High‐resolution nuclear magnetic resonance spectroscopy of biological tissues using projected magic angle spinning

Abstract

High-resolution NMR spectra of materials subject to anisotropic broadening are usually obtained by rotating the sample about the magic angle, which is 54.7°to the static magnetic field. In projected magic angle spinning (p-MAS), the sample is spun about two angles, neither of which is the magic angle. This provides a method of obtaining isotropic spectra while spinning at shallow angles. The p-MAS experiment may be used in situations where spinning the sample at the magic angle is not possible due to geometric or other constraints, allowing the choice of spinning angle to be determined by factors such as the shape of the sample, rather than by the spin physics. Proton (1 H) NMR studies of metabolites in biological tissue have been extensively used to monitor changes in cellular function as a result of disease, e.g., (1-5). In tissue, resolving signals for the metabolites of interest can be difficult because the lines are often broad and overlapping. The primary mechanism of this broadening is local magnetic field gradients caused by variations in the bulk magnetic susceptibility (6,7). This effect can be alleviated by magic angle spinning (MAS), a technique for averaging anisotropic interactions transforming under spatial motion as the second order Legendre polynomial, P 2 cos(). In MAS (8), the sample is rotated about an axis that makes an angle of cos Ϫ1 ͱ1/3Ϸ54.7°w ith respect to the static magnetic field B 0 .High-speed MAS is routinely used in high-resolution solid-state NMR studies of diverse materials such as inorganic solids, polymers, and macromolecular crystals. Several techniques have been developed for obtaining the same type of resolved isotropic spectra at the slower spinning speeds required for fragile samples such as intact organs or even living animals. When the spinning speed is less than the width of the anisotropic spectrum, spinning sidebands appear at multiples of the spinning speed, obscuring the spectrum and lowering the effective signal-tonoise ratio. Sideband suppression by rotor synchronized pulse sequences such as 1D-TOSS (9) and 2D-PASS (10) has been used in slow-spinning studies of excised human tissue (4,11). Magic angle hopping, in which the sample hops among three discrete positions during the pulse sequence (12), and magic angle turning (MAT), its continuous rotation analog (13-15), are related techniques for acquiring isotropic-anisotropic correlations. MAT spectra of metabolites in tissues and organs have been recently acquired for both excised tissues (16) and a living mouse (17). In these applications, rotor synchronized pulses produce an isotropic spectrum in one dimension and a manifold of sidebands covering the full powder pattern in a second dimension, correlating isotropic and anisotropic information in a two-dimensional NMR experiment. This allows the isotropic spectrum to be recovered without the need for fast spinning, as well as retaining potentially valuable information derived from anisotropic interactions.The purpose of the p-MAS experiment is to obtain highreso...

New Methodology For Use in Rotating Field Nuclear MagneticResonance

“…The p-MAS instead of having to use all angles only needs two for isotropic spectrum recovery. [30] These experiments create the same data set as VACSY but incorporate a second time domain to do it. This is done by incrementing the time domain of one of the angles and collecting at the second as in Figure 4.12.…”

Characterization of frequency-chirped dynamic nuclear polarization in rotating solids

“…Frequency-swept DNP at higher magnetic fields has also been shown to improve DNP performance [30,31], but has only recently been implemented with MAS [32,33]. MAS improves the sensitivity and resolution of solid-state NMR [34][35][36][37][38] by partially averaging anisotropic interactions of the magnetic resonance Hamiltonian, and is a crucial aspect of applying DNP to systems of interest.…”

Characterization of Frequency-Chirped Dynamic Nuclear Polarization in Rotating Solids

“…Frequency-swept DNP at higher magnetic fields has also been shown to improve DNP performance [30,31], but has only recently been implemented with MAS [32,33]. MAS improves the sensitivity and resolution of solid-state NMR [34][35][36][37][38] by partially averaging anisotropic interactions of the magnetic resonance Hamiltonian, and is a crucial aspect of applying DNP to systems of interest.…”