Experimental and theoretical study of the hydration of phosphate groups in esters of biological interest

We have studied the influence of different groups esterified to phosphates on the strength of the interaction of the PO bond with one water molecule. Experimental vibrational spectra of PO(4)3-, HPO4(2-), H2PO4-, phosphoenolpiruvate (PEP) and ortho-phosphocholamine (o-PC) were obtained by means of FTIR spectroscopy. Geometry calculations were performed using standard gradient techniques and the default convergence criteria as implemented in GAUSSIAN 98 Program. In order to assess the behaviour of such DFT theoretical calculations using B3LYP with 6-31G* and 6-311++G** basis sets, we carried out a comparative work for those compounds. The results were then used to predict the principal bands of the vibrational spectra and molecular parameters (geometrical parameters, stabilisation energies, electronic density). In this work, the relative stability and the nature of the PO bond in those compounds were systematically and quantitatively investigated by means of Natural Bond Order (NBO) analysis. The topological properties of electronic charge density are analysed employing Bader's Atoms in Molecules theory (AIM). The hydrogen bonding of phosphate groups with water is highly stable and the PO bond wavenumbers are shifted to lower experimental and calculated values (with the DFT/6-311++G** basis set). Accordingly, the predicted order of the relative stability of the hydrogen bonding of the water molecule to the PO bond of the investigated compounds is: PO(4)3->HPO4(2-)>H2PO4->phosphoenolpiruvate>phosphocholamine for the two basis sets used.