Ion-binding study by 17O solid-state NMR spectroscopy in the model peptide gly-gly-gly at 19.6 T

Li+ and Ca2+ binding to the carbonyl oxygen sites of a model peptide system has been studied by O-17 solid-state NMR spectroscopy. O-17 chemical shift (CS) and quadrupole coupling (QC) tensors are determined in four Gly-(Gly-O-17)-Gly polymorphs by a combination of stationary and fast magic-angle spinning (MAS) methods at high magnetic field, 19.6 T. In the crystal lattice, the carbonyl oxygen of the central glycyl residue in two gly-gly-gly polymorphs form intermolecular hydrogen bonds with amides, whereas the corresponding carbonyl oxygens of the other two polymorphs form interactions with Li+ and Ca2+ ions. This permits a comparison of perturbations on O-17 NMR properties by ion binding and intermolecular hydrogen bonding. High quality spectra are augmented by density functional theory (DFT) calculations on large molecular clusters to gain additional theoretical insights and to aid in the spectral simulations. Ion binding significantly decreases the two 17O chemical shift tensor components in the peptide plane, delta(11) and delta(22), and, thus, a substantial change in the isotropic chemical shift. In addition, quadrupole coupling constants are decreased by up to 1 MHz. The effects of ion binding are found to be almost an order of magnitude greater than those induced by hydrogen bonding.