Proton-detected NMR Spectroscopy Research Articles

The Structure of Molecular and Surface Platinum Sites Determined by DNP-SENS and Fast MAS 195Pt Solid-State NMR Spectroscopy.

The molecular level characterization of heterogeneous catalysts is challenging due to the low concentration of surface sites and the lack of techniques that can selectively probe the surface of a heterogeneous material. Here, we report the joint application of room temperature proton-detected NMR spectroscopy under fast magic angle spinning (MAS) and dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP-SENS), to obtain the 195Pt solid-state NMR spectra of a prototypical example of highly dispersed Pt sites (single site or single atom), here prepared via surface organometallic chemistry, by grafting [(COD)Pt(OSi(OtBu)3)2] (1, COD = 1,5-cyclooctadiene) on partially dehydroxylated silica (1@SiO2). Compound 1@SiO2 has a Pt loading of 3.7 wt %, a surface area of 200 m2/g, and a surface Pt density of around 0.6 Pt site/nm2. Fast MAS 1H{195Pt} dipolar-HMQC and S-REDOR experiments were implemented on both the molecular precursor 1 and on the surface complex 1@SiO2, providing access to 195Pt isotropic shifts and Pt-H distances, respectively. For 1@SiO2, the measured isotropic shift and width of the shift distribution constrain fits of the static wide-line DNP-enhanced 195Pt spectrum, allowing the 195Pt chemical shift tensor parameters to be determined. Overall the NMR data provide evidence for a well-defined, single-site structure of the isolated Pt sites.

Studying intact bacterial peptidoglycan by proton-detected NMR spectroscopy at 100 kHz MAS frequency

The bacterial cell wall is composed of the peptidoglycan (PG), a large polymer that maintains the integrity of the bacterial cell. Due to its multi-gigadalton size, heterogeneity, and dynamics, atomic-resolution studies are inherently complex. Solid-state NMR is an important technique to gain insight into its structure, dynamics and interactions. Here, we explore the possibilities to study the PG with ultra-fast (100 kHz) magic-angle spinning NMR. We demonstrate that highly resolved spectra can be obtained, and show strategies to obtain site-specific resonance assignments and distance information. We also explore the use of proton-proton correlation experiments, thus opening the way for NMR studies of intact cell walls without the need for isotope labeling.

J Cross-Polarization Methods for Localized NMR without Magnetic Field Gradients

This work predominantly refers to 13C in vivo spectroscopy and has two objectives. First, heteronuclear editing is considered for proton-detected NMR spectroscopy of rare nuclei. The second aspect refers to localized heteronuclear NMR spectroscopy implying the editing option. Both purposes are achieved by employing J cross polarization (or coherent rotating-frame polarization transfer). The spatial selectivity is due to different in-homogeneities of the superimposed double-resonance radiofrequency fields of the J cross-polarization experiment. A 13C surface coil, for instance, was combined with a 1H birdcage resonator, so that J cross polarization is restricted to the region where the Hartmann/Hahn condition is approached. Heteronuclear editing of NMR lines for proton-detected 13C spectroscopy was performed by the aid of a cyclic cross-polarization pathway. Test experiments demonstrating the volume selectivity of the transfer process and the editing efficiency of the cyclic cross-polarization procedure are reported.

Cyclic and Slice-Selective J Cross Polarization for Heteronuclear Editing and Localized NMR

This work has two objectives. First, heteronuclear editing was considered, with particular aim at proton-detected NMR spectroscopy of rare nuclei. The second aspect refers to localized heteronuclear NMR spectroscopy, implying the editing option. The common basis of both techniques is J cross polarization (or coherent rotating-frame polarization transfer). In the presence of magnetic field gradients this process is slice selective and is introduced as a principle for localized liquid-state NMR, particularly of heteronuclear spins. Heteronuclear editing of NMR lines is performed by the aid of a cyclic cross-polarization pathway. Both methods can be combined to a technique for localized proton-detected NMR spectroscopy of rare nuclei. Test experiments demonstrating the slice selectivity of the transfer process and the efficiency of the cyclic cross-polarization editing procedure are reported. Applications to localized homo- or heteronuclear spectroscopy and imaging are discussed.

Measurement of relaxation time constants for methyl groups by proton-detected heteronuclear NMR spectroscopy

Experimental schemes are described that allow proton-detected NMR spectroscopy to measure accurately 13C spin—lattice relaxation time constants of methyl groups. By using magic angle pulses in DEPT-base sequences, or analogously tuned delays in INEPT-based sequences, contributions to the observed NMR signal are eliminated that would otherwise complicate the measurement of the desired time constants. Experimental verification of the proposed techniques is provided by measurements on sodium acetate and on a peptide.

Improvements in proton-detected NMR spectroscopy using spin-flip decoupling. An application to heteronuclear chemical-shift correlation

Improvements in proton-detected NMR spectroscopy using spin-flip decoupling. An application to heteronuclear chemical-shift correlation