Study of the peak effect phenomenon in single crystals of 2H-NbSe2

The weakly pinned single crystals of the hexagonal 2H-NbSe₂ compound have emerged as prototypes for determining and characterizing the phase boundaries of the possible order-disorder transformations in the vortex matter. We present here a status report based on the ac and dc magnetization measurements of the peak effect phenomenon in three crystals of 2H-NbSe₂, in which the critical current densities vary over two orders of magnitude. We sketch the generic vortex phase diagram of a weakly pinned superconductor, which also utilizes theoretical proposals. We also establish the connection between the metastability effects and pinning.     

INACTIVATTON OF S-RNA BY ULTRAVIOLET RADIATIONS

Abstract. Various S-RNA's from E. coli show wide differences in sensitivity to ultraviolet (u.v.) radiation as measured by their amino acid acceptor capacities. S-RNA for valine is least sensitive and phenylalanine and lysine are most sensitive. S-RNA's for valine, isoleucine, glycine, aspartic acid, serine, and arginine give exponential type dose-effect curves. Those for alanine, glutamic acid, lysine, and phenylalanine are rapidly inactivated at low doses and only at higher doses give exponential type inactivation curves having lower slopes than seen at low doses. Penylalanine and glutamic acid S-RNA inactivated by 2650 or 2804 Å radiations are not reactivated by 2380 Å radiation, indicating that uracil dimers are not involved in the inactivation process. Heating of irradiated S-RNA to bring about rehydration of uracil residues is not effective in restoring acceptor activity of phenylalanine and leucine S-RNA. Melting curve experiments suggest that photochemical products other than uracil diniers and hydrated uracil residues are formed and result in changes in the secondary structure of S-RNA.     

Oxygen-independent inactivation of Haemophilus influenzae transforming DNA by monochromatic radiation: action spectrum, effect of histitine and repair.

Abstract— The action spectrum for the oxygen-independent inactivation of native transforming DNA from Haemophilus influenzae with near-UV radiation revealed a shoulder beginning at 334 and extending to 460 nm. The presence of 0.2 M histidine during irradiation produced a small increase in inactivation at 254, 290 and 313 nm, a large increase at 334 nm and a decrease in inactivation at 365, 405 and 460 nm. Photoreactivation did not reverse the DNA damage produced at pH 7.0 at 334, 365, 405 and 460 nm, but did reactivate the DNA after irradiation at 254, 290 and 313 nm. The inactivation of DNA irradiated at 254, 290 and 313 nm was considerably greater when the transforming ability was assayed in an excision-defective mutant compared with the wild type, although DNA irradiated at 334, 365, 405 and 460 nm showed smaller differences. These results suggest that the oxygen-independent inactivation of H. influenzae DNA at pH 7 by irradiation at 334, 365, 405 and 460 nm is caused by lesions other than pyrimidine dimers.     

An efficient synthesis of 3-(N-piperidinemethyl)-2,2,5,5-tetramethyl-1-oxy-3-pyrroline,a promising radioprotector for cancer radiotherapy

Nitroxides can ameliorate the toxic effects of radiation during cancer therapy. Nitroxides are paramagnetic and can be used in magnetic resonance imaging (MRI) and electron paramagnetic resonance imaging (EPRI) to monitor in vivo oxidative stress status. Compound 5 (3-(N-piperidinemethyl)-2,2,5,5-tetramethyl-1-oxy-3-pyrroline) was found to be the most effective nitroxide radioprotector. An efficient synthesis for this promising radioprotector was developed.