Glycine aggregates in water and saltwater probed by polarized second harmonic scattering

The formation of crystals has been a long-standing unresolved question while its understanding is crucial for rational design of the crystallization process and controlling polymorphism. Glycine is a well-studied model system as it has been known to form three polymorphs, depending on crystallization conditions. Recently, a new hypothesis has been suggested that glycine forms pre-nucleation aggregates, both in water and saltwater, before nucleating as a metastable β-glycine polymorph. Molecular dynamics simulations proposed the structure of the pre-nucleation aggregates in water as a linear hydrogen-bonded network of glycine molecules. Herein, polarized second harmonic scattering (pol-SHS) was used to test this hypothesis by probing the structural details of glycine aggregates in water and saltwater. Depolarization ratio extracted from a series of concentrations of glycine solutions strongly indicates the presence of aggregates both in water and saltwater and their structure evolves from linear (dipolar) networks to more branched (less dipolar) networks. Retardation ratio of pol-SHS data provided the molecular level details of how water organizes around a glycine molecule/aggregate in water or saltwater. In an aqueous solution, water reorganizes from azimuthal to radial pattern when glycine is dissolved. In saltwater, water organizes in a radial pattern around salt ions and the addition of glycine does not alter the structural organization of water. Glycine aggregates grow while the long-range molecular correlation of the sodium chloride-water networks is maintained. This study demonstrates the potency of pol-SHS in investigating the structure of molecular aggregates in solution in the context of understanding the crystal nucleation mechanism.