Oct. 22, 2021

As the building blocks of proteins, amino acids are a key ingredient of life. Glycine, as the simplest form of amino acids, together with water, are the most "wanted" molecules when scientists search for evidence of life in other Earth-like planets, and the origin of life on Earth. If they do exist in outer space, in which form are they stacked? A recent study suggests that glycine and water will fall in love with each other and form a hydrate.

As one of the most abundant amino acids in nature, glycine is a fascinating molecule for scientists. "This (glycine) fun, cute, small, famous biomolecule fascinates me with respect to its structure diversity, said Chunhua (Tony) Hu, a senior crystallographer at the Department of Chemistry of New York University. "Whenever you have some new ideas or tools, you want to test them on glycine first."  

In 2001, a group of researchers at the University of Minnesota observed a new phase when they quenched the aqueous glycine solution in liquid nitrogen and kept the solid at 208 Kelvin. The new phase was named "X-phase" by a research group at the Novosibirsk State University in Russia when they revisited this unknown phase.

Puzzled by the X-phase, Hu attempted to solve its structure by preparing the sample with a method called flash cooling. Having tried numerous recipes, he figured out the optimal conditions to reproduce the X-phase. The same sample was then sent to the Advanced Photon Source, Argonne National Laboratory for further X-ray diffraction analysis. The beamline scientist, Wenqian Xu, examined the sample, and said, "The diffraction data looks pretty good, and it is sufficient to determine its unit cell parameters. However, we could not find any appropriate solutions regarding the molecular arrangement in the unit cell. We need help from theory."

Qiang Zhu, an assistant professor in UNLV’s Department of Physics and Astronomy, is leading a research team to develop a novel method for crystal structure prediction, which allows them to find optimal structures and compositions of new compounds. By considering the possibilities suggested by prior XRD analysis, the search immediately yielded a dihydrate model possessing the best agreement with the observed diffraction pattern.

"It is quite amazing that such a crystal structure search only took a few days even on an ordinary desktop," Zhu said. "The crystal structure looks beautiful. It contains one glycine molecule and two water molecules in an asymmetric unit. Each glycine molecule is surrounded by seven water molecules via hydrogen bonds. Glycine molecules are well separated by water. The absence of strong glycine-glycine and water-water interactions make the hydrate rather unique. Such a feature has not been found in other amino acids hydrates discovered so far. This is perhaps the only way to make such harmony possible between glycine and water."

More interestingly, the establishment of GDH might have greater implications in planetary science. In 2009, the presence of glycine sampled in 2004 from comet Wild 2 by NASA spacecraft Stardust was confirmed. In 2016, glycine was again detected in Comet 67P/Chuyumov-Gerasimenko by Rosetta spacecraft. The comets, popularly described as "dirty snowballs," are at low temperature with massive ice, providing the right conditions to stabilize GDH. Thus it is likely that glycine could be embedded in the cometary ice and then released together with other volatiles.

"Initially, we just attempted to solve a crystal structure accidentally found in pharmaceutical engineering processes." says Zhu. "We didn’t expect it might have greater value in planetary sciences. If the results prove to be useful, why not apply the same approaches to study other life-relevant organic molecules?"

The results of the research, "Structure of Glycine Dihydrate: Its implications to Crystallization of Glycine from Solution and Modification of Glycine in Space" were recently published in Angewandte Chemie.