Solution-state cross-polarization for selective excitation of scalar-coupled quadrupolar nuclei S>1/2 in the presence of a residual quadrupolar splitting

Multinuclear Residual Quadrupolar Couplings for Structure and Assignment

Most stable isotopes have a nuclear spin >1/2, but the quadrupole interaction poses challenge on their detection by nuclear magnetic resonance (NMR). On the other hand, the quadrupole interaction is a rich source of structural information that may be exploited for solution NMR in the form of residual quadrupolar couplings (RQCs) of weakly oriented samples. While 2H RQCs are now well established for structure verification and enantiomeric discrimination of organic molecules, we will in this article highlight some recent work on RQCs of other nuclei (especially 7Li and 11B).

Harnessing Water to Enhance Quadrupolar NMR Spectroscopy and Imaging

¹⁷ O and ¹⁴ N are attractive targets for in vivo NMR spectroscopy and imaging, but low gyromagnetic ratios γ and fast spin relaxation complicate observations. This work explores indirect ways of detecting some of these sites with the help of proton-detected double resonance techniques. As standard coherence transfer methods are of limited use for such indirect detection, alternative routes for probing the quadrupolar spectra on ¹ H were tested. These centered on modulating the broadening effects imparted onto protons adjacent to the low-γ species through J couplings through either continuous wave or spin-echo double-resonance decoupling/recoupling sequences. As in all cases, the changes imparted by these double-resonance strategies were small due to the fast relaxation undergone by the quadrupoles, the sensitivity of these approaches was amplified by transferring their effects onto the abundant water ¹ H signal. These amplifications were mediated by the spontaneous exchanges that the labile ¹ Hs bound to ¹⁷ O or ¹⁴ N undergo with the water protons. In experiments designed on the basis of double-resonance spin echoes, these enhancements were imparted by looping the transverse encodings together with multiple longitudinal storage periods, leading to decoupling-recoupling with exchange (D-REX) sequences. In experiments designed on the basis of continuous on/off quadrupolar decoupling, these solvent exchanges were incorporated into chemical-exchange saturation transfer schemes, leading to decoupling-recoupling with saturation transfer (D-REST) sequences. Both of these variants harnessed sizable proportions of the easily detectable water signals, in order to characterize the NMR spectra and/or to image with atomic-site specificity the ¹⁷ O and ¹⁴ N species.

Identification of beryllium fluoride complexes in mechanically distorted gels using quadrupolar split 9Be NMR spectra resolved with solution-state selective cross-polarization

The uniformly anisotropic media afforded by hydrogels are being increasingly exploited in analytical (structure elucidation) nuclear magnetic resonance (NMR) spectroscopy, and in studies of mechanosensitive biophysical and biochemical properties of living cells. The 9Be NMR parameters of beryllium fluoride complexes formed in aqueous solutions are sensitive markers of the anisotropic molecular environments produced by gelatin gels. The electric quadrupole moment of the 9Be nucleus (spin I = 3/2) interacts with the electric field gradient tensor in a stretched (or compressed) gel, giving rise to the splitting of peaks in 9Be NMR spectra. These are in addition to those produced by scalar coupling to the 19F nuclei. Thus, an equilibrium mixture of beryllofluoride complexes (BeF2, BeF3-, and BeF42-) in mechanically distorted gels generates an envelope of overlapping 9Be NMR multiplets. In the present work, the multiplets were dissected apart by using selective excitation of 9Be-19F cross-polarization; and the spectral components were quantified with multi-parameter line-shape decomposition, coupled with SpinDynamica simulations. The effects of gel density and Bloom number (a measure of gelatin-gel rigidity under standard conditions of sample preparation) on residual quadrupolar splittings were examined. Cross-polarization experiments revealed a bimodal distribution of  residual quadrupolar coupling constants (RQC) of the BeF3- complexes. The average RQC of the dominant BeF3- population was ∼3 times larger than that of BeF42-. The secondary BeF3- population existed in a tetrahedral configuration. It was attributed to BeF3- complexes associated with negatively charged -COO- groups of the denatured collagen matrix.