UC Berkeley researchers observe like-charge attraction between ions

It seems that the conventional scientific concept that opposites attract and likes repel may not be as simple as it appears.

In a paper published July 19 in the Journal of Chemical Physics, UC Berkeley researchers proved that certain positively charged ions, or cations, can attract each other when hydrated. Generally, only positively and negatively and charged ions bond — not those of like charges.

Although cation-cation pairing models were predicted in theory prior to this research, scientists were never able to observe the mechanisms of such an interaction. According to David Prendergast, corresponding author of the paper and staff scientist at the Lawrence Berkeley National Laboratory, this research is the first time scientists have been able to observe conclusively that such pairings are possible since cation-cation interactions are very weak.

The researchers injected small amounts of a sample containing guanidinium cations, a side group of common amino acids found in proteins, into a small opening and analyzed the samples with X-ray in a particle accelerator.

According to Richard Saykally, campus professor of chemistry and a corresponding author of the paper, the research team observed a direct interaction between the two positively charged guanidinium ions.

“The study basically tells us how strong hydrogen bonding is in water — that it is strong enough hold two positively charged molecules,” Prendergast said.

According to Prendergast, the results give biologists a deeper understanding of the functions of proteins.

“This shows that seemingly unlikely processes like cation-cation pairing … can actually become quite prominent,” Saykally said in an email. “Guanidinium is the side chain of arginine, which is one of the 20 natural amino acids. Hence, such cation-cation pairing could actually be important in biology.”

Even with the significance of the finding, the researchers say there is still much more to explore.

Saykally intends to investigate such interactions in biological components while Prendergast said he would like to discover exactly how the size and shape of these cations contribute to their pairing.