Accuracy of 1H–1H distances measured using frequency selective recoupling and fast magic-angle spinning

Abstract

Selective recoupling of protons (SERP) is a method to selectively and quantitatively measure magnetic dipole–dipole interaction between protons and, in turn, the proton–proton distance in solid-state samples at fast magic-angle spinning. We present a bimodal operator-based Floquet approach to describe the numerically optimized SERP recoupling sequence. The description calculates the allowed terms in the first-order effective Hamiltonian, explains the origin of selectivity during recoupling, and shows how different terms are modulated as a function of the radio frequency amplitude and the phase of the sequence. Analytical and numerical simulations have been used to evaluate the effect of higher-order terms and offsets on the polarization transfer efficiency and quantitative distance measurement. The experimentally measured 1H–1H distances on a fully protonated thymol sample are ∼10%–15% shorter than those reported from diffraction studies. A semi-quantitative model combined with extensive numerical simulations is used to rationalize the effect of the third-spin and the role of different parameters in the experimentally observed shorter distances. Measurements at high magnetic fields improve the match between experimental and diffraction distances. The measurement of 1H–1H couplings at offsets different from the SERP-offset has also been explored. Experiments were also performed on a perdeuterated ubiquitin sample to demonstrate the feasibility of simultaneously measuring multiple quantitative distances and to evaluate the accuracy of the measured distance in the absence of multispin effects. The estimation of proton–proton distances provides a boost to structural characterization of small pharmaceuticals and biomolecules, given that the positions of protons are generally not well defined in x-ray structures.

Refined Structures of O-Phospho-l-serine and Its Calcium Salt by New Multinuclear Solid-State NMR Crystallography Methods

“…(ii) The (large) spin system was approximated by a smaller counterpart of 2–4 spins, ,,, with the internuclear distances varied to fit the NMR-signal buildup to the experimental counterpart. Such internuclear-distance extraction procedures have been applied extensively to “isolated” spin pairs, such as 13 C– 13 C pairs in organic/biological samples, ,,, where specific 13 C-site labeling in conjunction with diluting the 13 C– 13 C pairs by co-crystallization with a natural abundance material is often straightforward. , Alternatively, selective dipolar recoupling specific to one spin pair may be employed. − …”

Solid-State NMR Dipolar and Chemical Shift Anisotropy Recoupling Techniques for Structural and Dynamical Studies in Biological Systems

“…Apart from R 2 -based experiments, other types of selective recoupling strategies encoded with RF field manipulations have also been proposed, including ZQ − and DQ ,,,,− homonuclear dipolar recoupling schemes, where the chemical shift/resonance offset was used to truncate the recoupled homonuclear dipolar couplings into desired forms. However, most of these sequences have not been used in practical biomolecules.…”

Solid-State NMR: Methods for Biological Solids

“…The observed 1 H– 1 H restraints with BASS-SD have been mapped onto the X-ray structure of GB1 (Figure D). SERP (SElective Recoupling of Protons, Figure E) and SPR (Selective Proton Recoupling) are two new first-order recoupling sequence that selectively recouple proton spin-pairs in fully protonated and perdeuterated molecules. − SERP allows measuring quantitative 1 H– 1 H distances both in fully protonated and perdeuterated proteins with exchangeable protons. , However, in fully protonated proteins the measured 1 H– 1 H distances are ∼10–15% shorter than the diffraction distances . In comparison to SERP, the frequency selective-CN n ν sequence (section ), in particular C6 4 1 , was shown to provide improved recoupling efficiency at significantly lower RF fields .…”