Solubility of Hydrophobes into Macrocyclic Hosts

The intrinsic nature of macrocyclic molecules to preferentially absorb a specific solute has been opening up supramolecular chemistry. Nevertheless, the determinant factor with molecular perspectives in promoting host-guest complexations remains inconclusive, due to the lack of rigorous thermodynamic examination on the guest solubility inside the host. Here, we quantify the solute-solvent energetic and entropic contributions between the end states and on the docking route during inclusion of noble gases in cucurbit[5]uril, cucurbit[6]uril, and α-cyclodextrin, using molecular dynamics simulations in combination with the potential distribution theorem. Results show that in all of the pairs examined both the solute-solvent energy and entropy favor the inclusion, while the former is rather dominant. The frequency of interior drying, which pertains to the entropic contribution, differs between the hosts and is controlled by the existence of lid water at portal and the flexibility of host framework. Moreover, the hosts exhibit various types of absorption manners, involving non-, single-, and double-free-energy barriers.

On-Off of Co-non-solvency for Poly(N-vinylcaprolactam) in Alcohol-Water Mixtures: A Molecular Dynamics Study

Poly(N-vinylcaprolactam) (PVCL) exhibits co-non-solvency in aqueous solutions of 2-propanol but not in methanol. What distinguishes the impact of these two cosolvents on the polymer conformational stability? We report a molecular dynamics simulation study on PVCL 50-mer and monomers dissolved in methanol-water and 2-propanol-water mixtures. We show that the alcohol-concentration dependence of the effective attraction between a pair of PVCL monomers closely resembles the conformational changes in a single PVCL 50-mer as well as the experimentally observed behavior for PVCL chains. We also found that, at the co-non-solvency maximum, the monomer-monomer attraction works over a long-range beyond the solvent-separated distance. Then, we correlate the long-range attraction to the appearance of a dense alcohol concentration accumulated between the monomers. Furthermore, we distinctly demonstrate that the co-non-solvency of PVCL monomers can be switched on/off by artificially tuning the alcohol size while keeping the energetic parameters. Thus, we conclude that the magnitude of the excluded volume effect in alcohol accompanying the gain in translational entropy of the solvent is crucial to the occurrence of PVCL polymer co-non-solvency.

Reduction of water-mediated repulsion drives poly(N-vinylcaprolactam) collapse upon heating

Thermo-sensitive aqueous polymers undergo a coil-to-globule transition on heating, with drastic chemical and structural changes. We performed molecular dynamics simulations for PVCL in water to study the driving forces for the polymer's collapse.

Antifreeze Glycoproteins Bind Reversibly to Ice via Hydrophobic Groups

Antifreeze molecules allow organisms to survive in subzero environments. Antifreeze glycoproteins (AFGPs), produced by polar fish, are the most potent inhibitors of ice recrystallization. To date, the molecular mechanism by which AFGPs bind to ice has not yet been elucidated. Mutation experiments cannot resolve whether the binding occurs through the peptide, the saccharides, or both. Here, we use molecular simulations to determine the mechanism and driving forces for binding of AFGP8 to ice, its selectivity for the primary prismatic plane, and the molecular origin of its exceptional ice recrystallization activity. Consistent with experiments, AFGP8 in simulations preferentially adopts the PPII helix secondary structure in solution. We show that the segregation of hydrophilic and hydrophobic groups in the PPII helix is vital for ice binding. Binding occurs through adsorption of methyl groups of the peptide and disaccharides to ice, driven by the entropy of dehydration of the hydrophobic groups as they nest in the cavities at the ice surface. The selectivity to the primary prismatic plane originates in the deeper cavities it has compared to the basal plane. We estimate the free energy of binding of AFGP8 and the longer AFGPs4-6, and find them to be consistent with the reversible binding demonstrated in experiments. The simulations reveal that AFGP8 binds to ice through a myriad of conformations that it uses to diffuse through the ice surface and find ice steps, to which it strongly adsorbs. We interpret that the existence of multiple, weak binding sites is the key for the exceptional ice recrystallization inhibition activity of AFGPs.

On the cononsolvency behaviour of hydrophobic clusters in water–methanol solutions

The solvent-excluded volume effect cannot rationalize the cononsolvency phenomenon of polymers, such as PNIPAM, in water–methanol solutions.