Nuclear magnetic resonance (NMR) can probe the local structure and dynamic properties of liquids and solids, making it one of the most powerful and versatile analytical methods available today. However, its intrinsically low sensitivity precludes NMR analysis of very small samples-as frequently used when studying isotopically labelled biological molecules or advanced materials, or as preferred when conducting high-throughput screening of biological samples or 'lab-on-a-chip' studies. The sensitivity of NMR has been improved by using static micro-coils, alternative detection schemes and pre-polarization approaches. But these strategies cannot be easily used in NMR experiments involving the fast sample spinning essential for obtaining well-resolved spectra from non-liquid samples. Here we demonstrate that inductive coupling allows wireless transmission of radio-frequency pulses and the reception of NMR signals under fast spinning of both detector coil and sample. This enables NMR measurements characterized by an optimal filling factor, very high radio-frequency field amplitudes and enhanced sensitivity that increases with decreasing sample volume. Signals obtained for nanolitre-sized samples of organic powders and biological tissue increase by almost one order of magnitude (or, equivalently, are acquired two orders of magnitude faster), compared to standard NMR measurements. Our approach also offers optimal sensitivity when studying samples that need to be confined inside multiple safety barriers, such as radioactive materials. In principle, the co-rotation of a micrometre-sized detector coil with the sample and the use of inductive coupling (techniques that are at the heart of our method) should enable highly sensitive NMR measurements on any mass-limited sample that requires fast mechanical rotation to obtain well-resolved spectra. The method is easy to implement on a commercial NMR set-up and exhibits improved performance with miniaturization, and we accordingly expect that it will facilitate the development of novel solid-state NMR methodologies and find wide use in high-throughput chemical and biomedical analysis.
We show how high-resolution NMR spectra can be obtained for solids for which the spectra are normally broadened due to structural disorder. The method relies on correlations in the chemical shifts between pairs of coupled spins. It is found experimentally that there are strong correlations in the chemical shifts between neighboring spins in both phosphorus-31 and carbon-13 spectra. These correlations can be exploited not only to provide resolution in two-dimensional spectra, but also to yield "chains" of correlated chemical shifts, constituting a valuable new source of structural information for disordered materials.
Understanding
the molecular determinants underlying protein function
requires the characterization of both structure and dynamics at atomic
resolution. Nuclear relaxation rates allow a precise characterization
of protein dynamics at the Larmor frequencies of spins. This usually
limits the sampling of motions to a narrow range of frequencies corresponding
to high magnetic fields. At lower fields one cannot achieve sufficient
sensitivity and resolution in NMR. Here, we use a fast shuttle device
where the polarization builds up and the signals are detected at high
field, while longitudinal relaxation takes place at low fields 0.5
< B0 < 14.1 T. The sample is propelled
over a distance up to 50 cm by a blowgun-like system in about 50 ms.
The analysis of nitrogen-15 relaxation in the protein ubiquitin over
such a wide range of magnetic fields offers unprecedented insights
into molecular dynamics. Some key regions of the protein feature structural
fluctuations on nanosecond time scales, which have so far been overlooked
in high-field relaxation studies. Nanosecond motions in proteins may
have been underestimated by traditional high-field approaches, and
slower supra-τc motions that have no effect on relaxation
may have been overestimated. High-resolution relaxometry thus opens
the way to a quantitative characterization of nanosecond motions in
proteins.
We present a new solid-state nuclear magnetic resonance experiment that yields, under CRAMPS decoupling conditions, a significant reduction in proton line widths for powdered organic solids. This experiment which relies on a constant-time acquisition of the proton transverse magnetization, removes the contribution of nonrefocusable broadening from the proton line widths. Although this new technique suffers from relatively low sensitivity, we demonstrate in this paper its feasibility on two model samples, L-alanine and the dipeptide Ala-Asp. In both cases a factor of between 2 and 3 in line width reduction is obtained for most of the proton resonances.
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