14N overtone NMR under MAS: Signal enhancement using cross-polarization methods

Abstract

Polarization transfer methods are widely adopted for the purpose of correlating different nuclear species as well as to achieve signal enhancement. Polarization transfer from 1 H to the 14 N overtone transition (Δ =2) can be achieved using cross polarization methods under magic-angle spinning conditions, where spin locks of the order of several milliseconds can be obtained on common bio-solids (a-glycine and N-acetylvaline). Signal enhancement factors up to 4.4 per scan, can be achieved under favorable conditions, despite MHz-sized quadrupolar interaction. Moreover, we present a detailed theoretical treatment and accurate numerical simulations which are in excellent agreement the unusual experimental matching conditions observed for cross-polarization to 14 N overtone.

Selective 1H-14N Distance Measurements by 14N Overtone Solid-State NMR Spectroscopy at Fast MAS

“…However, the 1 H- 15 N experiments on that work only allowed the determination of the shortest 1 H-15 N distance due to the dipolar truncation effect. On the other hand, 14 N isotope benefits from the high natural abundance (99.6%), but it suffers from the severe quadrupolar broadening and complicated spin dynamics because 14 N is an integer quadrupolar nucleus (spin I 1). For protons, the intense 1 H-1 H homonuclear dipolar couplings in the solid state cause 1 H line broadening and shorten the 1 H coherence time.…”

“…The development of fast magic-angle spinning (MAS, ] R ≥ 60 kHz) with proton detection has made 14 N NMR spectroscopy a routinely used method, overcoming the difficulty associated with quadrupolar interaction (Cavadini et al, 2006;Gan et al, 2007;Cavadini, 2010;Nishiyama et al, 2011;Brown, 2014;Pandey and Nishiyama, 2015;Shen et al, 2015;Pandey et al, 2016;Carnevale et al, 2017;Hung et al, 2019;Jarvis et al, 2019;Rankin et al, 2019;Wijesekara et al, 2020). Furthermore, under fast MAS conditions, the strong 1 H-1 H dipolar network is largely suppressed (Nishiyama, 2016).…”

“…These two advantages potentially facilitate the 1 H-14 N distance measurement. Recently, our group have introduced a combination of phase-modulated (PM) pulse (Nimerovsky et al, 2014;Makrinich et al, 2017;Makrinich et al, 2018), SR4 2 1 recoupling (Brinkmann and Kentgens, 2006), and rotational-echo saturation-pulse double-resonance (RESPDOR) (Gan, 2006;Chen et al, 2010a;Chen et al, 2010b;Lu et al, 2011) (PM-S-RESPDOR) that can extract multiple 1 H- 14 N distances with high accuracy at fast MAS of 70 kHz (Duong et al, 2019). Such success mainly comes from the robustness that universal fraction curves can be obtained for the distance measurement under the saturation by the PM pulse for a wide range of 14 N quadrupolar coupling constant (C Q ) and 1 H-14 N dipolar coupling.…”

“…This broadband characteristics of PM pulse is useful when we work on systems containing a single 14 N site, as shown in the previous study. However, complications may arise for systems where multiple 14 N sites are present, as shown below.…”

“…An alternative approach is the 14 N overtone (OT) NMR spectroscopy, where the forbidden transitions |Δm| 2 are weakly allowed (m is the energy level) (Bloom and LeGros, 1986;Tycko and Opella, 1987;Jayanthi and Ramanathan, 2011;O'Dell and Ratcliffe, 2011;Nishiyama et al, 2013;O'Dell and Brinkmann, 2013;Haies et al, 2015a;Haies et al, 2015b;Shen et al, 2017;Concistré et al, 2018;Gan et al, 2018;Pandey and Nishiyama, 2018). Because it is twice the fundamental frequency, 14 N OT frequency is more available to commercial MAS probes since many probes are not designed to FIGURE 1 | The 5-spin system with two N and three H atoms complicating the 1 H-14 N distance measurements by PM-S-RESPDOR.…”