Piotr Paluch

Study of Intermolecular Interactions in the Corrole Matrix by Solid‐State NMR under 100 kHz MAS and Theoretical Calculations

Recent progress in solid-state (SS)NMR spectroscopic methods based on fast magic angle spinning (MAS) has enabled new opportunities for the structural study of small quantities (< 5 mg) of natural abundance samples. Utilizing throughspace and through-bond polarization transfer, indirect detection of low-g nuclei, and suitable homoand heteronuclear decoupling, oneand two-dimensional (1D and 2D) spectra of such samples can be measured with excellent sensitivity and resolution. However, determination of the short-range intermolecular order often remains elusive. Such analyses can be well-served by studying heteronuclear correlations that take advantage of the large chemical shift range of most low-g nuclei (for example, C or N). Indeed, heteronuclear correlation (HETCOR) NMR spectroscopy and measurements of internuclear distances, often in concert with theoretical calculations, have provided structural details of complex hydrogen-bonded systems in chemistry and biology, blended materials, and host–guest pairs. Still, intermolecular polarization transfers to low-g nuclei are often hampered by low sensitivity. A promising solution to this challenge is offered by homonuclear H–H 2D correlation methods, such as double-quantum (DQ)MAS or spin-diffusion (NOESYlike) experiments, provided that sufficient resolution is achieved in both dimensions. One of the possible approaches is the use of H CRAMPS decoupling in concert with fast MAS to boost resolution in these experiments. The recent development of ultrafast MAS (at 100 kHz and more) provides access to appropriate H resolution without RF decoupling. Herein, we report the first application of H 2D SSNMR measurements under MAS at 100 kHz, which are used in combination with indirectly detected H{C} and H{N} HETCOR experiments and theoretical calculations to scrutinize the interactions within a host–guest (HG) system consisting of 5,10,15-tris(pentafluorophenyl)corrole 1, and toluene (Scheme 1).

Simple and accurate determination of X-H distances under ultra-fast MAS NMR.

We demonstrate that a very simple experiment, Cross-Polarization with Variable Contact-time (CP-VC), is very efficient at ultra-fast MAS (νR ≥ 60 kHz) to measure accurately the C-H and N-H distances, and to analyze the dynamics of bio-molecules. This experiment can be performed with samples that are either (13)C or (15)N labeled or without any labeling. The method is very robust experimentally with respect to imperfect Hartman-Hahn setting, and presents a large scaling factor allowing a better dipolar determination, especially for long C-H or N-H distances, or for CH3 or NH3 moieties with three-site hopping. At ultra-fast MAS, it can be used quantitatively in a 2D way, because its scaling factor is then little dependent on the offsets. This robustness with respect to offset is related to the ultra-fast spinning speed, and hence to the related small rotor diameter. Indeed, these two specifications lead to efficient n = ±1 zero-quantum Hartman-Hahn CP-transfers with large RF-fields on proton and carbon or nitrogen channels, and large dipolar scaling factor.

Ibuprofen in mesopores of Mobil Crystalline Material 41 (MCM-41): a deeper understanding.

In this work, we compared two methods (incipient wetness and melting) for the encapsulation of ibuprofen in the pores of Mobil Crystalline Material 41 (MCM-41) through NMR (nuclear magnetic resonance) spectroscopy. (1)H NMR spectra were recorded under very fast MAS (sample spinning 60 kHz) conditions in both 1D and 2D mode (NOESY sequence). We also performed (13)C cross-polarization magic angle spinning (CP/MAS) experiments, (13)C single pulse experiments (SPE), and (1)H-(13)C HSQC HR/MAS (heteronuclear single quantum coherence high resolution) HR/MAS correlations. Evaluation of the encapsulation methods included an analysis of the filling factor of the drug into the pores. The stability of Ibu/MCM in an environment of ethanol or water vapor was tested. Our study showed that melting a mixture of Ibu and MCM is a much more efficient method of confining the drug in the pores compared to incipient wetness. The optimal experiments for the former method achieved a filling factor of approximately 60%. We concluded that the major limitation to the applicability of the incipient wetness method (filling factor ca. 20%) is the high affinity of solvent (typically ethanol) for MCM-41. We found that even ethanol vapor can remove Ibu from the pores. When a sample of Ibu/MCM was stored for a few hours in a closed vessel with ethanol vapor, Ibu was transported from the pores to the outer walls of MCM. We observed a similar phenomenon with water vapor, although this process is slower compared to the analogous procedure using ethanol. Our study clearly demonstrates that existing methods used to encapsulate drugs in mesoporous silica nanoparticles (MSNs) require reevaluation.

Insights into the Tautomerism in meso‐Substituted Corroles: A Variable‐Temperature 1H, 13C, 15N, and 19F NMR Spectroscopy Study

Tris(pentafluorophenyl)corrole and its (15)N-enriched isotopomer were studied in [D8]toluene solution by 1D and 2D variable-temperature NMR techniques to establish the mechanisms of tautomerization of the NH protons inside the interior of the corrole macrocycle. Three such rate processes could be identified of which two modulate the spectral line shapes at temperatures above 205 K and the third is NMR-inaccessible as it is very fast. The latter involves the proton engaged in an unsymmetrical proton sponge unit formed by two pyrrole nitrogen atoms. Temperature and concentration dependences of the two remaining processes were determined. One of them is purely intramolecular and the other is intermolecular at low temperatures, with growing contribution of an intramolecular mechanism at elevated temperatures. The proposed microscopic mechanisms of all these processes are semi-quantitatively confirmed by quantum chemical calculations using density functional theory.

Theoretical study of CP-VC: a simple, robust and accurate MAS NMR method for analysis of dipolar C-H interactions under rotation speeds faster than ca. 60 kHz.

We show that Cross-Polarization with Variable Contact-time (CP-VC) allows an accurate determination of C-H dipolar interactions, which permits an easy detailed analysis of bond lengths and local dynamics, e.g. in biomolecules. The method presents a large dipolar scaling factor of 1/√2, leading to a better determination of dipolar interactions, especially for long C-H distances, and it allows the observation of very small local details such as those related either to CH(2) three spin systems, or even to hydrogen bonds. CP-VC is very simple to set up and very robust with respect to most experimental parameters, such as: rf-offsets, chemical-shift anisotropies, imperfect Hartmann-Hahn setting, and rf-inhomogeneity. The only required condition is the use of a sufficiently fast MAS spinning speed of at least ca. 60 kHz.

Accurate NMR determination of C-H or N-H distances for unlabeled molecules.

Cross-Polarization with Variable Contact-time (CP-VC) is very efficient at ultra-fast MAS (νR ≥ 60 kHz) to measure accurately the dipolar interactions corresponding to C-H or N-H short distances, which are very useful for resonance assignment and for analysis of dynamics. Here, we demonstrate the CP-VC experiment with (1)H detection. In the case of C-H distances, we compare the CP-VC signals with direct ((13)C) and indirect ((1)H) detection and find that the latter allows a S/N gain of ca. 2.5, which means a gain of ca. 6 in experimental time. The main powerful characteristics of CP-VC methods are related to the ultra-fast spinning speed and to the fact that most of the time only the value of the dipolar peak separation has to be used to obtain the information. As a result, CP-VC methods are: (i) easy to set up and to use, and robust with respect to (ii) rf-inhomogeneity thus allowing the use of full rotor samples, (iii) rf mismatch, and (iv) offsets and chemical shift anisotropies. It must be noted that the CP-VC 2D method with indirect (1)H detection requires the proton resolution and is thus mainly applicable to small or perdeuterated molecules. We also show that an analysis of the dynamics can even be performed, with a reasonable experimental time, on unlabeled samples with (13)C or even (15)N natural abundance.

Study of the thermal processes in molecular crystals of peptides by means of NMR crystallography

1D and 2D Very Fast Magic Angle Spinning (VF MAS) NMR experiments with sample rotation up to 55 kHz were applied to study both the dihydrate form of Tyr–(D)Ala–Phe–Gly (N-terminal sequence of opioid peptide dermorphin) and the anhydrous form, which was obtained by thermal treatment. Employing both homo-nuclear (1H–1H BABA, 13C–13C SHANGHAI) and hetero-nuclear 2D correlations (1H–13C and 1H–15N) with inverse detection, it was shown that removing water from the crystal lattice of this tetrapeptide does not destroy its subtle pseudo-cyclic structure, and its supramolecular array is preserved. The GIPAW method was employed to compute the geometry of the peptides and calculate the 13C σii principal elements of the NMR shielding tensor parameters and 1H isotropic NMR shifts. The theoretical values of 13C σii were compared with the experimental 13C δii chemical shift tensor values obtained by a 2D PASS experiment. The correlations 13C σii versus δii and 1H σiso versus δiso were used to evaluate the quality of the computational approach. With the new set of coordinates obtained by the GIPAW method, the crystal and molecular structure of the dehydrated Tyr–(D)Ala–Phe–Gly peptide that was obtained by thermal treatment was constructed. Methodology used in this project combining NMR measurements, analysis of X-ray powder diffraction data and advanced quantum mechanical calculations is known as NMR crystallography.

Fine refinement of solid state structure of racemic form of phospho-tyrosine employing NMR Crystallography approach

We present step by step facets important in NMR Crystallography strategy employing O-phospho-dl-tyrosine as model sample. The significance of three major techniques being components of this approach: solid state NMR (SS NMR), X-ray diffraction of powdered sample (PXRD) and theoretical calculations (Gauge Invariant Projector Augmented Wave; GIPAW) is discussed. Each experimental technique provides different set of structural constraints. From the PXRD measurement the size of the unit cell, space group and roughly refined molecular structure are established. SS NMR provides information about content of crystallographic asymmetric unit, local geometry, molecular motion in the crystal lattice and hydrogen bonding pattern. GIPAW calculations are employed for validation of quality of elucidation and fine refinement of structure. Crystal and molecular structure of O-phospho-dl-tyrosine solved by NMR Crystallography is deposited at Cambridge Crystallographic Data Center under number CCDC 1005924.

Chiral crystals from porphyrinoids possessing a very low racemization barrier

The unique solid-state NMR technique Very Fast MAS with a sample spinning rate over 40 kHz was employed for screening and selection of crystals of porphyrinoid derivatives – corroles, suitable for X-ray diffraction studies. The host–guest approach and formation of inclusion complexes were efficient methods for the synthesis of chiral crystals from free base corroles possessing an extremely low racemization barrier. Our data show that the intrinsic features of corroles, including the dihedral angle saddling effect and distortion of the macrocyclic rings out of planarity, have no crucial influence on the tendency for symmetry breaking.

Study of the Mechanism of Thermal Chemical Processes in the Crystals of YAF Tripeptides by Means of Mass Spectrometry and Solid State NMR

Thermal reactions in two Tyr-Ala-Phe (YAF) tripeptide crystals with different molecular packing (monoclinic and hexagonal), distinct stereochemistry of central amino acid (D or L alanine) and specific arrangement of molecules in the crystal lattice (head-to-tail) were investigated. Samples were heated up to 180 °C, while the melting point for YAF crystals is above the 220 °C. Below the melting temperature, in both cases the chemical reactions leading to formation of cyclic dipeptides (YA diketopiperazine) and leaving of phenylalanine were observed. Two possible mechanisms of chemical reaction in the crystal lattice assuming intra- and/or intermolecular pathways were considered. (13)C and (15)N enriched YAF samples were employed to study of mechanism of solid state reactivity using mass spectrometry and advanced solid state NMR techniques (2D DARR (Dipolar Assisted Rotational Resonance) and 2D Double CP (Cross-Polarization) correlations).