Graph-Theoretic Analysis of Monomethyl Phosphate Clustering in Ionic Solutions.

All-atom molecular dynamics simulations combined with graph-theoretic analysis reveal that clustering of monomethyl phosphate dianion (MMP) is strongly influenced by the types and combinations of cations in the aqueous solution. Although Ca promotes the formation of stable and large MMP clusters, K alone does not. Nonetheless, clusters are larger and their link lifetimes are longer in mixtures of K and Ca. This "synergistic" effect depends sensitively on the Lennard-Jones interaction parameters between Ca and the phosphorus oxygen and correlates with the hydration of the clusters. The pronounced MMP clustering effect of Ca in the presence of K is confirmed by Fourier transform infrared spectroscopy. The characterization of the cation-dependent clustering of MMP provides a starting point for understanding cation-dependent clustering of phosphoinositides in cell membranes.