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

Abstract

Recent progress in solid-state (SS)NMR spectroscopic methods based on fast magic angle spinning (MAS) [1] 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 homo-and heteronuclear decoupling, one-and two-dimensional (1D and 2D) spectra of such samples can be measured with excellent sensitivity and resolution.[2] 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, 13 C or 15 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.[3] Still, intermolecular polarization transfers to low-g nuclei are often hampered by low sensitivity. A promising solution to this challenge is offered by homonuclear 1 H-1 H 2D correlation methods, such as double-quantum (DQ)MAS [4] or spin-diffusion (NOESYlike) experiments, [5] provided that sufficient resolution is achieved in both dimensions. One of the possible approaches is the use of 1 H CRAMPS decoupling in concert with fast MAS to boost resolution in these experiments.[6] The recent development of ultrafast MAS (at 100 kHz and more [7] ) provides access to appropriate 1 H resolution without RF decoupling.Herein, we report the first application of 1 H 2D SSNMR measurements under MAS at 100 kHz, which are used in combination with indirectly detected 1 H{ 13 C} and 1 H{ 15 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).Corroles are aromatic macrocycles composed of four pyrrolic rings connected by three meso carbons and bearing one direct pyrrole-pyrrole link. The first synthesis of these materials was a multi-step process with low overall yield.[8] As synthetic methods have improved, there has been increased interest in potential applications of corroles in catalysis, sensors, imaging, and medicinal chemistry.[9] The coordination chemistry of corroles has attracted particular attention, and new metal-corrole systems possessing intriguing properties are being continuously reported.[10] Studies of solid-state structures of corroles and their interactions with other aromatic compounds pose challenges for diffraction and spectroscopic methods. Owing to the difficulties involved in growing X-ray quality crystals of the corroles, which tend to form disordered host(corrole)-guest(solvent) systems, only a few X-ray structures of unsubstituted metal-free corrole have been published to date, including that of corrole 1.[11]Herein, we present the results ...

Structure of fully protonated proteins by proton-detected magic-angle spinning NMR

“…1 H, 13 C, and 15 N backbone and side-chain resonances were assigned using a suite of proton detected 3D spectra correlating backbone and sidechain chemical shifts.…”

“…High-resolution proton-detected methods were first demonstrated with modest spinning frequencies by today's standards (∼10 kHz) and relied on a reduction of 1 H-1 H couplings by high levels of dilution with deuterium, typically perdeuteration, and complete (8, 9) or partial (10-12) protonation at exchangeable sites. The need for narrow proton resonances without such extreme levels of deuteration has motivated a continuous technological development, resulting in a dramatic increase in the available spinning frequency (13)(14)(15)(16)(17)(18)(19)(20).At MAS frequencies of 40-60 kHz, deuteration and 100% reprotonation at exchangeable sites, primarily amide protons, result in resolved and sensitive spectra, similar in quality to the case of higher dilution levels and lower spinning frequencies (21-23). This opens the way to rapid sequential assignment of backbone resonances (24-27), as well as to the unambiguous measurement of detailed structural and dynamical parameters (28-32).…”

“…High-resolution proton-detected methods were first demonstrated with modest spinning frequencies by today's standards (∼10 kHz) and relied on a reduction of 1 H-1 H couplings by high levels of dilution with deuterium, typically perdeuteration, and complete (8,9) or partial (10)(11)(12) protonation at exchangeable sites. The need for narrow proton resonances without such extreme levels of deuteration has motivated a continuous technological development, resulting in a dramatic increase in the available spinning frequency (13)(14)(15)(16)(17)(18)(19)(20).…”

“…To our knowledge, we present the first two examples of such structure determinations, the small model protein GB1 (52) and the de novo determination of Acinetobacter phage 205 (AP205) coat protein (AP205CP), a 28-kDa dimer in a 2.5-MDa viral capsid assembly (53). In both cases, we used a single 0.5-mg sample of uniformly 13 C, 15 N-labeled protein for backbone and side-chain resonance assignment, and for collection of a dense network of proton-proton distance restraints. Each structure was calculated from data acquired in less than 2 weeks, and making use of unsupervised analysis algorithms as implemented in UNIO (54).…”

Constant-time 2D and 3D through-bond correlation NMR spectroscopy of solids under 60 kHz MAS

“…In addition to the suppression of 1 H− 1 H dipolar couplings, ultrafast-MAS dramatically enhances spectral resolution of protons. [45][46][47][48][49] Because of this reason and due to the enhanced signal-to-noise ratio rendered by proton detection, the development of protonbased solid-state NMR techniques has been the main focus of recent ultrafast-MAS studies. [50][51][52][53][54][55][56][57][58][59][60] Recent studies from our laboratory have demonstrated the feasibility of recoupling proton-proton dipolar couplings, 14,[61][62][63] protonbased multidimensional experiments, [64][65][66][67] spectral-editing techniques, 68,69 and proton-proton distance measurements 69 under ultrafast-MAS conditions, whereas effective use of deuteration has also very recently been shown to be useful in proton-proton distance measurements by Reif et al 70,71 We have also shown the role of dipolar couplings that control the efficiency of refocused-INEPT pulse sequence under ultrafast-MAS.…”