(M1230-05-25) A Molecular Dynamics Study of the Impact of Water Content on Pre-nucleation Aggregates in Amorphous 2-Phenylamino Nicotinic Acid

Purpose: Amorphous solid state has become increasingly important for oral drug delivery of poorly water-soluble drug molecules. However, amorphous materials are thermodynamically unstable and revert to the crystalline form upon storage. While several factors control crystallization from amorphous solids, one important environmental factor impacting their stability is water. Water has been shown to act as a plasticizer or an anti-plasticizer of amorphous materials with its ability to change the glass transition temperature. However, the role of water in promoting or inhibiting the crystallization of specific polymorphs from amorphous solid state has not been explored. In this study, the role of water on crystallization of amorphous 2-phenyl amino nicotinic acid (2PNA) was examined (Figure 1a). Our previous report demonstrated that amorphous 2PNA contains both carboxylic acid dimer (-COOH^…-COOH) and carboxylic acid-pyridine N (-COOH^…pyridine N) hydrogen-bonded molecules with the crystallization outcome dependent on relative humidity. In the current study, the role of water content on evolution of -COOH^…pyridine N pre-nucleation aggregates in amorphous 2PNA and the crystallization of δ polymorph has been explored.

Methods: Using molecular dynamics simulations, the impact of water content on inter-molecular interactions and structural evolvement in amorphous 2PNA was investigated. In silico amorphous model for 2PNA consisted of 200 molecules and was generated using the Disordered System Builder module in Maestro. The dimensions of the amorphous cell were 3.5 nm × 3.5 nm× 3.5 nm, with periodic boundary conditions (Figure 1b). 2PNA−water mixtures were generated to obtain models with 1%, 2.5%, 5%, and 11% w/w water, respectively. After allowing the model system to relax, the MD simulations were carried out in two phases, the equilibration phase and the production phase at 300K. In the first part of the equilibration phase, the amorphous cell was allowed to relax for 1000 ps under isothermal (NVT) conditions to obtain a structure with equilibrated temperature. In the second part of the equilibration phase, the system was allowed to equilibrate under isothermal-isobaric (NPT) conditions to obtain a system with equilibrated density. In the production phase, the system was subjected to MD simulations at 300K for 1 ns. The Nose-Hoover thermostat, and Martyna-Tobias-Klein barostat, were used to maintain the temperature and pressure, respectively with a time step of 2 fs. The production run was used for data analysis.

Results: The influence of water content on hydrogen bonding interactions was assessed using radial distribution functions. The nomenclature used to name atoms in RDF analysis is shown in Figure 1a and the nomenclature used for water oxygen is O_w. The RDF for O_w-N1 (Figure 2a), shows a sharp peak at 2.8 Å, which is indicative of strong hydrogen bonding between water and the pyridine nitrogen. There seems to be a strong radial water structuring around N1, and a substantial shielding between the N1 atoms and the surrounding -COOH groups. With increasing water content, this shielding is dramatically reduced. With 11%w/w water, the secondary broad peaks begin to disappear indicating loss of long-range water structure around N1. At the same time, the RDF for N1-O16 (Figure 2b) shows greater structuring of -COOH hydroxyl groups around N1 with a long-range structure beginning to emerge as the water content in the amorphous phase increases. The radial distribution function for O_w-O15 (Figure 2c) shows a peak at 3.4 Å. This is indicative of weak interaction of water with -COOH group in 2PNA. Another broad peak is observed at 5.8 Å. This hydration shell structure decreases in intensity with increasing water content. At 11%w/w concentration, the peak at 5.8 Å completely disappears indicating loss of long-range hydration structure around O15. The RDF of H26-O15 (Figure 2d) does not show a significant difference with increasing water content. These results shed light on the pre-nucleation events involving -COOH^..pyridine N hydrogen-bonded aggregates. At low concentrations of water, pyridine nitrogen is strongly hydrogen-bonded to water with multiple hydration shells shielding it from -COOH groups. As the water concentration increases, the shielding decreases considerably, allowing greater number of interactions between -COOH and pyridine functional groups. Furthermore, at higher concentrations, the water molecules begin to show long-range structure or “water clustering”. The RDF of O_w-O_w  (Figure 3) demonstrates this behavior. As the water concentration increases, two broad peaks at approximately 4.8 Å and 7 Å can be seen to develop. This water re-distribution provides the mobility in the 2PNA system to accelerate the crystallization of δ polymorph.

Conclusion: The study underlines the structural evolvement of pre-nucleation aggregates in amorphous samples on interacting with water molecules. This internal restructuring with increasing water concentration is motivated by lowering of total energy of the system.

Acknowledgements: The authors do not claim any conflict of interest.


Figure 1: (a) Chemical Structure of 2PNA. The numbering of the atoms has been specified with carbons in grey, hydrogens in white, nitrogens in blue, and oxygens in red color. (b) Amorphous model for 2PNA consisting of 200 molecules with dimensions of 3.5 nm × 3.5 nm × 3.5 nm.


Figure 2. Radial distribution function for (a) Ow-N1, (b) N1-O16, (c) Ow-O15, and (d) H26-O15, at 1%, 2.5%, 5% and 11%w/w water content.


Figure 3. Radial distribution function for Ow- Ow, at 1%, 2.5%, 5% and 11%w/w water content.