Elucidation of structural restraints for phosphate residues with different hydrogen bonding and ionization states

Solid state NMR spectroscopy and gauge including atomic orbital (GIAO) theoretical calculations were employed to establish structural restraints (ionization, hydrogen bonding, spatial arrangement) for O-phosphorylated l-threonine derivatives in different ionization states and hydrogen bonding strengths. These structural restraints are invaluable in molecular modeling and docking procedures for biological species containing phosphoryl groups. Both the experimental and the GIAO approach show that 31P delta ii chemical shift tensor parameters are very sensitive to the ionization state. The negative values found for the skew kappa are typical for -2 phosphates. The distinct span Omega values reflect the change of strength of hydrogen bonding. For species in the -1 ionization state, engaged in very strong hydrogen bonds, Omega is smaller than for a phosphate group involved in weak hydrogen bonding. For phosphates in the -2 ionization state, Omega is significantly smaller compared to -1 species, although the kappa for -1 samples never reaches negative values. For -1 phosphate residues, in the case when 1H one pulse and/or combined rotation and multiple pulse spectroscopy (CRAMPS) sequences fail and assignment of proton chemical shift is ambiguous, a combination of 1H-(13)C and 1H-(31)P 2D heteronuclear correlation (HETCOR) correlations is found to be an excellent tool for the elucidation of 1H isotropic chemical shifts. In addition, a 2D strategy using 1H-(1)H double quantum filter (DQF) correlations [a back-to-back (BABA) sequence in this work] is useful for analyzing the topology of hydrogen bonding. In the case of a multicenter phosphorus domain, 2D 31P-(31)P proton driven spin diffusion experiments give information about the spatial arrangement of the phosphate residues.