The structural properties of melamine-uric acid complexes (which are responsible for kidney stones) with compositional variations are examined using a series of classical molecular dynamics simulations. The preferential interaction parameters imply that melamine interacts more strongly with uric acid than with other melamine molecules present in the system, whereas uric acid preferentially interacts with other uric acid molecules rather than with melamine. The stronger interactions among uric acid molecules produce higher-order uric acid clusters, which “drag” neighboring melamine molecules to be added to a cluster. Determination of orientational preferences between aromatic planes reveals that π–π stacking is responsible for uric acid self-association but less significant for melamine-melamine and melamine-uric acid accumulation. Cluster structure analyses suggest that higher concentrations of melamine, uric acid, or both result in a large insoluble melamine-uric acid complex cluster. Molecular mechanics-Poisson Boltzmann surface area calculations give a negative binding energy, indicating favorable complexation between melamine and uric acid molecules. Moreover, the overall complexation energy [ΔG0(mel-mel)+ ΔG0(uri-uri)+ ΔG0(mel-uri)] is more negative than ΔG0bind(mel-uri). The lifetime of melamine dimers is quite low compared with those of uric acid-uric acid and melamine-uric acid dimers, resulting in a low percentage of larger clusters for melamine-melamine interaction and a significant percentage of higher-order melamine-uric acid and uric acid-uric acid clusters with longer lifetimes. Furthermore, melamine and uric acid form strong hydrogen bonds, and melamine-melamine interactions are dominated by hydrogen bonding, whereas uric acid forms only a small number of hydrogen bonds with other uric acid molecules.

Drug molecules’ therapeutic efficacy depends on their bioavailability and solubility. But more than 70% of the formulated drug molecules show limited effectiveness due to low water solubility. Thus, the water solubility enhancement technique of drug molecules becomes the need of time. One such way is hydrotropy. The solubilizing agent of a hydrophobic molecule is generally referred to as a hydrotrope, and this phenomenon is termed hydrotropy. This method has high industrial demand, as hydrotropes are noninflammable, readily available, environmentally friendly, quickly recovered, cost-effective, and not involved in solid emulsification. The endless importance of hydrotropes in industry (especially in the pharmaceutical industry) motivated us to prepare a feature article with a clear introduction, detailed mechanistic insights into the hydrotropic solubilization of drug molecules, applications in pharma industries, and some future directions of this technique. Thus, we believe that this feature article will become an adequate manual for the pharmaceutical researchers who want to explore all of the past perspectives of the hydrotropic action of hydrotropes in pharmaceutics.

Theobromine, a naturally occurring substance, can be conceived as a prospective inhibitor for uric acid clustering. In aqueous solution, aggregates of π-stacked uric acid molecules with the larger size of clusters are modified into lower-order clusters with a substantial percentage of monomer by the incorporation of theobromine. The composite made of theobromine–uric acid is expected to have enhanced water solubility, allowing stable kidney stones to be excreted through urine. Interestingly, the strategy for the decomposition with feasible modifications in melamine–uric acid composites (that are hydrogen-bonded) is developed (by implementing the cluster structure analysis technique and binding free energies). The all-atom molecular dynamics (MD) data provides new insights into the structure and dynamics of uric acid along with melamine molecules in the context of aggregation. The simulation in the present study is supported further by structural and dynamical property calculations. The calculations of hydrogen bond dynamics, the average number of hydrogen bonds, dimer existence autocorrelation functions, umbrella sampling, and coordination number theorize that the incorporation of theobromine significantly modifies the aggregated structure of uric acid. The overall complexation energy, along with the quantum chemical calculations, further explains the alternation of aggregated structure. Furthermore, the preferential interaction parameter describes at which concentration theobromine–uric acid interaction (which is π-stacked) predominates over uric acid–uric acid interactions. Interestingly, the interactions between theobromine–melamine and melamine–melamine (which are hydrogen-bonded) are not relevant here. Thus, melamine–uric acid cluster size is reduced owing to the disintegration of self-aggregated uric acid clusters by the involvement of theobromine. Moreover, an excellent agreement is observed between present MD results and experimentally obtained data.

In this study, classical molecular dynamics simulation of eight melamine molecules is carried out in water over a temperature range of 300 K to 380 K at an ambient pressure to examine the molecular details of melamine aggregation along with the impact of temperature on the aggregated state of melamine in water. It is found that the hydrogen bonds formed between sp3 N-sp2 N of melamine, which is mainly responsible for the aggregation over the sp3 N-sp3 N, are disturbed mainly by the rise in temperature. These outcomes are complemented by the consideration of an average number of hydrogen bonds per melamine and preferential interaction parameter calculations. The impact of temperature is negligible on the orientational probability between the two triazine cores. The π–π stacking interaction between the two triazine rings plays a less significant role on melamine aggregation. Dynamical calculations, by considering cluster structure analyses and dimer existence autocorrelation function, strengthen the fact of destabilization of aggregated melamine in water with the rise in temperature. With free energy of solvation, association constant along with the binding free energy between a melamine pair gives the thermodynamic point of view of the impact of elevated temperature on melamine aggregation. Interestingly, the potential of mean force calculation using an umbrella sampling technique explains the reasons, in depth, of how do sp3 N-sp2 N interactions confirm the decrease in the initial probability of growth of higher order clusters with the increase in temperature.

The aggregation propensity of melamine molecules in aqueous solutions in a range of melamine concentrations is investigated by means of a combination of theoretical and experimental approaches. It is observed that the hydrogen bonding interactions of sp nitrogen atoms of one melamine with sp nitrogen atoms of another melamine play a major role in the melamine association. This finding is complemented by the observed favorable electrostatic energies between melamine molecules. The estimation of the orientational probability of melamine aromatic ring rules out any role of π-π interaction in melamine association. Further, the quantum chemical calculations suggest that a melamine molecule prefers to bind with another like molecule with a dihedral angle ranging from 36° to 46°. We have also determined the dimer existence autocorrelation functions to investigate the melamine-dimer stability with time in aqueous solution. Our results are well validated by the experimental findings (Chapman, R. P.; Averell, P. R.; Harris, R. R. Solubility of Melamine in Water. Ind. Eng. Chem. 1943, 35, 137-138. Ahromi, A. J.; Moosheimer, U. Oxygen Barrier Coatings Based on Supramolecular Assembly of Melamine. Macromolecules 2000, 33, 7582-7587. Yang, C.; Liu. Y. Studying on the Steady-State and Time-Resolved Fluorescence Characteristics of Melamine. Spectrochim. Acta A 2010, 75, 1329-1332. Mircescu, N. E.; Oltean, M.; Chis, V.; Leopold, N. FTIR, FT-Raman, SERS and DFT study on Melamine. Vib. Spectrosc. 2012, 62, 165-171. Makowski. S. J.; Lacher. M.; Schnick. W. Supramolecular Hydrogenbonded Structures between Melamine and N-Heterocycles. J. Mol. Struct. 2012, 1013, 19-25. Li, Z.; Chen, G.; Xu, Y.; Wang, X.; Wang, Z. Study of the Structural and Spectral Characteristics of CN(NH)(n = 1-4) Clusters. J. Phys. Chem. A 2013, 117, 12511-12518. Li, P.; Arman, D. H.; Wang, H.; Weng, L.; Alfooty, K.; Angawi, R. F.; Chen. B. Solvent Dependent Structures of Melamine: Porous or Non-porous. Cryst. Growth Des. 2015, 15, 1871-1875. Chen, J.; Lei, X.; Peng, B. Study on the Fluorescence Spectra of Melamine in Pure Milk. J. Opt. 2017, 46, 183-186.). Moreover, the thermodynamics of melamine association reveals that the association process is essentially driven by enthalpy, and this enthalpy-driven phenomenon is also confirmed by the experimental isothermal titration calorimetry measurements.