Concentration-dependent like-charge pairing of guanidinium ions and effect of guanidinium chloride on the structure and dynamics of water from all-atom molecular dynamics simulation

An all-atom molecular dynamics simulation shows concentration-dependent like-charge ion pairing of the guanidinium ion in an aqueous solution of guanidinium chloride. We have observed two types of like-charge ion pairing for guanidinium ions, namely, stacked ion pairs and solvent-separated ion pairs. Interestingly, both of these like-charge ion-pair formations are dependent on the concentration of guanidinium chloride in water. The probability of stacked like-charge ion-pair formation decreases, whereas, the probability of solvent-separated like-charge pairing increases as the concentration of guanidinium chloride increases, which is shown from radial distribution functions and is confirmed from the energy calculations. Besides like-charge ion-pair formation, we also investigated guanidinium chloride induced changes in water structure. Hydrogen-bond analysis indicates that guanidinium chloride does not alter the strict-hydrogen-bonding patterns of water, whereas, it breaks the bend-hydrogen bond and the non-hydrogen-bonding patterns. Tetrahedral order, nearest neighbor orientation, and distance distribution of water molecules around a probe water molecule show the extent of water structure distortion.