In Water as in Love, Likes Can Attract

            Early on in our knowledge of science we learned that opposite charges attract and like charges repel. This isn't always the case though. A research team led by theorist David Predergast and by Berkeley Lab chemist Richard Saykally, working at Advance Light Source have discovered that positively charged ions called cations can pair up with one another when placed in hydrated water. They observed guanidinium cations pairing with each other in aqueous solution. They observed this using  liquid microjets and a combination of X-ray spectroscopy. Cation-to-cation pairing has been predicted before but it has never been observed. Richard Saykally thinks that if guanidinium cations can pair this way then perhaps other similar cations can as well. Guanidinium is an ionic compound of hydrogen, carbon, and nitrogen atoms. The salt guanidinium chloride is often used by scientists for the denaturing of proteins.

          Orion Shih, who is a graduate of Saykally's research group, wrote a paper explaining this discovery of cation-to-cation pairing. Shih explains in his paper that guanidinium ions form strong donor hydrogen bond in the plane of the molecule, but it also forms weak acceptor hydrogen bonds with the pi electrons that are orthogonal (perpendicular) to the plane. It's because of this release of weakly interacting water molecules that contact pairing between the guanidinium cations occur.

This model of the guanidinium chloride salt (blue and silver) in solution shows carbon (yellow) and water (green) surrounding the cations and demonstrates cation-cation pairing. (Credit: Image courtesy of DOE/Lawrence Berkeley National Laboratory)


DOE/Lawrence Berkeley National Laboratory. "In water as in love, likes can attract." ScienceDaily, 19 Sep. 2013. Web. 28 Sep. 2013.