Temperature dependence of molecular motions in bulk polystyrene

Time-resolved optical spectroscopy is used to investigate the reorientation of three rigid probes and one labeled chain in bulk polystyrene. Orientational correlation times for these probes and labels are found to be in the range of 10^?8–10^?10 s at temperatures of 180–300°C. Consistent with previous studies, the attachment of a chromophore into the chain backbone slows its dynamics by about an order of magnitude. The temperature dependences of the correlation times are similar to the temperature dependence of the viscosity. When combined with probe reorientation times near and below T_g, these results indicate that probe reorientation tracks the temperature dependence of the viscosity quite well over twelve decades in time. In contrast, literature results for the translational diffusion of similarly sized probes indicates a substantially weaker temperature dependence near T_g. Thus it appears that a fundamental change in the mechanism of probe motion occurs near T_g. © 1994 John Wiley & Sons, Inc.     

Effect of blockage on spanwise correlation in a circular cylinder wake

A short series of experiments was conducted with the aim of assessing the possible effect of tunnel blockage on spanwise correlation lengths measured in the near-wake of a circular cylinder. The results indicate that increasing blockage acts to increase spanwise correlation. This finding has important implications for the conduct and reporting of both physical and numerical experiments on bluff-body wake flows.     

Characterization of Standard Reference Material 2941, uranyl-ion-doped glass, spectral correction standard for fluorescence

Standard Reference Material^® (SRM^®) 2941 is a cuvette-shaped, uranyl-ion-doped glass, recommended for use for relative spectral correction of emission and day-to-day performance validation of fluorescence spectrometers. Properties of this standard that influence its effective use or contribute to the uncertainty in its certified emission spectrum have been explored here. These properties include its photostability, absorbance, dissolution rate in water, anisotropy, temperature coefficient of fluorescence intensity, and fluorescence lifetimes. The expanded uncertainties in the certified spectrum are about 4% around the peak maximum at 526 nm, using an excitation wavelength of 427 nm. The SRM also exhibits a strong resistance to photodegradation, with no measurable decrease in fluorescence intensity even after 8 h of laser irradiation.     

New Tripodal, “Supercharged” Analogues of Adenosine Nucleotides: Inhibitors for the Fhit Ap3A Hydrolase

Methanetrisphosphonic acids provide a branch point for synthetic nucleotide analogues which can be exploited either to generate novel tripodal nucleotides or to incorporate additional negative charge into linear analogues relative to the parent nucleotide, as exemplified in the picture for ATP and diadenosine tetraphosphate (Ap₄A). These compounds show valuable discriminatory behavior as competitive inhibitors for the tumor suppressor protein Fhit and a second Ap_nA pyrophosphohydrolase. X=H, Cl, F.