Molecular dynamics simulation of aqueous solutions of glycine betaine

Molecular dynamics simulation is used to investigate the hydration properties of glycine betaine in a large range of solute concentration. Statistical analyses of the system trajectories evidence microscopic details suggesting an interpretation of experimental results recently obtained for aqueous solutions of trimethylamine-N-oxide, a bioprotectant closely related to glycine betaine

Molecular Dynamics Simulation of Aqueous Solutions of Trimethylamine-N-oxide and tert-Butyl Alcohol

In this work we have investigated hydration properties of aqueous solutions up to a solute molar fraction X2 = 0.125 of two isosteric molecules \u2013 the bioprotectant trimethylamine-N-oxide (TMAO) and the denaturant tert-butyl alcohol (TBA) \u2013 using molecular dynamics simulation at 298 K. Statistical analyses of the trajectories show in particular that as the solute concentration increases the number of the water molecules in the first hydration shell decreases uniformly for TMAO, while for TBA it decreases more rapidly in a concentration range where experiments indicate that TBA starts to self-aggregate. No appreciable solute segregation occurs for TMAO even in the most concentrated solution, where on the average each water molecule is shared by two solutes. This result parallels what has been recently found for glycine betaine, an organic osmolyte closely related to TMAO

Structural organization of surfactant aggregates in vacuo: a molecular dynamics and well-tempered metadynamics study

Experimental investigations using mass spectrometry have established that surfactant molecules are able to form aggregates in the gas phase. However, there is no general consensus on the organization of these aggregates and how it depends on the aggregation number and surfactant molecular structure. In the present paper we investigate the structural organization of some surfactants in vacuo by molecular dynamics and well-tempered metadynamics simulations to widely explore the space of their possible conformations in vacuo. To study how the specific molecular features of such compounds affect their organization, we have considered as paradigmatic surfactants, the anionic single-chain sodium dodecyl sulfate (SDS), the anionic double-chain sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and the zwitterionic single-chain dodecyl phosphatidyl choline (DPC) within a wide aggregation number range (from 5 to 100). We observe that for low aggregation numbers the aggregates show in vacuo the typical structure of reverse micelles, while for large aggregation numbers a variety of globular aggregates occur that are characterized by the coexistence of interlaced domains formed by the polar or ionic heads and by the alkyl chains of the surfactants. Well-tempered metadynamics simulations allows us to confirm that the structural organizations obtained after 50 ns of molecular dynamics simulations are practically the equilibrium ones. Similarities and differences of surfactant aggregates in vacuo and in apolar media are also discussed

Molecular dynamics and metadynamics simulations of electrosprayed water nanodroplets including sodium bis(2-ethylhexyl)sulfosuccinate micelles

The behavior of aqueous solutions of sodium bis(2-ethylhexyl)sulfosuccinate (AOTNa) under conditions of electrospray ionization (ESI) has been investigated by molecular dynamics (MD) and well-tempered metadynamics (WTM) simulations at 300 K and 400 K. We have examined water droplets with initial fixed numbers of water molecules (1000) and AOT(-) anions (100), and with sodium cations in the range of 70-130. At 300 K, all charged droplets show the water evaporation rate increasing with the absolute value of the initial droplet charge state (Z), accompanied by ejection of an increasing number of solvated sodium ions or by expulsion of AOT(-) anions depending on the sign of Z and by fragmentation in the case of high |Z|. At 400 K, the water evaporation becomes more rapid and the fission process more extensive. In all cases, the AOTNa molecules, arranged as a direct micelle inside the aqueous system, undergo a rapid inversion in vacuo so that the hydrophilic heads and sodium ions surrounded by water molecules move toward the droplet interior. At the end of the 100-ns MD simulations, some water molecules remain within the aggregates at both temperatures. The subsequent metadynamics simulations accelerate the droplet evolution and show that all systems become anhydrous, in agreement with the experimental results of ESI mass spectrometry. This complete water loss is accompanied by sodium counterion emission for positively charged aggregates at 300 K. The analysis shows how the temperature and droplet charge state affect the populations of the generated surfactant aggregates, providing information potentially useful in designing future ESI experimental conditions