The Hydrophobic Effect in Solute Partitioning and Interfacial Tension

The Correlated States Theory of the Hydrophobic Effect

Starting from Frank and Evans' "iceberg" model of hydrophobic hydration of small molecules (the "microscopic" hydrophobic effect, HE) published in 1945, much has been done with respect to understanding the nature of HE and elaborating a quantitative theory able to describe the thermodynamic profile (the "signature" of HE) for the large volume of experimental data accumulated to date. Generally, three sets of approaches addressing this issue were suggested, ranging from the approval of the central role of the water shell to the complete denial of its role, with the focus placed on the solute and its interactions with surrounding water. For this reason, some controversy is still present in understanding the fundamental nature of HE, even at the "microscopic" scale. Nevertheless, the general tendency of the past decade seems to have shifted toward a greater role of the water environment in determining the thermodynamic profile of HE, with a designated place for solute-water interactions as a fine-tuning of thermodynamic observables. In the present work, we developed a novel view on HE at the microscopic scale, appearing as a consequence of solute-water correlated translational and orientational vibration motion, emerging as a new property of hydrophobic hydration/solvation (the Correlated States Theory of HE). We built a fully analytical description of this process, which has enabled us to quantify the "signature" of HE for extended thermodynamic data sets without employing molecular simulations or any numerical functions in the core of the theory. As a consequence, our approach provides a self-consistent view on the known major experimental manifestations of HE across an extended temperature range, addresses some controversial issues existing to date, and creates a new augmentation to current knowledge. Most importantly, the suggested approach offers a paradigm shift from the currently dominating views on HE as a consequence of water-water interactions and the "excluded volume effect" toward the central role of solute-water interactions, and provides the first nonempirical proof of the validity of SASA-based computational models of hydrophobic hydration/solvation, which have been utilized on an empirical basis for more than 40 years.

A Molecular Dynamic Study of the Effects of Surface Partitioning on the OH Radical Interactions with Solutes in Multicomponent Aqueous Aerosols

The surface-bulk partitioning of small saccharide and amide molecules in aqueous droplets was investigated using molecular dynamics. The air-particle interface was modeled using a 80 Å cubic water box containing a series of organic molecules and surrounded by gaseous OH radicals. The properties of the organic solutes within the interface and the water bulk were examined at a molecular level using density profiles and radial pair distribution functions. Molecules containing only polar functional groups such as urea and glucose are found predominantly in the water bulk, forming an exclusion layer near the water surface. Substitution of a single polar group by an alkyl group in sugars and amides leads to the migration of the molecule toward the interface. Within the first 2 nm from the water surface, surface-active solutes lose their rotational freedom and adopt a preferred orientation with the alkyl group pointing toward the surface. The different packing within the interface leads to different solvation shell structures and enhanced interaction between the organic molecules and absorbed OH radicals. The simulations provide quantitative information about the dimension, composition, and organization of the air-water interface as well as about the nonreactive interaction of the OH radicals with the organic solutes. It suggests that increased concentrations, preferred orientations, and decreased solvation near the air-water surface may lead to differences in reactivities between surface-active and surface-inactive molecules. The results are important to explain how heterogeneous oxidation mechanisms and kinetics within interfaces may differ from those of the bulk.

Effect of Aqueous Anions on Graphene Exfoliation

Ion partitioning behavior in electrolyte solutions plays an important role in drug delivery and therapeutics, protein folding, materials science, filtration, and energy applications such as supercapacitors. Here, we show that the segregation of ions in solutions also plays an important role in the exfoliation of natural flake graphite to pristine graphene. Polarizable anions such as iodide and acetate segregate to the interfacial region of the aqueous phase during solvent interfacial trapping exfoliation of graphene. Ordered water layers and accumulated charges near the graphene surface aid in separating graphene sheets from bulk graphite, and, more importantly, reduce the reversibility of the exfoliation event. The observed phenomenon results not only in the improved stability of graphene-stabilized emulsions but also in a low-cost and environmentally friendly way of enhancing the production of graphene.

Evaluation of Protein-Ligand Docking by Cyscore

Protein-ligand docking is a powerful method in drug discovery. The reliability of docking can be quantified by RMSD between a docking structure and an experimentally determined one. However, most experimentally determined structures are not available in practice. Evaluation by scoring functions is an alternative for assessing protein-ligand docking results. This chapter first provides a brief introduction to scoring methods used in docking. Then details are provided on how to use Cyscore programs. Finally it describes a case study for evaluation of protein-ligand docking.

Fusion pores and their control of neurotransmitter and hormone release

Chang et al. review fusion pore structure and dynamics and discuss the implications for hormone and neurotransmitter release

Ca²⁺-triggered exocytosis functions broadly in the secretion of chemical signals, enabling neurons to release neurotransmitters and endocrine cells to release hormones. The biological demands on this process can vary enormously. Although synapses often release neurotransmitter in a small fraction of a millisecond, hormone release can be orders of magnitude slower. Vesicles usually contain multiple signaling molecules that can be released selectively and conditionally. Cells are able to control the speed, concentration profile, and content selectivity of release by tuning and tailoring exocytosis to meet different biological demands. Much of this regulation depends on the fusion pore—the aqueous pathway by which molecules leave a vesicle and move out into the surrounding extracellular space. Studies of fusion pores have illuminated how cells regulate secretion. Furthermore, the formation and growth of fusion pores serve as a readout for the progress of exocytosis, thus revealing key kinetic stages that provide clues about the underlying mechanisms. Herein, we review the structure, composition, and dynamics of fusion pores and discuss the implications for molecular mechanisms as well as for the cellular regulation of neurotransmitter and hormone release.