a molecular-scale heterogeneity in a raw elastomer (butadiene-acrylonitrile copolymer elastomer, NBR), a microcrystalline polymer (poly(vinyl chloride), PVC), and their 50/50 blend. The presence of the microcrystalline heterogeneity in PVC and in the blend was characterized by the temperature dependence of the frequency-swept dynamic mechanical behavior. The NMR T_1ρ relaxation experiments with cross-polarization (CP) and magic-angle spinning (MAS) revealed that (1) NBR contained a substantial fraction (ca. 27%) of a molecular-scale heterogeneity identified as butadiene blocks, (2) the fraction of microcrystallites in PVC was ca. 14%, (3) pure phases of both component polymers were present in the blend, dispersed in the mixed matrix, (4) the upper limit of the heterogeneous domains was estimated to be ca. 2.4 nm, and (5) fractions of heterogeneity tend to increase upon blending, indicating that the solubility of the butadiene block and syndiotactic PVC block decreases in the blend. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys, 35: 709–716, 1997     

Characterization of the molecular reorientational dynamics of the neat ionic liquid 1-butyl-3-methylimidazolium bromide in the super cooled state using 1H and 13C NMR spectroscopy

The (13)C spectra and (13)C longitudinal relaxation times (T(1)) were measured to investigate the segmental motion of the neat ionic liquid 1-butyl-3-methylimidazolium bromide ([bmim]Br) in the super cooled state. The (13)C signals of the imidazole ring significantly broadened at 283 K, whereas many other signals were unchanged. In the process of lowering temperature, the [bmim]Br changed to the solid state at ca 273 K without showing the rapid phase transition. Only the (13)C signal of the terminal methyl group in the butyl chain was still observed at 263 K, indicating that the motion of the imidazole ring was extremely restricted, whereas the methyl group in the butyl chain was active even in the solid state. The (13)C-T(1) values measured as a function of temperature also supported the discrete segmental motions of the [bmim](+) cation.     

Dynamic and magnetic susceptibility effects on the MAS NMR linewidth of a tetrapeptide bound to different resins

Under magic angle spinning, the NMR spectrum of the tetrapeptide Ala‐Ile‐Gly‐Met bound to a Wang resin, and swollen in DMF, exhibits proton and carbon linewidths that are sharp enough to allow the complete characterization of the peptide using classical liquid‐state NMR methods. The proton linewidths of the bound peptide remain, however, about three times larger than those of the free peptide in solution. The residual NMR linewidth originates essentially from incompletely averaged magnetic susceptibility effects due to the Wang resin. Replacing the aromatic Wang resin with a PEGA or POEPOP resin removes this effect. To investigate the contribution to line broadening of the peptide dynamics, relaxation studies were performed on the peptide bound to Wang and POEPOP resins. Copyright © 2001 John Wiley & Sons, Ltd.     

Characterization of the complex formation of 1,6‐anhydro‐β‐maltotriose with potassium using 1H and 39K NMR spectroscopy

The ¹H and ³⁹K longitudinal relaxation times (T₁) and ¹H diffusion coefficients were measured to investigate the complex formation of 1,6‐anhydro‐β‐maltotriose and potassium ions. Although the ¹H‐T₁ values of H3′, H5′, H1″ and H4″ decreased in the presence of potassium ions, ¹H chemical shifts and ¹H diffusion coefficients did not show significant changes. The long‐range coupling constants of ³J_C?H around the glycosyl bonds did not show significant changes either. In the measurements of ³⁹K spectra, the ³⁹K signal obviously broadened and the ³⁹K‐T₁ values decreased in the presence of 1,6‐anhydro‐β‐maltotriose, indicating the complex formation of 1,6‐anhydro‐β‐maltotriose and potassium ions. These results indicate that the conformation and molecular volume were unaffected in the complex formation. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis of a new quaternary phosphonium salt: NMR study of the conformational structure and dynamics

A novel phosphonium salt based on pyridoxine was synthesized. Conformational analysis of the compound in solution was performed using dynamic NMR experiments and calculations. The obtained results revealed some differences in the conformational transitions and the energy parameters of the conformational exchange of the studied compound in comparison to previously reported data for other phosphorus‐containing pyridoxine derivatives. It was shown that increasing the substituent at the C‐11 carbon leads to greater differences in the populations of stable states and the corresponding equilibrium energies. Copyright © 2015 John Wiley & Sons, Ltd.     

Testing the ONIOM G2R3 model against donor-acceptor dissociation energies of group 13-15 complexes: accuracy comparable to CCSD(T)/aug-CC-pVTZ at a fraction of the resource cost

The ability of the composite three-layer ONIOM G2R3 (OG2R3) method to match experimental dissociation energies for group 13-15 donor-acceptor complexes was examined for a database of 34 complexes. The composite approach provides energies that agree reasonably with experiment, performing nearly as well as both the CCSD(T)/aug-CC-pVTZ and CCSD(T)/6-311+G(2df, 2p) models for small molecules and nearly as well as the latter for slightly larger ones. Broadly, all three models exhibit average absolute errors of ～3 kcal mol(-1) , and root mean square errors of ～4 kcal mol(-1) . The average signed error suggest that the OG2R3 approach systematically underbinds by ～2.3 kcal mol(-1) ; if this is used as a general correction, the approach performs as well or better than the pure CCSD(T) models. However, the OG2R3 model can be applied to molecules too large to be studied by the other CCSD(T) methods, as it requires only a fraction of the time and computer resources.     

Improved resolution in two-dimensional 1H NMR spectra of peptides by band-selective, homonuclear decoupling during both the evolution and acquisition periods: application to characterization of the binding of peptides by heparin

Two-dimensional 1H NMR experiments that achieve band-selective, homonuclear decoupling in both the indirectly detected F1 and directly detected F2 dimensions were used to assign the highly overlapped 1H NMR spectrum of the peptide Ac-SRGKARVRAKVKDQTK-NH2, both free in solution and bound to heparin. Band-selective, homonuclear decoupling during the evolution period was achieved using a double pulsed field gradient spin-echo (DPFGSE) with semi-selective shaped pulses; band-selective, homonuclear decoupling during the acquisition period was achieved by time-shared semi-selective shaped pulse decoupling. Regular TOCSY, ROESY and NOESY spectra and TOCSY, ROESY and NOESY spectra measured with band-selective, homonuclear decoupling in the evolution (F1) dimension (BASHD-TOCSY, ROESY and NOESY spectra) and with band-selective, homonuclear decoupling in both the F1 and F2 dimensions (D-(or Double)-BASHD-TOCSY, ROESY and NOESY spectra) are reported and compared for the peptide and its heparin complex. Complete assignment of the 1H-NMR spectra of the free and heparin-complexed peptide was achieved with the high resolution of the D-BASHD-TOCSY, ROESY and NOESY spectra. Characterization of the heparin-complexed peptide is of interest because of the ability of the peptide to neutralize the anticoagulant activity of heparin.     

Physicochemical characterization of the dimeric lanthanide complexes [en{Ln(DO3A)(H2O)}2] and [pi{Ln(DTTA)(H2O)}2]2-: a variable-temperature 17O NMR study

The Gd(III) complexes of the two dimeric ligands [en(DO3A)2] {N,N'-bis[1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecan-10-yl-methylcarbonyl]-N,N'-ethylenediamine} and [pi(DTTA)2]8- [bisdiethylenetriaminepentaacetic acid (trans-1,2-cyclohexanediamine)] were synthesized and characterized. The 17O NMR chemical shift of H2O induced by [en{Dy(DO3A)}2] and [pi{Dy(DTTA)}2]2- at pH 6.80 proved the presence of 2.1 and 2.2 inner-sphere water molecules, respectively. Water proton spin-lattice relaxation rates for [en{Gd(DO3A)(H2O)}2] and [pi{Gd(DTTA)(H2O)}2]2- at 37.0 +/- 0.1 degrees C and 20 MHz are 3.60 +/- 0.05 and 5.25 +/- 0.05 mM(-1) s(-1) per Gd, respectively. The EPR transverse electronic relaxation rate and 17O NMR transverse relaxation time for the exchange lifetime of the coordinated H2O molecule and the 2H NMR longitudinal relaxation rate of the deuterated diamagnetic lanthanum complex for the rotational correlation time were thoroughly investigated, and the results were compared with those reported previously for other lanthanide(III) complexes. The exchange lifetimes for [en{Gd(DO3A)(H2O)}2] (769 +/- 10 ns) and [pi{Gd(DTTA)(H2O)}2]2- (910 +/- 10 ns) are significantly higher than those of [Gd(DOTA)(H2O)]- (243 ns) and [Gd(DTPA)(H2O)]2- (303 ns) complexes. The rotational correlation times for [en{Gd(DO3A)(H2O)}2] (150 +/- 11 ps) and [pi{Gd(DTTA)(H2O)}2]2- (130 +/- 12 ps) are slightly greater than those of [Gd(DOTA)(H2O)]- (77 ps) and [Gd(DTPA)(H2O)]2- (58 ps) complexes. The marked increase in relaxivity (r1) of [en{Gd(DO3A)(H2O)}2] and [pi{Gd(DTTA)(H2O)}2]2- result mainly from their longer rotational correlation time and higher molecular weight.     

Biomimetic Interaction between FeII and O2: Effect of the Second Coordination Sphere on O2 Binding to FeII Complexes: Evidence of Coordination at the Metal Centre by a Dissociative Mechanism in the Formation of μ‐Oxo Diferric Complexes

We report that the formation of μ‐oxo diferric compounds from O₂ and FeCl₂ complexes within the tris(2‐pyridylmethyl)amine series (N. K. Thallaj et al. Chem. Eur. J., 2008 , 14, 6742–6753) involves coordination of O₂ to the metal centre and that this reaction occurs following initial dissociation of the bound equatorial chloride anion. We also report evidence of the formation of a reduced form of dioxygen by an inner‐sphere mechanism, thus leading to modification of the ligand. The solid‐state structures of [FeCl₂L] complexes (L¹=mono(α‐pivalamidopyridylmethyl)bis(2‐pyridylmethyl)amine, L²=mono(α‐pivalesteropyridylmethyl)bis(2‐pyridylmethyl)amine, L³=bis(α‐pivalamidopyridylmethyl)mono(2‐pyridylmethyl)amine are described, and spectroscopic data support the structural retention in solution. In [FeCl₂L³], the two amide hydrogen atoms stabilise the equatorial chloride anion in such a way that its exchange by a weak ligand is impossible: [FeCl₂L³] is perfectly oxygen‐stable. In [FeCl₂L²], the equatorial chloride anion is completely free to move and coordination of O₂ can take place. The reaction product with [FeCl₂L²] is a μ‐oxo diferric complex in which the ester function has been transformed into a phenol group. This conversion can be seen as a hydrolysis reaction in basic medium, hence supporting the initial formation of a reduced form of dioxygen in the medium. Complex [FeCl₂L¹] exhibits a very weak reactivity with O₂, in line with a semistabilised equatorial chloride counteranion.     

The application of (199)Hg NMR and (199m)Hg perturbed angular correlation (PAC) spectroscopy to define the biological chemistry of Hg(II): a case study with designed two- and three-stranded coiled coils

The use of de novo designed peptides is a powerful strategy to elucidate Hg(II)-protein interactions and to gain insight into the chemistry of Hg(II) in biological systems. Cysteine derivatives of the designed alpha-helical peptides of the TRI family [Ac-G-(L(a)K(b)A(c)L(d)E(e)E(f)K(g))(4)-G-NH(2)] bind Hg(II) at high pH values and at peptide/Hg(II) ratios of 3:1 with an unusual trigonal thiolate coordination mode. The resulting Hg(II) complexes are good water-soluble models for Hg(II) binding to the protein MerR. We have carried out a parallel study using (199)Hg NMR and (199m)Hg perturbed angular correlation (PAC) spectroscopy to characterize the distinct species that are generated under different pH conditions and peptide TRI L9C/Hg(II) ratios. These studies prove for the first time the formation of [Hg{(TRI L9C)(2)-(TRI L9C-H)}], a dithiolate-Hg(II) complex in the hydrophobic interior of the three-stranded coiled coil (TRI L9C)(3). (199)Hg NMR and (199m)Hg PAC data demonstrate that this dithiolate-Hg(II) complex is different from the dithiolate [Hg(TRI L9C)(2)], and that the presence of third alpha-helix, containing a protonated cysteine, breaks the symmetry of the coordination environment present in the complex [Hg(TRI L9C)(2)]. As the pH is raised, the deprotonation of this third cysteine generates the trigonal thiolate-Hg(II) complex Hg(TRI L9C)(3)(-) on a timescale that is slower than the NMR timescale (0.01-10 ms). The formation of the species [Hg{(TRI L9C)(2)(TRI L9C-H)}] is the result of a compromise between the high affinity of Hg(II) to form dithiolate complexes and the preference of the peptide to form a three-stranded coiled coil.