Protein crystal nucleation: is the pair interaction potential the primary determinant of kinetics?

Fundamental understanding of protein crystal nucleation facilitates crystallization of biological macromolecules for structure determination and control of crystal size distribution. In the studies presented here, nucleation kinetics of hen egg-white lysozyme crystals were measured at solution conditions that exhibited equal solubility by adjusting pH, temperature, or sodium chloride concentration. It was observed that solution conditions that lead to equal solubility resulted in equal nucleation rates and hence kinetic parameters. Since the solubility of globular proteins correlates with the osmotic second virial coefficient, B(22), an integral measure of the protein pair interaction potential, this observation indicates that the protein pair interaction plays a key role in determining nucleation kinetic parameters.     

Hydrogen bonding in macrocyclic receptor systems

Abstract

The interactions of macrocyclic polyethers with alkali and alkaline earth cations have been well studied and much about their chemistry is now well understood. Less well examined or comprehended are hydrogen bond interactions. A combination of ion selective electrode binding constant determination techniques and fast atom bombardment mass spectrometry are brought to bear on this problem. It is found that all-oxygen crown ethers and their derivatives exhibit quite different complexation behaviour with ammonium salts than do their various azacrown counterparts.     

Photoexcited GaAs surfaces studied by transient terahertz time-domain spectroscopy

The transmission characteristics of an air-GaAs interface and the transient absorption and index spectra of the thin, photoexcited surface layer are investigated subsequent to excitation by a femtosecond laser pulse. We find that the total phase change and transmission of a terahertz (THz) probe pulse are dominated by interface effects. This observation has important implications in the interpretation of THz time-domain spectroscopy data of absorbing media. We also observe that the THz pulse apparently arrives at the detector as much as 60 fs earlier when it is transmitted through an optically excited GaAs wafer. This effect is fully explained in terms of a frequency-dependent transmission and phase shift at the air-GaAs interface and is not associated with superluminal propagation.     

Die Bestimmung von Aceton und Acetessigs?ure mit dem Autenriethschen Kolorimeter

Ohne Zusammenfassung     

Excitation in low-energy Li+ collisions with cesiated W(110) surfaces: alignment and orientation of the collisionally excited Li(2p) state

The linear and circular polarization of the Li(2p?2s) emission has been measured for Li⁺ collisions with cesiated and oxydized cesiated W(110) surfaces, Cs-W(110) and O/Cs-W(110), at impact energies below 1 keV for an incidence angle of 5 degrees with respect to the surface plane. The cesium coverage has been varied between zero and the saturation coverage at room temperature. Although the excitation of the projectile seems to occur in a binary collision with only one Cs adatom, the presence of the surface influences the total photon yield by Auger deexcitation, and the circular polarization by affecting the energetic separation of the Li(2p) substates close to the surface.