Proton-Selective ¹⁷O−H Distance Measurements in Fast Magic-Angle-Spinning Solid-State NMR Spectroscopy for the Determination of Hydrogen Bond Lengths

Abstract

We present a proton-selective method to determine 17 O-1 H distances in organic, biological and biomimetic materials by fast magic-angle-spinning (MAS) solid-state NMR spectroscopy. This method allows the determination of internuclear distances between specific ( 17 O, 1 H) spin pairs selectively. In this way medium-range 17 O‚‚‚ 1 H distances across hydrogen bonds can be estimated in the presence of short-range 17 O-1 H contacts sharing the same 17 O site. The method employs the newly developed symmetry-based radiofrequency (rf) pulse sequence SR4 1 2 applied to the protons to achieve heteronuclear dipolar recoupling, while simultaneously decoupling the homonuclear proton dipolar interactions. Fast MAS (50 kHz) and high static magnetic fields (18.8 T) achieve the required proton spectral resolution.Hydrogen bonding is an essential component of the structure and functioning of bio(mimetic) materials. 1 17 O solid-state NMR has started to develop into a powerful tool to study hydrogen bonding in nucleic acids, amino acids, peptides, and proteins, 2 as it gives direct access to one of the hydrogen-bonding partners in X-H‚‚‚O (X ) O, N, C) hydrogen bonds. So far, mainly the indirect correlation of 17 O chemical shifts and quadrupolar couplings has been used for this purpose. However, the most direct and accurate method to characterize these types of hydrogen bonds is to measure O‚‚‚H distances across the hydrogen bonds by determining 17 O-1 H heteronuclear dipolar couplings.Important examples of moderate hydrogen bonds 1 in biological systems are N-H‚‚‚OdC hydrogen bonds in -sheets and R-helices and O-H‚‚‚O hydrogen bonds in polysaccharides. O‚‚‚H distances d across moderate hydrogen bonds are in the medium-range order of 150-220 pm, hence significantly longer than O-H interbond distances (98-99 pm). O-H groups can act simultaneously as proton donors and acceptors, for example, in the H‚‚‚O-H‚‚‚O type hydrogen bonds in polysaccharides such as cellulose. 3 In the crystal structure of L-tyrosine‚HCl the O η -H η group acts as donor in the O η -H η ‚‚‚Cl -hydrogen bond, and simultaneously the O η acts as acceptor in the intermolecular O′′ -H′′‚‚‚O η hydrogen bond 4 (shown in Figure 1).The determination of dipolar couplings under MAS conditions requires the application of recoupling rf pulse sequences in order to suspend the MAS averaging of these couplings over a defined time interval. The rotational echo double resonance (REDOR) sequence 6 applied to the 1 H possesses the properties ii-iv; however, under fast MAS and the usual rf field limitations, substantial recoupling of the 1 H homonuclear dipolar interactions occurs. The symmetrybased R18 2 5 heteronuclear recoupling sequence fulfills the criteria i and iv and has been used to determine 17 O-1 H interbond distances. 7 Recently, we presented supercycled symmetry-based RN n V pulse sequences achieving heteronuclear longitudinal twospin-order (I z S z ) recoupling that possess all properties i-iv. 8 These sequences were used to determine the short-range O η -H η in...

Measuring proton shift tensors with ultrafast MAS NMR

“…Other experimental parameters are given in the caption. The high-field ultrafast MAS 1 H spectrum (top) exhibits two well-resolved peaks which have been assigned previously by measurement of 1 H-17 O internuclear distances using heteronuclear recoupling experiments [23]. The neutron structure of tyrosine.HCl [24] contains two short hydrogen bonds both involving the phenolic OH (designated O η H η in Ref.…”

Solid‐State NMR Studies on Supramolecular Chemistry

“…46 A recent review from Brown 22 elegantly collects some examples of the use of SEDOR (spin-echo double resonance), REDOR, and REAPDOR techniques for the determination of 13 C-17 O and 15 N-17 O dipolar couplings. 47 However, these represent rare cases since for the oxygen atom there is only one NMR active isotope, oxygen-17, which has a natural abundance of 0.038% and is a spin I = 5/2. Thus its study absolutely requires isotopic enrichment.…”

Recent Advances in Solid‐State MAS NMR Methodology for Probing Structure and Dynamics in Polymeric and Supramolecular Systems

“…However, with isotopic enrichment, 17 O NMR is being increasingly used in the study of organic and biological molecules, [132] as well as inorganic materials. [133] Recently, Brinkmann and Kentgens [134] Figure 9 shows the change in 17 From fits to numerical 2-spin simulations (solid lines in Figure 9), the 17 [124,125] and REAPDOR [136][137][138] techniques have been reported for CO chemisorbed on Pd metal, [139] L-tyrosine [137] and asparagine monohydrate, [140] as well as the differentiation of parallel and anti-parallel b-sheets formed by L-alanyl-alanyl-alanine. [141] The latter is illustrated in Figure 10, where a faster dephasing is evident for the parallel form on account of the closer intrasheet carbon-oxygen distances.…”

“…Indeed, J couplings are not normally resolved in ordinary one-dimensional solid-state S. P. Brown H resonances. [134] The solid and dotted lines correspond to fits to numerical two-spin simulations and average Hamiltonian calculations (using the fit parameters), respectively. [Reprinted with permission from ref.…”

“…[Reprinted with permission from ref. [134] Copyright ( NMR spectra, since the observed linewidth is usually larger than the magnitude of the coupling. However, a simple t/ 2-p-t/2 spin-echo MAS experiment can be used to refocus evolution due to all terms that appear as offsets, e.g.…”