The conformational states of Mg.ATP in water.

The conformational equilibria of Mg.ATP in solution is studied using molecular dynamics (MD) augmented with umbrella sampling methods. Free energy comparisons show that the Mg(2+) ion is equally likely to coordinate the oxygens of the two end phosphates, or of all three phosphates. The MD trajectories reveal two major degrees of freedom of the Mg.ATP molecule in solution, and we compute the free energy as a function of these variables, and determine its elastic properties. Comparing the free energy function with several crystallographic structures of ATP analogs, we find that the crystal structures correspond to states where ATP would be elastically strained. The average water density around Mg.ATP is investigated to show the average number of hydrogen bonds and the hydrophobicity.