Claudiu Filip

Determining relative proton–proton proximities from the build-up of two-dimensional correlation peaks in 1H double-quantum MAS NMR: insight from multi-spin density-matrix simulations

The build-up of intensity-as a function of the number, n(rcpl), of POST-C7 elements used for the excitation and reconversion of double-quantum (DQ) coherence (DQC)-is analysed for the fifteen distinct DQ correlation peaks that are observed experimentally for the eight separate (1)H resonances in a (1)H (500 MHz) DQ CRAMPS solid-state (12.5 kHz MAS) NMR spectrum of the dipeptide beta-AspAla (S. P. Brown, A. Lesage, B. Elena, and L. Emsley, J. Am. Chem. Soc., 2004, 126, 13230). The simulation in SPINEVOLUTION (M. Veshtort and R. G. Griffin, J. Magn. Reson., 2006, 178, 248) of t(1) ((1)H DQ evolution) FIDs for clusters of eight dipolar-coupled protons gives separate simulated (1)H DQ build-up curves for the CH(2)(a), CH(2)(b), CH(Asp), CH(Ala), NH and OH (1)H single-quantum (SQ) (1)H resonances. An analysis of both the simulated and experimental (1)H DQ build-up leads to the following general observations: (i) considering the build-up of (1)H DQ peaks at a particular SQ frequency, maximum intensity is observed for the DQC corresponding to the shortest H-H distance; (ii) for the maximum intensity (1)H DQ peak at a particular SQ frequency, the recoupling time for the observed maximum intensity depends on the corresponding H-H distance, e.g., maximum intensity for the CH(2)(a)-CH(2)(b) (H-H distance = 1.55 A) and OH-CH(Asp) (H-H distance = 2.49 A) DQ peaks is observed at n(rcpl) = 2 and 3, respectively; (iii) for DQ peaks involving a CH(2) proton at a non-CH(2) SQ frequency, there is much reduced intensity and a maximum intensity at a short recoupling time; (iv) for the other lower intensity (1)H DQ peaks at a particular SQ frequency, maximum intensity is observed for the same (or close to the same) recoupling time, but the relative intensity of the DQ peaks is a reliable indicator of the relative H-H distance-the ratio of the maximum intensities for the peaks at the CH(Ala) SQ frequency due to the two DQCs with the NH and OH protons are found to be approximately in the ratio of the squares of the corresponding dipolar coupling constants. While the simulated (1)H DQ build-up curves reproduce most of the features of the experimental curves, maximum intensity is often observed at a longer recoupling time in simulations. In this respect, simulations for two to eight spins show a trend towards a faster decay for an increasing number of considered spins. Finally, simulations show that increasing either the Larmor frequency (to 1 GHz) or the MAS frequency (to 125 kHz) does not lead to changes in the marked differences between the (1)H DQ build-up curves at the CH(Asp) SQ frequency for DQCs to the CH(2)(a) and OH protons that correspond to similar H-H distances (2.39 A and 2.49 A, respectively).

Multiple-quantum magic-angle spinning spectroscopy using nonlinear sampling.

NMR spectroscopy is a relatively insensitive technique and many biomolecular applications operate near the limits of sensitivity and resolution. A particularly challenging example is detection of the quadrupolar nucleus 17O, due to its low natural abundance, large quadrupole couplings, and low gyromagnetic ratio. Yet the chemical shift of 17O spans almost 1000 ppm in organic molecules and it serves as a potentially unique reporter of hydrogen bonding in peptides, nucleic acids, and water, and as a valuable complement to 13C and 15N NMR. Recent developments including the multiple-quantum magic-angle spinning (MQMAS) experiment have enabled the detection of 17O in biological solids, but very long data acquisitions are required to achieve sufficient sensitivity and resolution. Here, we perform nonlinear sampling in the indirect dimension of MQMAS experiments to substantially reduce the total acquisition time and improve sensitivity and resolution. Nonlinear sampling prevents the use of the discrete Fourier transform; instead, we employ maximum entropy (MaxEnt) reconstruction. Nonlinearly sampled MQMAS spectra are shown to provide high resolution and sensitivity in several systems, including lithium sulfate monohydrate (LiSO(4)-H(2)17O) and L-asparagine monohydrate (H(2)17O). The combination of nonlinear sampling and MaxEnt reconstruction promises to make the application of 17O MQMAS practical in a wider range of biological systems.

Spin dynamics under magic angle spinning by Floquet formalism

A new analytical method is presented for studying in a uniform way the spin dynamics in NMR experiments performed under the conditions of magic angle spinning. It was derived on the basis of the formalized Floquet theory and consists in transforming the Fourier-state representation of the NMR signal into an integral one. The integral representation proves to be well suited in combination with Rayleigh-Schrodinger perturbation theory for both the fast and the slow spinning regimes. The corresponding perturbation expansions can be readily extended to higher-order correction terms, which also allows the inclusion of more moderate spinning speeds. Explicit expressions of the perturbation series were derived for both spinning regimes and applied to the case of isolated dipolar-coupled spin-1/2 pairs for which the results can be compared with those obtained by the exact evaluation of the equation of motion.

Testing the limits of sensitivity in a solid-state structural investigation by combined X-ray powder diffraction, solid-state NMR, and molecular modelling

A solid state structural investigation of ethoxzolamide is performed on microcrystalline powder by using a multi-technique approach that combines X-ray powder diffraction (XRPD) data analysis based on direct space methods with information from (13)C((15)N) solid-state Nuclear Magnetic Resonance (SS-NMR) and molecular modeling. Quantum chemical computations of the crystal were employed for geometry optimization and chemical shift calculations based on the Gauge Including Projector Augmented-Wave (GIPAW) method, whereas a systematic search in the conformational space was performed on the isolated molecule using a molecular mechanics (MM) approach. The applied methodology proved useful for: (i) removing ambiguities in the XRPD crystal structure determination process and further refining the derived structure solutions, and (ii) getting important insights into the relationship between the complex network of non-covalent interactions and the induced supra-molecular architectures/crystal packing patterns. It was found that ethoxzolamide provides an ideal case study for testing the accuracy with which this methodology allows to distinguish between various structural features emerging from the analysis of the powder diffraction data.

NMR crystallography methods to probe complex hydrogen bonding networks: application to structure elucidation of anhydrous quercetin

Despite the simple molecular structure of quercetin, the exact conformation of the hydroxyl groups and the induced hydrogen bonding network in its anhydrous form could not be determined if starting from crystal structure models provided by X-ray powder diffraction (XRPD) alone. If instead a multi-technique approach that combines XRPD data analysis with information from 13C and 1H solid-state Nuclear Magnetic Resonance (ss-NMR) and molecular modeling is employed the ambiguities could be largely reduced. A conformational analysis was performed on the isolated molecule at the molecular mechanics (MM) level of theory, whereas quantum chemical computations of the full crystal were employed for geometry optimization and chemical shift calculation based on the Gauge Including Projector Augmented-Wave (GIPAW) method. 13C and ultra-fast MAS (60 kHz) 1H ss-NMR data were used at 11.75 T to assess the reliability of the derived structure solutions, and to obtain important insights into the relationship between the hydrogen bonding network and the induced supra-molecular architectures/crystal packing patterns. A direct comparison with the data recorded on quercetin dihydrate, of which single-crystal X-ray structure is known, proved useful for increasing the confidence level in the derived conclusions. Also, an ultra-fast MAS 1H double-quantum (DQ) excitation scheme is employed, which has the advantage of providing better resolution at long DQ excitation times compared with conventional multiple-pulse homonuclear decoupling: this may be relevant for further methodological development in NMR crystallography, especially when dealing with the effects of increased lattice disorder.

CHHC and 1H–1H magnetization exchange: Analysis by experimental solid-state NMR and 11-spin density-matrix simulations

A protocol is presented for correcting the effect of non-specific cross-polarization in CHHC solid-state MAS NMR experiments, thus allowing the recovery of the (1)H-(1)H magnetization exchange functions from the mixing-time dependent buildup of experimental CHHC peak intensity. The presented protocol also incorporates a scaling procedure to take into account the effect of multiplicity of a CH(2) or CH(3) moiety. Experimental CHHC buildup curves are presented for l-tyrosine.HCl samples where either all or only one in 10 molecules are U-(13)C labeled. Good agreement between experiment and 11-spin SPINEVOLUTION simulation (including only isotropic (1)H chemical shifts) is demonstrated for the initial buildup (t(mix)<100micros) of CHHC peak intensity corresponding to an intramolecular close (2.5A) H-H proximity. Differences in the initial CHHC buildup are observed between the one in 10 dilute and 100% samples for cases where there is a close intermolecular H-H proximity in addition to a close intramolecular H-H proximity. For the dilute sample, CHHC cross-peak intensities tended to significantly lower values for long mixing times (500micros) as compared to the 100% sample. This difference is explained as being due to the dependence of the limiting total magnetization on the ratio N(obs)/N(tot) between the number of protons that are directly attached to a (13)C nucleus and hence contribute significantly to the observed (13)C CHHC NMR signal, and the total number of (1)H spins into the system. (1)H-(1)H magnetization exchange curves extracted from CHHC spectra for the 100% l-tyrosine.HCl sample exhibit a clear sensitivity to the root sum squared dipolar coupling, with fast buildup being observed for the shortest intramolecular distances (2.5A) and slower, yet observable buildup for the longer intermolecular distances (up to 5A).

Optimizing structure determination from powders of crystalline organic solids with high molecular flexibility: the case of lisinopril dihydrate

Structure determination of lisinopril dihydrate is conducted using NMR crystallography methods. The analysis is centred on the optimization of a strategy capable of providing key structural features that are inaccessible by XRPD alone even under high-resolution conditions.

Can the conformation of flexible hydroxyl groups be constrained by simple NMR crystallography approaches? The case of the quercetin solid forms

Hydrogen atoms in systems with many flexible hydroxyl side-groups are difficult to be exactly located from experimental X-Ray diffraction and/or solid-state NMR data, thus often leading to wrong conclusions with respect to the hydrogen bonding network established in crystal lattice. A simple computational method is proposed in the present work to tackle this problem, which may be readily incorporated in conventional NMR crystallography protocols. The method is based on ranking all possible conformations of the flexible hydroxyls according to their lattice energy in crystalline environments. Its effectiveness is investigated on two distinct solid forms of quercetin, for which only two out of the five hydroxyl side-groups can be well constrained from experimental/theoretical data. For this purpose, first-principle quantum-mechanical computations were combined with calculations at the molecular mechanics (MM) level of theory, and previous ss-NMR and X-Ray diffraction data. To assess accuracy in ranking the identified conformers, tests have been performed first on quercetin dihydrate, for which an X-Ray single-crystal structure is available. The possibility of applying this method in a real NMR crystallography context has been investigated finally on anhydrous quercetin, for which only powder X-Ray crystal structure has been reported so far.

Structure of N-(5-ethyl-[1,3,4]-thiadiazole-2-yl)toluenesulfonamide by combined X-ray powder diffraction, 13C solid-state NMR and molecular modelling.

The crystal structure solution of the title compound is determined from microcrystalline powder using a multi-technique approach that combines X-ray powder diffraction (XRPD) data analysis based on direct-space methods with information from (13)C solid-state NMR (SSNMR), and molecular modelling using the GIPAW (gauge including projector augmented-wave) method. The space group is Pbca with one molecule in the asymmetric unit. The proposed methodology proves very useful for unambiguously characterizing the supramolecular arrangement adopted by the N-(5-ethyl-[1,3,4]-thiadiazole-2-yl)toluenesulfonamide molecules in the crystal, which consists of extended double strands held together by C-H···π non-covalent interactions.

Crystal Structure and Desolvation Behaviour of the Tadalafil Monosolvates with Acetone and Methyl Ethyl Ketone

Crystal structures of Tadalafil (TDF) monosolvated forms with acetone (ACE) and methyl ethyl ketone (MEK) were determined by single-crystal X-ray diffraction in which same persistent chains of TDF molecules are present as in the reported structures. The solvates crystallize in a higher orthorhombic symmetry than the known forms with monoclinic structures. Weak interactions between TDF and solvent molecules are present in both solvates, leading to slight conformational distortions of TDF molecules. The MEK solvate showed slightly higher stability than the ACE solvate, regardless of their highly similar molecular conformations and crystal packing. Desolvation into anhydrous TDF was achieved by heating, exposure to temperature and relative humidity and by mechanical stress. The high solubility of TDF in ACE and MEK solvents combined with the ease of desolvation of the resulting solvated forms indicates the viability of the solvates use as intermediates in the TDF crystallization process.