Clustering in Aqueous Glycine Solutions

Presence of nanodroplets in undersaturated aqueous glycine solutions were recently observed in experimental studies. These nanodroplets re-appeared even after careful efforts for their removal and purification of the glycine solutions. The composition of these clusters is not clear, although it has been suggested that they are mainly composed of glycine. To gain insights into this phenomenon, the aggregation of glycine in aqueous solutions at concentrations below the experimental solubility limit using large-scale molecular dynamics simulations under ambient conditions was studied. Three protonation states of glycine (zwitterion = GLZ, anion = GLA, and cation = GLC) are simulated using the OPLS (glycine) and TIP3P (water) molecular force fields.

Further information

When initiated from dispersed states, it was found that giant clusters do not form (Case A1) in the simulations unless salt impurities are present (Case B1). Moreover, if simulations are initiated from a giant cluster, it was found that the cluster tends to dissolve in the absence of salt impurities (Case A2) and prefers to remain in the presence of salt impurities (Case B2).

These simulation results provide little support for the possibility that the giant clusters seen in experiments are composed purely of glycine (and water). Considering that strenuous efforts are made in experiments to remove impurities such as salt, it is proposed here that the giant clusters observed might instead result from the aggregation of reaction products of aqueous glycine, such as diketopiperazine or other oligoglycines which may be difficult to separate from glycine using conventional methods, or their co-aggregation with glycine.

Employed Research Services

"Classical molecular dynamics simulations were carried out on the Eddie cluster, using the large-scale atomic/molecular massively parallel simulator (LAMMPS) code. The LAMMPS code was compiled using Intel compiler available on Eddie. The optional “INTEL acceleration package” available in LAMMPS was included during the compilation process. This has significantly improved the simulation speed. Each molecular dynamics simulation utilised 16-core Eddie computing node for several days." (Dr Nasser Afify)

The Digital Research Services employed were the following:

• Cloud Storage - To store all data collected during the study
• Eddie - To conduct all the analysis

Dr Nasser Afify is Research Associate in the Institute for Materials and Processes (IMP). You can find him on ResearchGate or LinkedIn.

• Afify, Nasser D., Carlos A. Ferreiro-Rangel, and Martin B. Sweatman. "Molecular Dynamics Investigation of Giant Clustering in Small-Molecule Solutions: The Case of Aqueous PEHA." The Journal of Physical Chemistry B 126.43 (2022): 8882-8891.
• McHale, Glen, et al. "The liquid Young’s law on SLIPS: Liquid–liquid interfacial tensions and Zisman plots." Langmuir 38.32 (2022): 10032-10042.
• Tan, Jiazheng, et al. "Cluster formation in symmetric binary SALR mixtures." The Journal of Chemical Physics 154.7 (2021): 074504.
• Afify, N. D., and M. B. Sweatman. "Preferential heating of aqueous amine solutions using infrared radiation at selected vibrational frequencies: A molecular dynamics study." The Journal of Chemical Physics 151.2 (2019): 024503.
• Afify, N. D., and Martin B. Sweatman. "Molecular dynamics simulation of microwave heating of liquid monoethanolamine (MEA): An evaluation of existing force fields." The Journal of Chemical Physics 148.20 (2018): 204513.
• Afify, Nasser D., and Martin B. Sweatman. "Classical molecular dynamics simulation of microwave heating of liquids: the case of water." The Journal of Chemical Physics 148.2 (2018): 024508.

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