Pierre Thureau

Quantitative Structural Constraints for Organic Powders at Natural Isotopic Abundance Using Dynamic Nuclear Polarization Solid‐State NMR Spectroscopy

A straightforward method is reported to quantitatively relate structural constraints based on (13)C-(13)C double-quantum build-up curves obtained by dynamic nuclear polarization (DNP) solid-state NMR to the crystal structure of organic powders at natural isotopic abundance. This method relies on the significant gain in NMR sensitivity provided by DNP (approximately 50-fold, lowering the experimental time from a few years to a few days), and is sensitive to the molecular conformation and crystal packing of the studied powder sample (in this case theophylline). This method allows trial crystal structures to be rapidly and effectively discriminated, and paves the way to three-dimensional structure elucidation of powders through combination with powder X-ray diffraction, crystal-structure prediction, and density functional theory computation of NMR chemical shifts.

Determining chemical exchange rates of the uracil labile protons by NMR diffusion experiments

The exchange rates of the amido-protons of uracil with water were determined by NMR diffusion experiments and the results showed a factor 2 difference in lability between them, which was confirmed by more classical 2D-NMR exchange experiments.

Probing the Motional Behavior of Eumelanin and Pheomelanin with Solid-State NMR Spectroscopy: New Insights into the Pigment Properties

Melanin is the most widespread pigment in the animal kingdom. Despite its importance, its detailed structure and overall molecular architecture remain elusive. Both eumelanin (black) and pheomelanin (red) occur in the human body. These two melanin compounds show very different responses to UV-radiation exposure, which could relate to their microscopic features. Herein, the structural properties and motional behavior of natural eu- and pheomelanin extracted from black and red human hair are investigated by means of solid-state NMR spectroscopy. Several 1D and 2D NMR spectroscopic techniques were combined to highlight the differences between the two forms of the pigment. The quantitative analysis of the (1) H NMR wide-line spectra extracted from 2D (1) H-(13) C LG-WISE experiments revealed the presence of two dynamically distinguishable components in both forms. Remarkably, the more mobile fraction of the pigment showed a higher mobility with respect to the proteinaceous components that coexist in the melanosome, which is particularly evident for the red pigment. An explanation of the observed effects takes into account the different architecture of the proteinaceous matrix that constitutes the physical substrate onto which melanin polymerizes within the eu- and pheomelanosomes. Further insight into the molecular structure of the more mobile fraction of pheomelanin was also obtained by means of the analysis of 2D (1) H-(13) C INEPT experiments. Our view is that not only structural features inherent in the pure pigment, but also the role of the matrix structure in defining the overall melanin supramolecular arrangement and the resulting dynamic behavior of the two melanin compounds should be taken into account to explain their functions. The reported results could pave a new way toward the explanation of the molecular origin of the differences in the photoprotection activity displayed by black and red melanin pigments.

NMR diffusion and nuclear Overhauser investigation of the hydration properties of thymine : influence of the methyl group

The absence of preferential hydration in thymine and its lowest water accessibility with respect to uracil were evidenced by NMR diffusion and HOESY experiments; the hydration differences observed between these pyrimidine bases were attributed to the electronic rather than steric properties of the methyl group.

Towards measurement of homonuclear dipolar couplings in 1H solid-state NMR: recoupling with a rotor-synchronized decoupling scheme

We describe a magic-angle spinning NMR experiment for (1)H-(1)H homonuclear dipole-dipole coupling estimations in organic solids. The methodology involves reintroducing dipolar interactions with rotor-synchronized homonuclear decoupling pulse sequences. Frequency-selective DANTE pulses are used to isolate a specific spin pair from a natural isotopic abundance sample. The coupling of interest, between the selected spin pair, may be extracted by a non linear least-squares fit of the experimentally observed modulation of the signal intensity to an exact analytical formula. The experiment is demonstrated on natural isotopic abundance glycine and alanine powder samples.

Selective measurements of long-range homonuclear J-couplings in solid-state NMR

We demonstrate here that the principle of frequency-selective spin-echoes can be extended to the measurements of long-range homonuclear scalar J-couplings in the solid-state. Singly or doubly frequency-selective pulses were used to generate either a J-modulated experiment (S) or a reference experiment (S0). The combination of these two distinct experiments provides experimental data that, in favorable cases, are insensitive to incoherent relaxation effects, and which can be used to estimate long-range homonuclear J-couplings in multiple spin-systems. The concept is illustrated in the case of a uniformly (13)C and (15)N labeled sample of L-histidine, where the absolute value of homonuclear J-couplings between two spins separated by one, two or three covalent bonds are measured. Moreover, we show that a (2)J((15)N-C-(15)N) coupling as small as 0.9 Hz can be precisely measured with the method presented here.

Acquisition strategy to obtain quantitative diffusion NMR data

Pulsed Gradient Spin Echo (PGSE) diffusion NMR experiments constitute a powerful tool for analyzing complex mixtures because they can in principle separate the NMR spectra of each mixture component. However, because these experiments intrinsically rely on spin echoes, they are traditionally regarded as non-quantitative, due to the signal attenuation caused by longitudinal (T(1)) and transverse (T(2)) nuclear magnetic relaxation during the rather long delays of the pulse sequence. Alternatively to the quantitative Direct Exponential Curve Resolution Algorithm (qDECRA) approach proposed by Antalek (J. Am. Chem. Soc. 128 (2006) 8402-8403), this work presents an acquisition strategy that renormalizes this relaxation attenuation using estimates of the T(1) and T(2) relaxation times for all the nuclei in the mixture, as obtained directly with the pulse sequence used to record the PGSE experiment. More specifically, it is shown that only three distinct PGSE experiments need to be recorded, each with a specific set of acquisition parameters. For small- and medium-sized molecules, only T(1) is required for obtaining accurate quantification. For larger molecular weight species, which typically exhibit short T(2) values, estimates of T(2) must also be included but only a rough estimation is required. This appears fortunate because these data are especially hard to obtain with good accuracy when analyzing homonuclear scalar-coupled systems. Overall, the proposed methodology is shown to yield a quantification accuracy of ±5%, both in the absence and in the presence of spectral overlap, giving rise--at least, in our hands--to results that superseded those achieved by qDECRA, while requiring substantially less experimental time.

Up to 100% Improvement in Dynamic Nuclear Polarization Solid-State NMR Sensitivity Enhancement of Polymers by Removing Oxygen

High-field dynamic nuclear polarization (DNP) has emerged as a powerful technique for improving the sensitivity of solid-state NMR (SSNMR), yielding significant sensitivity enhancements for a variety of samples, including polymers. Overall, depending upon the type of polymer, the molecular weight, and the DNP sample preparation method, sensitivity enhancements between 5 and 40 have been reported. These promising enhancements remain, however, far from the theoretical maximum (>1000). Crucial to the success of DNP SSNMR is the DNP signal enhancement (εDNP ), which is the ratio of the NMR signal intensities with and without DNP. It is shown here that, for polymers exhibiting high affinity toward molecular oxygen (e.g., polystyrene), removing part of the absorbed (paramagnetic) oxygen from the solid-state samples available as powders (instead of dissolved or dispersed in a solvent) increases proton nuclear relaxation times and εDNP, hereby providing up to a two-fold sensitivity increase (i.e., a four-fold reduction in experimental time).

Solid-State 1H NMR Studies of Homonuclear Dipolar Couplings

Abstract We review here the different methodologies used to study 1H homonuclear dipolar couplings in solid-state NMR. Because most protonated powder samples contain clusters of strongly coupled 1H nuclei, structural studies based on the measurement of individual 1H homonuclear dipolar couplings have long been severely restricted. Driven by faster MAS frequencies, higher magnetic fields, pulse sequence and computing developments, major improvements have been achieved in this field over the last decade. We describe here the solid-state NMR techniques that have been developed recently, with special attention to the experiments based on 1H spin diffusion and double-quantum recoupling. For these experiments, a comprehensive description of the spin dynamics involved is provided. Finally, an overview of the most promising emerging techniques is also given.

Probing crystal packing of uniformly 13C-enriched powder samples using homonuclear dipolar coupling measurements

The relationship between the crystal packing of powder samples and long-range (13)C-(13)C homonuclear dipolar couplings is presented and illustrated for the case of uniformly (13)C-enriched L-alanine and L-histidine·HCl·H2O. Dipolar coupling measurement is based on the partial reintroduction of dipolar interactions by spinning the sample slightly off-magic-angle, while the coupling of interest for a given spin pair is isolated with a frequency-selective pulse. A cost function is used to correlate the so-derived dipolar couplings to trial crystal structures of the samples under study. This procedure allowed for the investigation of the l-alanine space group and L-histidine·HCl·H2O space group and unit-cell parameters.