(389ac) Classical Molecular Dynamics Simulations to Understand the Formation of Polymer-Amino Acid Assemblies

Amino acids, fundamental to living organisms, link biological processes with chemistry and materials science. Despite their simple chemistry, their diverse structures and functionalities enable the formation of biocrystals and complexes with remarkable properties, making them valuable for green and electronic applications. Recently, a breakthrough experimental study showed that hydroxyl-rich polyvinyl alcohol (PVA)-water interfaces can guide the formation of γ-glycine biocrystals. The γ-glycine polymorph is thermodynamically more stable but kinetically less favorable than the predominant α-glycine polymorph observed during homogeneous nucleation. Developing scalable production methods for γ-glycine biocrystals would be valuable for biomedical, semiconductor, and energy applications due to their exceptional piezoelectric properties, but the mechanism by which PVA promotes γ-glycine formation remains unknown, hindering further process development. The focus of the work is to study different polymers and planar self-assembled monolayers (SAMs) to understand how interactions between different functional groups and glycine promote the formation of γ-glycine.

Using classical molecular dynamics (MD) simulations, we investigated the orientations and interactions between glycine molecules and SAM surfaces that present distinct functional groups to relate these to structural observables for known polymorphs. Three order parameters were used for comparison: the angle that glycine molecules form with the normal to the SAM surface, the intermolecular angles of glycine molecules, and the number of close contacts between the glycine molecules and the SAM surface. These order parameters were used to compare six systems of aqueous glycine and SAMs with different functional groups at different pH values to identify conditions that promote the nucleation of γ-glycine as observed experimentally. We find that systems representing polymer templates in low and isoelectric pH ranges promote order parameters consistent with those found for γ-glycine polymorphs while enriching the local concentration of glycine, leading to conditions that facilitate nucleation. However, conditions that lead to strong template-glycine electrostatic interactions disrupt interfacial glycine structure, highlighting the delicate interplay of interactions required for polymorph selection. Understanding the role of interactions at the surface in promoting polymorph selection could help identify the most suitable polymer template to produce γ-glycine at a scalable level, and moreover lead to generalizable design guidelines to promote the nucleation of amino-acid biocrystals by extending this approach to other amino acid and polymer systems.