Tailored low-power cross-polarization under fast magic-angle spinning

Abstract

High static magnetic fields and very fast magic-angle spinning (MAS) promise to improve resolution and sensitivity of solid-state NMR experiments. The fast MAS regime has permitted the development of low-power cross-polarization schemes, such as second-order cross-polarization (SOCP), which prevent heat deposition in the sample. Those schemes are however limited in bandwidth, as weak radio-frequency (RF) fields only cover a small chemical shift range for rare nuclei (e.g. (13)C). Another consideration is that the efficiency of cross-polarization is very sensitive to magnetization decay that occurs during the spin-lock pulse on the abundant nuclei (e.g. (1)H). Having characterized this decay in glutamine at 60 kHz MAS, we propose two complementary strategies to tailor cross-polarization to desired spectral regions at low RF power. In the case of multiple sites with small chemical shift dispersion, a larger bandwidth for SOCP is obtained by slightly increasing the RF power while avoiding recoupling conditions that lead to fast spin-lock decay. In the case of two spectral regions with large chemical shift offset, an extension of the existing low-power schemes, called MOD-CP, is introduced. It consists of a spin-lock on (1)H and an amplitude-modulated spin-lock on the rare nucleus. The range of excited chemical shifts is assessed by experimental excitation profiles and numerical simulation of an I(2)S spin system. All SOCP-based schemes exhibit higher sensitivity than high-power CP schemes, as demonstrated on solid (glutamine) and semi-solid (hydrated, micro-crystalline ubiquitin) samples.

Spin-locking and cross-polarization under magic-angle spinning of uniformly labeled solids

“…Knowing the isotropic chemical shifts of the three 13 In all cases, a dip at m 1C = m r /2 is observed; this is the HORROR condition arising from intermolecular dipolar coupling for the on-resonance site, i.e., the CH site. This condition can even be seen in the 15 N spin-lock curves in Fig.…”

“…Spin-locking is useful to many NMR experiments, in particular, the simultaneous application of spin-lock pulses to two different spin species, I and S, enables mutual polarization transfer when their spin-lock rf field amplitudes (m 1 ) match the so-called Hartmann-Hahn (HH) condition [2], m 1I = m 1S , a process commonly known as cross-pola rization (CP) [3]. The transfer of polarization from abundant nuclei with high gyromagnetic ratios (c) like 1 H to nuclei with lower c (e.g., 13 C and 15 N) via CP is widely used for sensitivity enhancement [4]. Furthermore, CP with short contact times can be used for selective coherence transfer, for instance, it is the most commonly used method for establishing heteronuclear correlation (HETCOR) between 13 C and 1 H in solids.…”

“…The transfer of polarization from abundant nuclei with high gyromagnetic ratios (c) like 1 H to nuclei with lower c (e.g., 13 C and 15 N) via CP is widely used for sensitivity enhancement [4]. Furthermore, CP with short contact times can be used for selective coherence transfer, for instance, it is the most commonly used method for establishing heteronuclear correlation (HETCOR) between 13 C and 1 H in solids. In the case of uniformly 13 C and 15 N labeled proteins, CP between 13 C and 15 N with low rf fields can be used for coherence transfer between specific spin pairs to take advantage of the large 13 C and 15 N chemical shift ranges, namely SPECIFIC CP [5,6].…”

“…The efficiency of 13 C M 15 N SPECIFIC CP transfer is mostly determined by two factors, the T 1q of the two spin species and accurate matching of the CP condition within the magnitude of the 13 C-15 N dipolar coupling. The importance of matching CP conditions has been examined extensively, but the spin-lock near the conditions commonly used for 13 C M 15 N SPECIFIC CP has to our knowledge not been investigated in detail; specifically, the interference effects from chemical shift anisotropy (CSA), and hetero-and homo-nuclear dipolar interactions. This study was motivated by the fact that both the spin-lock and cross-polarization efficiencies directly impact the sensitivity of many solid-state NMR experiments applied to proteins and biomolecules.…”

“…Further, fast MAS can cause partial breakdown of the proton 'spin bath,' making the four primary HH matching conditions n = ±1, ±2 narrower than the single match condition observed for static samples. CP is usually mediated by spin flip-flop (I + S À + I À S + ), or the so-called zero-quantum (ZQ) dipolar Hamiltonian, but under MAS, flip-flip/flop-flop (I À S À + I + S + ) can also occur via the double-quantum (DQ) dipolar Hamiltonian mechanism at the conditions m 1I + m 1S = n Á m r , which can often be met under fast MAS and weak spin-locking rf fields [10][11][12][13]. In order to compensate mismatch of the CP conditions due to inhomogeneous rf fields, a sweep of the rf field amplitude [14][15][16] or frequency offset [17][18][19] across one of the matching conditions is usually applied to improve CP efficiency [14][15][16].…”