Electrostatic interactions via ion pairs are vital for biological macromolecules. Regarding the free energy of each ion pair as a function of the interionic distance, continuum electrostatic models predict a single energy minimum corresponding to the contact ion-pair (CIP) state, whereas atomically detailed theoretical hydration studies predict multiple energy minima corresponding to the CIP and solvent-separated ion-pair (SIP) states. Through a statistical analysis of high-resolution crystal structures, we present experimental evidence of the SIP as a metastable state. The histogram of interionic distances between protein side-chain NH3 + and DNA phosphate groups clearly shows two major peaks corresponding to the CIP and SIP states. The statistical data are consistent with the probability distribution of the CIP-SIP equilibria previously obtained with molecular dynamics simulations. Spatial distributions of NH3 + ions and water molecules around phosphates reveal preferential sites for CIP and SIP formations and show how the ions compete with water molecules.
ASJC Scopus subject areas
- Materials Science(all)
- Physical and Theoretical Chemistry