Glycine Open Dimers in Solution: New Insights into α-Glycine Nucleation and Growth

Abstract

The traditional view that α-glycine is preferentially nucleated from aqueous solution due to the presence of glycine cyclic dimers as precursors in the solution has been refuted in recent studies, which suggest that glycine molecules are present mainly as monomers in the solution. Our present work investigates the clustering behavior of glycine molecules in supersaturated aqueous solutions using molecular dynamics (MD) simulation, and our results show that glycine molecules exist predominantly as monomers at all concentrations studied for hydrogen-bond criteria of 2.2 Å/160° but that there are always at least 12% of glycine molecules existing as open dimers. Further, MD simulations are performed to study the effect of different building units on the growth of α-glycine crystal in supersaturated solutions. Our results suggest glycine open dimers to be the most favorable growth unit for α-glycine crystal. The presence of open dimers and the predominant monomers that are able to form open dimers in solution provides a convincing explanation for the preferential nucleation and growth of α-glycine.