Structure and conformation of the tripeptide: N-t-Boc-Prolyl-Phenylalanyl-Proline (Boc-Pro-Phe-Pro)

The structure and conformation of the tripeptide N-t-Boc-Prolyl-Phenylalanyl-Proline (Boc-Pro-Phe-Pro) (C₂₄H₃₃N₃O₆) have been investigated with X-ray crystallographic and spectroscopic methods. Two conformations of Boc-Pro-Phe-Pro crystallized in the space groupP2₁2₁2₁ with cell dimensionsa=11.912(1),b=14.256(1),c=30.402(3). The conformation of the backbone, the orientation of the aromatic side chain and the puckering modes for the pyrrolidine rings of these conformers differ significantly. The peptide bonds exist in the generally preferredtrans conformation being slightly non-planar. These two conformations reflect-helix- and collagen-type prolines. The crystal structures of both the conformers are stabilized by two, although different intermolecular hydrogen bonds. An intermolecular bond between the carbonyl oxygen of the carboxy terminal (³Pro) and the amide proton (²Phe) is observed for both the conformers. The second intermolecular hydrogen bond for conformer 1 is between the hydrogen of the carboxy terminal (³Pro) and the carbonyl oxygen of the N-t-Boc protecting group in contrast to the second hydrogen bond between the carbonyl oxygen of the (²Phe) residue and the hydrogen of the carboxy terminal (³Pro) for conformer 2. NMR spectroscopic studies indicated the presence of stereo-conformations due to thecis andtrans amide bonds similar to other proline-containing peptides.     

The Cassie equation: how it is meant to be used

A review of literature shows that the majority of papers cite a potentially incorrect form of the Cassie and Cassie-Baxter equations to interpret or predict contact angle data. We show that for surfaces wet with a composite interface, the commonly used form of the Cassie-Baxter equation, cosθ(c)=f(1)cosθ-(1-f), is only correct for the case of flat topped pillar geometry without any penetration of the liquid. In general, the original form of the Cassie-Baxter equation, cosθ(c)=f(1)cosθ(1)-f(2), with f(1)+f(2)≥1, should be used. The differences between the two equations are discussed and the errors involved in using the incorrect equation are estimated to be between ~3° and 13° for superhydrophobic surfaces. The discrepancies between the two equations are also discussed for the case of a liquid undergoing partial, but increasing, levels of penetration. Finally, a general equation is presented for the transition/stability criterion between the Cassie-Baxter and Wenzel modes of wetting.     

Compartmentalization of Chemically Separated Components into Droplets

Not merely a drop in the ocean : The integration of capillary electrophoresis (CE) with droplet generation driven by electroosmotic flow enabled the compartimentalization of molecular components separated by CE in a series of droplets (see picture; the green bars represent the separated analytes). The droplet‐confined bands can be docked and studied on a chip.

     

A quantum dot sensitized solar cell based on vertically aligned carbon nanotube templated ZnO arrays

We report on a quantum dot sensitized solar cell (QDSSC) based on ZnO nanorod coated vertically aligned carbon nanotubes (VACNTs). Electrochemical impedance spectroscopy shows that the electron lifetime for the device based on VACNT/ZnO/CdSe is longer than that for a device based on ZnO/CdSe, indicating that the charge recombination at the interface is reduced by the presence of the VACNTs. Due to the increased surface area and longer electron lifetime, a power conversion efficiency of 1.46% is achieved for the VACNT/ZnO/CdSe devices under an illumination of one Sun (AM 1.5G, 100 mW/cm²).     

Fifth-order Raman spectroscopy of liquid benzene: experiment and theory

The heterodyned fifth-order Raman response of liquid benzene has been measured and characterized by exploiting the passive-phase stabilization of diffractive optics. This result builds on our previous work with liquid carbon disulfide and extends the spectroscopy to a new liquid for the first time. The all-parallel and Dutch Cross polarization tensor elements are presented for both the experimental results and a finite-field molecular dynamics simulation. The overall response characteristics are similar to those of liquid carbon disulfide: a complete lack of signal along the pump delay, an elongated signal along the probe delay, and a short-lived signal along the time diagonal. Of particular interest is the change in phase between the nuclear and electronic response along the probe delay and diagonal which is not seen in CS2. Good agreement is achieved between the experiment and the finite-field molecular dynamics simulation. The measurement of the low-frequency Raman two-time delay correlation function indicates the intermolecular modes of liquid benzene to be primarily homogeneously broadened and that the liquid loses its nuclear rephasing ability within 300 fs. This rapid loss of nuclear correlations indicates a lack of modal character in the low-frequency motions of liquid benzene. This result is a validation of the general nature of the technique and represents an important step forward with respect to the use of nonlinear spectroscopy to directly access information on the anharmonic motions of liquids.     

Understanding (sessile/constrained) bubble and drop oscillations

The diffuse literature on drop oscillation is reviewed, with an emphasis on capillary wave oscillations of constrained drops. Based on the review, a unifying conceptual framework is presented for drop and bubble oscillations, which considers free and constrained drops/bubbles, oscillation of the surface or the bulk (i.e. center of mass) of the drop/bubble, as well as different types of restoring forces (surface tension, gravity, electromagnetic, etc). Experimental results (both from literature and from a new set of experiments studying sessile drops in cross flowing air) are used to test mathematical models from literature, using a novel whole profile analysis technique for the new experiments. The cause of oscillation (cross flowing air, vibrated surface, etc.) is seen not to affect oscillation frequency. In terms of models, simplified models are seen to poorly predict oscillation frequencies. The most advanced literature models are found to be relatively accurate at predicting frequency. However it is seen that no existing models are reliably accurate across a wide range of contact angles, indicating the need for advanced models/empirical relations especially for drops undergoing the lowest frequency mode of oscillation (the order 1 degree 1 non-axisymmetric ‘bending’ mode that corresponds to a lateral ‘rocking’ motion of the drop).