Improved magnetization transfer in solid-state NMR with fast magic angle spinning

Abstract

The efficiency of magnetization transfer between different spins S such as chemically inequivalent carbon-13 nuclei in solid samples that are spinning at high frequencies about the magic angle can be enhanced by a phase-alternated recoupling irradiation scheme (PARIS). Dipolar recoupling is assisted by a radio-frequency (rf) field applied to the abundant I (proton) spins. In contrast to rotary resonance-based recoupling schemes, the new method does not depend critically on the rf amplitude, which need not be matched with the spinning frequency. Modest rf amplitudes suffice to bring about efficient magnetization transfer even at high spinning speeds, thus avoiding excessive sample heating. The new method compensates efficiently for rf field inhomogeneity, so that the full sample volume is used more effectively.

Restoring symmetry in two-dimensional solid-state NMR correlation spectra

“…The remarkable agreement between these calculated rate constants and those extracted by fitting the experimental data of Figure 5 to Eqs. (1), (2), although it may be perhaps somewhat fortuitous considering the number of assumptions and approximations, suggests that rate constants retrieved from symmetric 2D exchange spectra are directly related to internuclear distances and may be exploited for structure determination. This is analogous to the measurement of internuclear distances between chemically equivalent sites in rotating solids [16,17].…”

“…In this work we used only N = 2, since the magnetization exchange between spectrally distant carbons such as carboxyl and aliphatic carbons (separated by ca. 100-150 ppm or 10-15 kHz at 9.4 T) is less efficient when using PARIS with N = 1/2 [2], while PARIS with N = 2 generates recoupling bands at m rot /2 on either side of resonance signals [2]. For PARISxy, the pulse duration s p must always be equal to half the rotor period (s p = s rot /2).…”

“…c o m / l o c a t e / c p l e t t to equilibrate the magnetization (dashed rectangles in Figure 1). Both recoupling methods use rf fields applied only to the protons with a moderate constant rf amplitude m 1 ( 1 H) that need not fulfill any rotary resonance condition [2,3]. For PARIS irradiation the phase is reversed from +x to Àx after a duration s p = Ns rot where N = ½ or 2.…”

“…The pulse sequences used in this work are shown in Figure 1. Either the phase-alternated recoupling irradiation scheme (PARIS) [2] or its phase-shifted counterpart PARISxy [3] Chemical Physics Letters j o u r n a l h o m e p a g e : w w w . e l s e v i e r .…”

“…Magnetization transfer can be brought about either via through-bond scalar couplings or via through-space dipolar interactions, provided the latter are 'recoupled' to cancel their elimination by magic angle spinning (MAS). To promote dipolar recoupling, we developed PARIS [2] and PARISxy [3] methods which permit one to achieve an efficient exchange of magnetization over a wide range of spinning frequencies and static magnetic fields and to record sensitive 2D correlation spectra of microcrystalline proteins [4], amyloid fibrils [5] and mixtures of crystallographic forms [6].…”