Investigation of molecular and mesoscale clusters in undersaturated glycine aqueous solutions

Georgina Zimbitas,Anna Jawor-Baczynska,Maria Jazmin Vesga,Nadeem Javid,Barry D Moore,John Parkinson,Jan Sefcik

doi:10.1016/j.colsurfa.2019.123633

Georgina Zimbitas, Anna Jawor-Baczynska + Show 5 more

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https://doi.org/10.1016/j.colsurfa.2019.123633

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Abstract

In this work DLS, NTA, SAXS and NMR were used to investigate populations, size distributions and structure of clusters in undersaturated aqueous solutions of glycine. Molecular and colloidal scale (mesoscale) clusters with radii around 0.3-0.5 nm and 100–150 nm, respectively, were observed using complementary experimental techniques. Molecular clusters are consistent with hydrated glycine dimers present in equilibrium with glycine monomers in aqueous solutions. Mesoscale clusters previously observed in supersaturated glycine solutions appear to be indefinitely stable, in mutual equilibrium within mesostructured undersaturated solutions across all glycine concentrations investigated here, down to as low as 1 mg/g of water.

Highlights

Aqueous solutions of highly soluble molecules, such as those of small amino acids, are usually assumed to be essentially homogenous systems with some degree of local structuring due to specific interactions on the sub-nanometre scale; these molecular structures usually do not exceed several solute molecules[1,2,3]
Our results show that undersaturated glycine aqueous solutions are mesostructrured liquid phases where hydrated glycine monomer and dimers are present alongside mesoscale clusters with radii in colloidal domain (100-150 nm) across a wide range of glycine concentrations (1-230 mg/g)
Molecular clusters were detected by three complementary experimental methods: Dynamic Light Scattering (DLS), Nuclear Magnetic Resonance (NMR)-Diffusion Ordered Spectroscopy (DOSY) and Small Angle X-ray Scattering (SAXS)

Summary

Introduction

Aqueous solutions of highly soluble molecules, such as those of small amino acids, are usually assumed to be essentially homogenous systems with some degree of local structuring due to specific interactions on the sub-nanometre scale (e.g. molecular clusters, hydration shells); these molecular structures (molecular clusters) usually do not exceed several solute molecules[1,2,3]. The presence of sub-micron size domains with liquid-like properties has been widely reported in concentrated solutions of large organic molecules such as proteins[11-13, 1627] as well as in those of smaller molecules[28,29,30,31,32,33,34,35,36,37,38,39,40,41] Detailed studies of both super- and undersaturated aqueous solutions of NaCl, (NH4)2SO4 and Na-citrate, using dynamic light scattering (DLS), revealed that such systems contain solvated ions (with radii below 1nm) and larger structures with radii varying from 50nm to 500nm[5]. Using static and dynamic light scattering (SLS and DLS, respectively) further investigation of undersaturated aqueous solutions of common small organic molecules, such as various amino acids and amines[42], citric acid[5, 43, 44], glucose[44], and urea[43, 44], revealed the presence of large-scale supermolecular structures with broad size distributions, within ranges of several hundred nanometers

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