Claudiu Filip | PubMed Health

PubMed2003

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.

Anisotropy↗

PubMed2002

3D TEDOR NMR experiments for the simultaneous measurement of multiple carbon-nitrogen distances in uniformly (13)C,(15)N-labeled solids.

We describe three-dimensional magic-angle-spinning NMR experiments for the simultaneous measurement of multiple carbon-nitrogen distances in uniformly (13)C,(15)N-labeled solids. The approaches employ transferred echo double resonance (TEDOR) for (13)C-(15)N coherence transfer and (15)N and (13)C frequency labeling for site-specific resolution, and build on several previous 3D TEDOR techniques. The novel feature of the 3D TEDOR pulse sequences presented here is that they are specifically designed to circumvent the detrimental effects of homonuclear (13)C-(13)C J-couplings on the measurement of weak (13)C-(15)N dipolar couplings. In particular, homonuclear J-couplings lead to two undesirable effects: (i) they generate anti-phase and multiple-quantum (MQ) spin coherences, which lead to spurious cross-peaks and phase-twisted lines in the 2D (15)N-(13)C correlation spectra, and thus degrade the spectral resolution and prohibit the extraction of reliable cross-peak intensities, and (ii) they significantly reduce cross-peak intensities for strongly J-coupled (13)C sites (e.g., CO and C(alpha)). The first experiment employs z-filter periods to suppress the anti-phase and MQ coherences and generates 2D spectra with purely absorptive peaks for all TEDOR mixing times. The second approach uses band-selective (13)C pulses to refocus J-couplings between (13)C spins within the selective pulse bandwidth and (13)C spins outside the bandwidth. The internuclear distances are extracted by using a simple analytical model, which accounts explicitly for multiple spin-spin couplings contributing to cross-peak buildup. The experiments are demonstrated in two U-(13)C,(15)N-labeled peptides, N-acetyl-L-Val-L-Leu (N-ac-VL) and N-formyl-L-Met-L-Leu-L-Phe (N-f-MLF), where 20 and 26 (13)C-(15)N distances up to approximately 5-6 A were measured, respectively. Of the measured distances, 10 in N-ac-VL and 13 in N-f-MLF are greater than 3 A and provide valuable structural constraints.

Carbon Isotopes↗