Effect of mixing, concentration and temperature on the formation of mesostructured solutions and their role in the nucleation of DL-valine crystals

We report investigations on the formation of mesostructured solutions in DL-valine-water-2-propanol mixtures, and the crystallization of DL-valine from these solutions. Mesostructured liquid phases, similar to those previously observed in aqueous solutions of glycine and DL-alanine, were observed using Dynamic Light Scattering and Brownian microscopy, in both undersaturated and supersaturated solutions below a certain transition temperature. Careful experimentation was used to demonstrate that the optically clear mesostructured liquid phase, comprising colloidal mesoscale clusters dispersed within bulk solution, is thermodynamically stable and present in equilibrium with the solid phase at saturation conditions. Solutions prepared by slow cooling contained mesoscale clusters with a narrow size distribution and a mean hydrodynamic diameter of around 200 nm. Solutions of identical composition prepared by rapid isothermal mixing of valine aqueous solutions with 2-propanol contained mesoscale clusters which were significantly larger than those observed in slowly cooled solutions. The presence of larger mesoscale clusters was found to correspond to faster nucleation. Observed induction times were strongly dependent on the rapid initial mixing step, although solutions were left undisturbed afterwards and the induction times observed were up to two orders of magnitude longer than the initial mixing period. We propose that mesoscale clusters above a certain critical size are likely to be the location of productive nucleation events

Investigation of molecular and mesoscale clusters in undersaturated glycine aqueous solutions

YesIn 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.Supported by EPSRC funding via the SynBIM project (Grant Reference EP/P0068X/1) and by the Synchrotron SOLEIL

Self-assembly of ultra-small micelles from amphiphilic lipopeptoids

Poly(N-substituted glycine) “peptoids” constitute a promising class of peptide-mimetic materials. We introduce the self-assembly of lipopeptoids into spherical micelles ca. 5 nm in diameter as well as larger assemblies by varying the peptoid sequence design. Our results point to design rules for the self-assembly of peptoid nanostructures, enabling the creation of stable, ultra-small peptidomimetic nanospheres

Pre-nucleation clusters as solute precursors in crystallisation

Crystallisation is at the heart of various scientific disciplines, but still the understanding of the molecular mechanisms underlying phase separation and the formation of the first solid particles in aqueous solution is rather limited. In this review, classical nucleation theory, as well as established concepts of spinodal decomposition and liquid–liquid demixing, is introduced together with a description of the recently proposed pre-nucleation cluster pathway. The features of pre-nucleation clusters are presented and discussed in relation to recent modifications of the classical and established models for phase separation, together with a review of experimental work and computer simulations on the characteristics of pre-nucleation clusters of calcium phosphate, calcium carbonate, iron(oxy)(hydr)oxide, silica, and also amino acids as an example of small organic molecules. The role of pre-nucleation clusters as solute precursors in the emergence of a new phase is summarized, and the link between the chemical speciation of homogeneous solutions and the process of phase separation via pre-nucleation clusters is highlighted

Nucleation mechanism of crystal formation during antisolvent or cooling induced crystallisation

This thesis was previously held under moratorium from 10th May 2011 until 10th May 2013.This project studied the nucleation mechanism of crystal formation during antisolvent or cooling crystallisation of simple amino acids: D,L-valine and glycine. These amino acids can co-precipitate with proteins to form Protein Coated Microcrystals (PCMCs) in which the crystals create a solid support and the biomacromolecules cover their surface while remaining in a native state. The understanding of the formation mechanism of small microcrystals would help to better control and manage the process which leads to ordered attachment of biomacromolecules on their surfaces. Spectrophotometry, 1H nuclear magnetic resonance (NMR), dynamic light scattering (DLS) and optical microscopy were used to probe the evolution of the system from the transparent solution to a suspension of microcrystals. The nucleation mechanism of antisolvent crystallisation was found to involve formation of a transparent nanoemulsion composied of sub-micron valine-rich liquid nanodroplets with an average size and size distribution depending on supersaturation and the mixing conditions used during sample preparation. The supersaturated solutions prepared by cooling crystallisation, without agitation produced smaller nanodroplets and resulted in formation of only a few large crystals with an extremely slow crystallisation rate compared to samples with identical composition prepared by antisolvent crystallisation. The following nucleation mechanism of amino acids crystals is proposed: dissolution of amino acid into an aqueous/2-propanol mixture at concentration close to saturation results in spontaneous formation of a thermodynamically stable system consisting of amino acid rich liquid nanodroplets dispersed in amino acid solution; above a particular amino acid composition (consistent with the crystal solubility limit) the dispersed nanodroplets become metastable and shear induced coalescence of nanodroplets can provide access to a fast crystallisation pathway (non-classical); in the absence of shear the nanodroplets are colloidally-stable and crystallisation follows a much slower pathway (classical). The spontaneous formation of solute-rich nanodroplets below the crystalline saturation limit as well as formation of metastable solute-rich nanodroplets above this limit provides a paradigm shift which can be potentially used to develop fundamental understanding of nonclassical crystallisation phenomena. It will be crucial for better design and control of crystallisation processes in pharmaceutical applications.This project studied the nucleation mechanism of crystal formation during antisolvent or cooling crystallisation of simple amino acids: D,L-valine and glycine. These amino acids can co-precipitate with proteins to form Protein Coated Microcrystals (PCMCs) in which the crystals create a solid support and the biomacromolecules cover their surface while remaining in a native state. The understanding of the formation mechanism of small microcrystals would help to better control and manage the process which leads to ordered attachment of biomacromolecules on their surfaces. Spectrophotometry, 1H nuclear magnetic resonance (NMR), dynamic light scattering (DLS) and optical microscopy were used to probe the evolution of the system from the transparent solution to a suspension of microcrystals. The nucleation mechanism of antisolvent crystallisation was found to involve formation of a transparent nanoemulsion composied of sub-micron valine-rich liquid nanodroplets with an average size and size distribution depending on supersaturation and the mixing conditions used during sample preparation. The supersaturated solutions prepared by cooling crystallisation, without agitation produced smaller nanodroplets and resulted in formation of only a few large crystals with an extremely slow crystallisation rate compared to samples with identical composition prepared by antisolvent crystallisation. The following nucleation mechanism of amino acids crystals is proposed: dissolution of amino acid into an aqueous/2-propanol mixture at concentration close to saturation results in spontaneous formation of a thermodynamically stable system consisting of amino acid rich liquid nanodroplets dispersed in amino acid solution; above a particular amino acid composition (consistent with the crystal solubility limit) the dispersed nanodroplets become metastable and shear induced coalescence of nanodroplets can provide access to a fast crystallisation pathway (non-classical); in the absence of shear the nanodroplets are colloidally-stable and crystallisation follows a much slower pathway (classical). The spontaneous formation of solute-rich nanodroplets below the crystalline saturation limit as well as formation of metastable solute-rich nanodroplets above this limit provides a paradigm shift which can be potentially used to develop fundamental understanding of nonclassical crystallisation phenomena. It will be crucial for better design and control of crystallisation processes in pharmaceutical applications

250 nm Glycine-Rich Nanodroplets Are Formed on Dissolution of Glycine Crystals But Are Too Small To Provide Productive Nucleation Sites

Recent theoretical and experimental studies have proposed a two-step mechanism for crystal formation in which crystal nucleation is preceded by formation of disordered molecular assemblies. Here, we investigated whether similar intermediates might also form as crystals dissolve, effectively the reverse process. A model system of glycine in water was studied, and the resultant solutions were characterized using small-angle X-ray scattering, dynamic light scattering, and nanoparticle tracking analysis. Invariably, dissolution of glycine crystals into water was observed to produce scattering nanospecies with liquid-like properties and a mean diameter of about 250 nm, at near saturation concentration. The nanospecies persisted indefinitely in solution in the presence of excess glycine crystals and were identified as glycine-rich nanodroplets with an equilibrium population of about 10⁹ per mL. The time to appearance of glycine crystals from quiescent supersaturated solution (<i>S</i> = 1.1) containing either a low population of nanodroplets (nanofiltered) or a high population of nanodroplets (unfiltered) was indistinguishable with typically only a single crystal forming after about 30 h. However, a very significant acceleration of crystal formation was observed whenever a gently tumbling stirrer-bar was introduced into the vial; thousands of microcrystals appeared after an incubation period of only 3–5 h. The possibility of this being caused by factors such as secondary nucleation, bubbles, or glass splinters or scratches was eliminated via control experiments. Further investigation of the glycine solution, just prior to appearance of microcrystals, revealed an additional subpopulation of extremely large glycine-rich nanodroplets (diameter >750 nm), not observed in quiescent solutions. It is proposed that productive nucleation of glycine crystals occurs exclusively within these larger glycine-rich nanodroplets because a critical mass of glycine is required to form nascent crystals large enough to survive exposure to bulk more dilute solution. We hypothesize that nucleation occurs frequently but nonproductively within subcritical mass nanodroplets and infrequently but productively within very rare critical mass solute-rich nanodroplets. Such a model provides a new compelling way of bridging classical mechanisms of crystal nucleation with the more recently proposed two-step processes