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

Abstract

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.