PHOTOTAUTOMERISM OF LUMICHROME IN METHANOL-ACETIC ACID MIXTURES. A STEADY-STATE AND TIME-RESOLVED FLUORESCENCE STUDY

Abstract— Fluorescence lifetimes of 3-methyllumichrome dual fluorescence at 440 nm (alloxazinic) and 520 nm (isoalloxazinic) and of 1,3-dimethyllumichrome, which is unable to phototautomerize, at the same wavelengths have been measured in methanol-acetic acid mixtures. The fluorescence decays of both lumichromes studied are exponential and the phototautomeric fluorescence of 3-methyllumichrome is created within approximately 50 ps. From the initial values of about 0.9 ns (alloxazinic) and 6.4 ns (isoalloxazinic) in 5% acetic acid the lifetimes shorten considerably with increasing acid concentration reaching 0.2 ns for 1,3-dimethyllumichrome and 80 ps for the alloxazinic and 2.4 ns for the isoalloxazinic form of 3-methyllumichrome in pure acid. Static and dynamic fluorescence quenching constants were estimated. The differences in the quenching rate and equilibrium constants of both lumichromes are interpreted in terms of different equilibria of hydrogen bond formation in ground and excited state as influenced by steric effects of the methyl substituent at the N-l position in 1,3-dimethyl. The hydrogen bonding atN–10 of 3-methyllumichrome with acetic acid is a prerequisite for additional hydrogen bonding at N-l enabling excited state proton transfer.     

Synthese und Viskositiitsverhalten von Poly(γ-p-biphenyl-methyl-L-glutamat) in Benzol/Dichloressigsäure-Mischungen; ein Vergleich mit Poly(ybenzyl-L-glutamat)

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Two-dimensional modeling of microscale transport and biotransformation in porous media

We develop a model for simulating the growth of a biofilm in a tortuous tube. The solutions to the Navier-Stokes equations and the advection-diffusion equation are calculated numerically using finite differences. These solutions are then coupled with a biofilm growth model. © 1994 John Wiley & Sons, Inc.     

Analysis of the complex permeability of monocrystalline MnZnFeII ferrite

A method of analysis is discussed by means of which the domain wall susceptibility and rotational susceptibility of monocrystalline MnZn ferrite can be distinguished. It is based upon a measurement of the frequency dependence of the disaccommodation occurring in the ferrite. From this analysis it appears that below 1 MHz the domain wall motion is the dominating magnetization process in MnZn ferrite, whereas above 3 MHz the rotational susceptibility prevails. Between 1 and 3 MHz, μ′ and μ″ are determined mainly by the rotational- and wall-susceptibility, respectively. Hence, when applied in magnetic video heads, the ferrite may generate electrical noise as a result of the dissipation accompanying the wall motion.     

Synthesis of a teichoic acid fragment of bacillus var.niger w.m. by a phosphotriester approach

Three properly-protected derivatives, one non-terminal and two terminal units, of 3-0-(2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyl)-sn-glycerol have been joined together by two interglyceridic (2→1) phosphotriester linkages to afford, after removal of all protective groups, a teichoic acid fragment.     

Transition probabilities in the Ni-Zn region

Lifetimes of low-lying nuclear levels of^61,63Ni,^60,62Cu,^63,65,67Zn and^65,66Ga have been measured by means of the delayed coincidence technique. A comparison of the results with available theoretical predictions is given. Properties of low-lying levels of odd Ni- and odd Zn-isotopes are compared.     

Atomic XAFS as a tool to probe the electronic properties of supported noble metal nanoclusters

Atomic XAFS is a very attractive technique for probing electronic properties of supported metal nanoclusters. For platinum nanoparticles on different supports, the technique is found to be in good agreement with infrared CO adsorption measurements. The advantages of AXAFS, however, are that no probe molecule is required and that real-time measurements under reaction conditions are possible.