Quantitative ¹H–¹H Distances in Protonated Solids by Frequency-Selective Recoupling at Fast Magic Angle Spinning NMR

Abstract

Nuclear magnetic resonance (NMR) spectroscopy of protons in protonated solids is challenging. Fast magic angle spinning (MAS) and homonuclear decoupling schemes, in conjunction, with high magnetic fields have improved the proton resolution. However, experiments to quantitatively measure 1 H− 1 H distances still remain elusive due to the dense proton−proton dipolar coupling network. A novel MAS solidstate NMR pulse sequence is proposed to selectively recouple and measure interproton distances in protonated samples. The phase-modulated sequence combined with a judicious choice of transmitter frequency is used to measure quantitative 1 H− 1 H distances on the order of 3 Å in Lhistidine•HCl•H 2 O, despite the presence of other strongly coupled protons. This method provides a major boost to NMR crystallography approaches for structural determination of pharmaceutical molecules by directly measuring 1 H− 1 H distances. The band-selective nature of the sequence also enables observation of selective 1 H− 1 H correlations (e.g., H N −H N /H N −H α /Η Ν −Η Methyl ) in peptides and proteins, which should serve as useful restraints in structure determination.

Structure Solution of Nano-Crystalline Small Molecules Using MicroED and Solid-State NMR Dipolar-Based Experiments

“…This strategy is feasible only when the magnetization transfer is selective otherwise the other 1 H- 1 H dipolar couplings would complicate this transfer. For example, under a broadband recoupling sequence such as fp-RFDR, the build-up curves show almost similar build-up rates for all 1 H atoms owing to the relayed magnetization transfer [ 40 ]. For this reason, we performed the 1 H- 1 H SERP experiments to selectively transfer the magnetization between a 1 H- 1 H pair, using the sequence in Figure S1d .…”

NMR Crystallography at Fast Magic-Angle Spinning Frequencies: Application of Novel Recoupling Methods

“…Table 1 lists these distances of the ABZ samples measured from the crystal structure of the forms (CCDC Deposition: 736,654 for ABZ I [24] and CCDC Deposition: 668,711 for ABZ II [23]). The positive contours are the consequence of zero-quantum recoupling happening during the P 0 P 180 P 90 P 270 ( Figure 2) blocks of the constant-time recoupling duration [21]. They give rise to positive cross peaks between adjacent resonances if they are strongly coupled, as shown in Figure 6.…”

“…The visualisation of 1 H -1 H interactions is particularly useful in small pharmaceutical molecules and reflects on intramolecular, intermolecular, molecular-packing and hydrogen-bonding interactions. Experimental methods such as BASS-SD [20], SERP [21] and 1 H single quantum-double quantum correlation experiments can provide these information [22]. Here, we showed that quantitative 1 H -1 H distances can be measured at fast MAS frequencies using SElective Recoupling of Protons (SERP) experiment.…”

“…Intensity of the recoupling curves was calculated relative to the intensity of the source magnetisation peak at the mixing time of 0 µs. Intensity of the recoupling curve from the simulations was scaled to match the experimental values, as suggested previously [21]. SIMPSON spin system containing H6, H9 and H10/H1 (intermolecular) were considered for fitting the build-up between H6 to H10 and spins for H3, and three spins for H14 with coupling corresponding to their distances were used for fitting the distance between H3 and H14.…”

Frequency Selective Phase-Optimized Recoupling for Protons in Ultra-Fast Solid-State Magic-Angle Spinning NMR

“…SPR is simple, robust and efficient in frequency selective recoupling, which serves as a potential technique to obtain useful restraints for structural determination. It can also be used in a constant-time way to quantitative measurements of distances 31,35 . Compared to the widely-used RFDR, SPR provides sensitivity gains up to 3 for selective 1 H− 1 H correlations (e.g.…”