The solvation of mono-, di- and trivalent metal ions in liquid ammonia is characterized from molecular simulations. A central focus of the analyses is given by metal ion acidity within the respective solvent complexes. For this purpose, a recently developed model for estimating the pK of the auto-protolysis reaction 2NH3 → NH4+ + NH2− is transferred to assessing the ‘local’ pK of ammonia molecules coordinating a metal ion. On this basis, we identify MI/II(NH3)n+/++ complexes (M = Na+, K+, Mg2+ with n = 6 and M = Ca2+ with n = 8, respectively) as predominant species unless extremely ammono-basic conditions are imposed. On the other hand, the trivalent Al3+ and Ga3+ species were found to favor [Al(NH2)3(NH3)2]0 and [Ga(NH2)4]− complexes, respectively. The negatively charged complexes dominate over a wide range of pH for gallium ion solvation in ammonia, whilst [Al(NH2)4(NH3)2]− complexes require moderately ammono-basic solutions.

•Quantum/molecular mechanics simulations predict the auto-protolysis constant of liquid ammonia.•Ammonium and amide ion solvation is assessed from ns-scale molecular dynamics simulations.•Dielectric permittivity and the pK are calculated as functions of temperature.A molecular mechanics simulation study of ammonium and amide ion solvation in ammonia is combined with quantum calculations to estimate the pK of the auto-protolysis reaction 2NH3 → NH4+ + NH2−. While the dielectric constant of liquid ammonia decays with increasing temperature, we find the auto-protolysis reaction to be shifted towards the product side. This is rationalized from discriminating the overall reduction of the polarity of liquid ammonia from local (nearest neighbor molecules) solvent binding to the ammonium and amide ions. Indeed, constant-volume simulations show that NH4+/NH2− ion coordination prevails heating up to 500 K thus leading to practically constant solvation energy.Download high-res image (27KB)Download full-size image

Transformations in the solid state are of considerable interest, both for fundamental reasons and because they underpin important technological applications. The interest spans a wide spectrum of disciplines and application domains. For pharmaceuticals, a common issue is unexpected polymorphic transformation of the drug or excipient during processing or on storage, which can result in product failure. A more ambitious goal is that of exploiting the advantages of metastable polymorphs (e.g. higher solubility and dissolution rate) while ensuring their stability with respect to solid state transformation. To address these issues and to advance technology, there is an urgent need for significant insights that can only come from a detailed molecular level understanding of the involved processes. Whilst experimental approaches at best yield time- and space-averaged structural information, molecular simulation offers unprecedented, time-resolved molecular-level resolution of the processes taking place. This review aims to provide a comprehensive and critical account of state-of-the-art methods for modelling polymorph stability and transitions between solid phases. This is flanked by revisiting the associated macroscopic theoretical framework for phase transitions, including their classification, proposed molecular mechanisms, and kinetics. The simulation methods are presented in tutorial form, focusing on their application to phase transition phenomena. We describe molecular simulation studies for crystal structure prediction and polymorph screening, phase coexistence and phase diagrams, simulations of crystal-crystal transitions of various types (displacive/martensitic, reconstructive and diffusive), effects of defects, and phase stability and transitions at the nanoscale. Our selection of literature is intended to illustrate significant insights, concepts and understanding, as well as the current scope of using molecular simulations for understanding polymorphic transitions in an accessible way, rather than claiming completeness. With exciting prospects in both simulation methods development and enhancements in computer hardware, we are on the verge of accessing an unprecedented capability for designing and developing dosage forms and drug delivery systems in silico, including tackling challenges in polymorph control on a rational basis.Download high-res image (90KB)Download full-size image

Polar crystals are characterized by an axis that has a nonzero dipole due to the nature of the molecular packing. For these crystals, the growth rates of the faces delineating the polar axis are generally expected to be equal. Recent experiments, however, have revealed a few exceptions where the growth of these faces from the vapor phase is asymmetric, a notable case being crystals of resorcinol. Here, we present the mechanics of resorcinol crystal growth from the melt for the hemihedral faces (011) and (01̅1̅) delineating the polar axis as revealed by molecular dynamics simulations. The simulations reveal asymmetric growth consistent with experiment. The asymmetry is attributed to the slow-growing (011) face being less able to direct the correct alignment of the oncoming molecules and the presence of an alternate resorcinol conformation that readily incorporates into the lattice at this surface, serving to poison and retard subsequent growth. Putting the issue of the rogue conformation aside, the identified factors that influence molecular recognition are considered to be applicable to other polar crystals, which suggest asymmetric growth along the polar axis to be a common feature.

We present an atomistic model of a full KRT35/KRT85 dimer, a fundamental building block of human hair. For both monomers initial structures were generated using empirical tools based on homology considerations, followed by the formulation of a naiïve dimer model from docking the monomers in vacuum. Relaxation in aqueous solution was then explored from molecular dynamics simulation. Driven by hydrophobic segregation and protein–protein hydrogen bonding relaxation dynamics result in a folded dimer arrangement which shows a striking encounter of cystein groups. Our simulations hence suggests that (i) cystein groups in the coil regions of keratin are well suited to establish disulfide bonds between the two monomers that constitute the dimer, and (ii) the particularly large number of cystein groups in the head and tail regions promotes the connection of dimers to establish meso- to macroscale fibers. Moreover, we show the molecular mechanisms of elastic and plastic deformation under tensile load. Upon elongation beyond the elastic regime, unfolding was identified as the exposure of hydrophobic moieties and the breaking of protein–protein hydrogen bonds. Therein, the step-wise character of the series of unfolding events leads to a broad regime of constant force in response to further elongation.

We present an atomistic model of a full KRT35/KRT85 dimer, a fundamental building block of human hair. For both monomers initial structures were generated using empirical tools based on homology considerations, followed by the formulation of a naiïve dimer model from docking the monomers in vacuum. Relaxation in aqueous solution was then explored from molecular dynamics simulation. Driven by hydrophobic segregation and protein–protein hydrogen bonding relaxation dynamics result in a folded dimer arrangement which shows a striking encounter of cystein groups. Our simulations hence suggests that (i) cystein groups in the coil regions of keratin are well suited to establish disulfide bonds between the two monomers that constitute the dimer, and (ii) the particularly large number of cystein groups in the head and tail regions promotes the connection of dimers to establish meso- to macroscale fibers. Moreover, we show the molecular mechanisms of elastic and plastic deformation under tensile load. Upon elongation beyond the elastic regime, unfolding was identified as the exposure of hydrophobic moieties and the breaking of protein–protein hydrogen bonds. Therein, the step-wise character of the series of unfolding events leads to a broad regime of constant force in response to further elongation.