Synthesis of Novel 4-Fluoro-2H-pyrazol-3-ylamines

A new and efficient synthesis for the preparation of novel 4-fluoro-2H-pyrazol-3-ylamines is described. It involves the reaction of an acyl chloride with fluoroacetonitrile and sequential ring closure of the α-fluoro-β-ketonitrile with hydrazine. Utilizing this synthetic protocol, we have synthesized a variety of 4-fluoro-2H-pyrazol-3-ylamines with different steric and electronic demands.     

TRIPLET-TRIPLET ENERGY TRANSFER IN P-TRYPSIN 1 THE ROLE OF THE INDOLE TRIPLET IN THE ULTRAVIOLET-INDUCED PHOTOLYSIS OF β-TRYPSIN

Detection of triplet-triplet energy transfer in an aqueous solution of P-trypsin is reported. This conclusion is based on the observation that a light excited phenolate side chain can sensitize the destruction of an adjacent indole side chain. The role that the indole triplet might play in the UV-induced photolysis of /l-trypsin is also investi-gated. The results suggest that the UV (309 nm)-induced inactivation of P-trypsin is not caused by indole ring destruction but by the disruption of disulfide bonds without thiol formation.     

TEMPERATURE AND VISCOSITY DEPENDENCE OF FLUORESCENCE QUENCHING BY OXYGEN IN MODEL SYSTEMS

Abstract— The bimolecular rate constant, k _q, for the quenching of the fluorescence of pyrenebutyric acid and naphthaleneacetic acid by molecular oxygen has been studied as a function of temperature and viscosity, η. Fluorescence lifetime measurements were used to monitor the degree of quenching. Glycerol-water and sucrose-water mixtures were used to increase the bulk viscosity. Plots of log k _q vs log η show deviations from Stokes-Einstein behavior, but the data are consistent with the patterns that have been observed for other diffusion limited reactions. This work provides background information, regarding the viscosity dependence of oxygen quenching reactions, which is essential for the correct interpretation of oxygen quenching/viscosity dependence studies with more complex systems (i.e. quenching of tryptophan residues in proteins).     

FLASH PHOTOLYSIS OF N-ACETYL-L-TRYPTOPHANAMIDE: THE RELATIONSHIP BETWEEN RADICAL YIELDS AND FLUORESCENCE QUENCHING

Abstract— The flash photolysis of N-acetyl-L-tryptophanamide (NATA) in the presence of the fluorescence quenchers, imidazole, acrylamide and trichloroethanol, has been investigated. Imadazole and acrylamide induce a decrease in the NATA radical yield which correlates with their NATA fluorescence quenching action. These observations suggest that the fluorescent state is primarily responsible for the monophotonic photoionization processes. The acrylamide data also suggest that 40–65% of the NATA radicals arise from a long-lived state (τ˜μs) which must originate from the fluorescent state. Unlike imidazole and acrylamide, trichloroethanol enhances the radical yield by reaction with excited state precursors. Mechanisms for the quenching of fluorescence and the long-lived states are discussed.     

ROLE OF THE TRYPTOPHAN FLUORESCENT STATE IN THE ULTRAVIOLET-INDUCED INACTIVATION OF β-TRYPSIN*

Abstract— Stern-Volmer quenching constants for β-trypsin at pH 3 were determined for fluorescence quenching by histidine, acrylamide, and nitrate ion. A modified Stern-Volmer plot (Lehrer, 1971) was employed to show that all of the fluorescent tryptophanyl residues of β-trypsin were equally susceptible to quenching by acrylamide at pH 3 when the enzyme was either in its native conformation or denatured in 6 M guanidine hydrochloride (GuHCl). Fluorescence lifetime measurements indicated that acrylamide quenched β-trypsin fluorescence by a purely collisional mechanism. Solvation of tryptophanyl residues of the protein was maximal at 2.5 M GuHCl, as monitored by fluorescence emission wavelength.
Investigations of the ultraviolet-induced inactivation of β-trypsin at 295 nm were performed in the presence of acrylamide at pH 3. The quantum yields for enzyme inactivation and indole destruction (determined using the PDAB reagent) were unchanged upon depopulation of the fluorescent state by 65 per cent, whether the enzyme was in its native conformation or denatured by 6 M GuHCl. It is concluded that the fluorescent state of tryptophanyl residues of β-trypsin is not involved in enzyme inactivation or tryptophan destruction.     

A COMPARISON OF FLUORESCENCE QUENCHING AND FLUORESCENCE DECAY STUDIES WITH TRYPTOPHAN

Abstract— The quenching of the fluorescence of aqueous tryptophan solutions has been studied as a function of emission wavelength using acrylamide as a collisional quencher. Our quenching studies are consistent with recent observations of the heterogeneity of tryptophan fluorescence, but they show a slight discrepancy when compared to certain analyses of the decay of tryptophan fluorescence in terms of two components.     

Reaching the unreachable: critical scaling at the superconducting transition

The large coherence length of conventional superconductors causes the critical region near T_c to be unobservably small, leading to the impression that mean-field theory can adequately explain properties near the transition. The high-temperature superconductors, however, have such strongly type-II character that the critical, or at least an intermediate critical, region is experimentally accessible. We demonstrate the applicability of XY-like critical scaling to magnetization, heat capacity and conductivity data and through the fitting parameters, make contact with a strong-coupling calculation of the critical temperature. Further, the scaled conductivity data show clear evidence for a thermodynamic transition along the freezing line H_m(T)∝(1 − T/T_c)^4/3.     

THE PHOTOINACTIVATION OF TRYPSIN AS SENSITIZED BY METHYLENE BLUE AND EOSIN Y*

Abstract— The kinetics of the photoinactivation of trypsin as sensitized by methylene blue and by eosin Y were investigated. The time-course of inactivation was first-order. The rates of inactivation were essentially identical as measured by three methods: the casein assay, the benzoyl-L-arginine ethyl ester assay, and the hemoglobin (in 5 M urea) assay. This is in sharp contrast to the results of earlier studies with flavins as photosensitizers, where the rate of inactivation was found to be much greater when measured by the hemoglobin assay than when measured by the other two assays. These differences are interpreted as indicating the production of a urea-sensitive 'damaged class' of trypsin molecules during photodynamic inactivation with flavins as sensitizers, but not with methylene blue and eosin Y. The results presented indicate that the mechanism of the photodynamic inactivation of enzymes might be different with different photosensitizing dyes.