Minimalist synthetic host with stacked guanidinium ions mimics the weakened hydration shells of protein-protein interaction interfaces.

Protein surfaces are complex solutes, and protein-protein interactions are specifically mediated by surface motifs that modulate solvation shells in poorly understood ways. We report herein a supramolecular host that is designed to mimic one of the most important recognition motifs that drives protein-protein interactions, the stacked arginine side chain. We show that it binds its guests and displays good selectivity in the highly competitive medium of pure, buffered water. We use a combination of experimental studies of binding and molecular dynamics simulations to build a cohesive picture of how this biomimetic host achieves the feat. The presence of the stacking element next to the guanidinium groups causes a decrease in the number of host-water hydrogen bonds, a decrease in the density of water around the host, and a decrease in water-water hydrogen bonds near the host. Experimental data using mixed organic/aqueous solvent systems confirm that this host relies on the hydrophobic effect in a way that the two control hosts do not. Our simulations and analysis provide detailed information on the linkage between (de)hydration and binding events in water in a way that could be applied to many aqueous supramolecular systems.

Combined Stokes vector and Mueller matrix polarimetry for materials characterization.

A robust way of measuring the optical properties of any material is to interrogate it with light of different polarizations. The 16-element Mueller matrix provides the most complete description of the optical properties of a sample based on its ability to alter the polarization state of transmitted or reflected light. This is valuable for ordered and isotropic materials alike. Similarly, the 4-element Stokes vector is the most complete description of the polarization of a light beam, including any depolarization effects. While the Mueller matrix offers the most chemical and physical insight, the Stokes vectors are easier to obtain, and there are more schemes for measuring them quickly in situations where time resolution is important. We describe a method where any Stokes polarimeter may be adapted to obtain Mueller matrices and discuss various approaches for achieving better time resolution.

Autocalibrating Stokes polarimeter for materials characterization.

The design and construction of an ellipsometer based on a fixed-wavelength rotating-retarder Stokes polarimeter is described. Details are provided for an automated calibration scheme that provides two advantages for its operation. The first allows the phase of the lock-in amplifiers to be set based on the raw data, without a known calibration sample. The second illustrates that the relative amplitude of the acquired signals may also be calibrated in a similar manner. As an illustration, the refractive index and thickness of a glass cover slide are determined over a range of incident angles.

An artificial primitive mimic of the Gramicidin-A channel.

Gramicidin A (gA) is the simplest known natural channel, and important progress in improving conduction activity has previously been obtained with modified natural gAs. However, simple artificial systems mimicking the gA functions are unknown. Here we show that gA can be mimicked using a simple synthetic triazole or 'T-channel' forming compound (TCT), having similar constitutional functions as the natural gAs. As in gA channels, the carbonyl moieties of the TCT, which point toward the T-channel core and surround the transport direction, are solvated by water. The net-dipolar alignment of water molecules along the chiral pore surfaces influences the conduction of protons/ions, envisioned to diffuse along dipolar hydrophilic pathways. Theoretical simulations and experimental assays reveal that the conduction through the T-channel, similar to that in gA, presents proton/water conduction, cation/anion selectivity and large open channel-conductance states. T-channels--associating supramolecular chirality with dipolar water alignment--represent an artificial primitive mimic of gA.