Photochemically induced fluorimetric detection of tianeptine and some of its metabolites. Application to pharmaceutical preparation.

The photochemically induced fluorescence (PIF) properties of tianeptine and some of its metabolites were investigated in acidic (pH 2.3) water-alcohol mixtures at room temperature. Two PIF methods were developed, including bulk solution and flow injection analysis (FIA). Linear calibration plots were established over a concentration range of more than one order of magnitude. Limits of detection ranged from 15 ng ml-1 for FIA-PIF to 25 ng ml-1 in bulk solution. The RSDs were between 3 and 5%. The PIF methods were applied to the determination of tianeptine in a pharmaceutical preparation with recoveries varying from 96 to 106% in bulk solutions and from 98 to 106% for FIA-PIF.     

Regioselective organocadmium alkylations of substituted quinones.

A series of substituted quinones was alkylated with diethylcadmium. Regiochemistry of addition shifted from quinol formation to conjugate addition as a function of the steric and electronic effects of the substituents.     

Evidence for vacancy percolation in highly-doped silicon

The extreme dopant diffusivities observed in silicon for donor concentrations in excess of 2×10²⁰cm^–3 have recently been explained by a vacancy percolation model proposing that a network of fast diffusion paths is formed when the average distance of the donors becomes equal to or less than the fifth nearest-neighbour distance. Here, we report on evidence by means of¹¹⁹Sn Mössbauer spectroscopy for the postulated high vacancy concentration in the percolation cluster.     

Origin of the conductivity minimum and the negative magnetoresistance in n-type sulpho-spinels

Electrical conductivity and magnetoresistance in Ga-doped and undoped sulpho-spinels have been studied. The influence of annealing treatments on the electrical properties is discussed and results are given. It is shown, that the conductivity minimum and the negative magnetoresistance found in Ga-doped samples at temperatures near T_c, are due to impurity conductivity of the magnetic impurity state type.     

High-temperature electrical properties of yttrium iron garnet under varying oxygen pressures

The d.c. conductivity and Seebeck coefficient of Ca and Mg doped YIG have been measured at 1200–1700 K. A change of p-type to n-type conductivity was accomplished by varying the oxygen pressure between 1 and 10^-8 atm. From the position of the conductivity minimum as a function of temperature the band gap energy is estimated to be 2.85 eV.     

A charge-scaling method to treat solvent in QM/MM simulations

 We present a method to treat the solvent efficiently in hybrid quantum mechanical/molecular mechanical simulations of chemical reactions in enzymes. The method is an adaptation of an approach developed for molecular-mechanical free-energy simulations. The charges of each of the exposed ionizable groups are scaled, and the system is simulated in the presence of a limited number of explicit solvent molecules to obtain a reasonable set of structures. Continuum electrostatics methods are then used to correct the energies. Variations in the procedure are discussed with an emphasis on modifications from the original protocol. We illustrate the method by applying it to the study of a hydrolysis reaction in a highly charged system comprising a complex between the base excision repair enzyme uracil-DNA glycosylase and double-stranded DNA. The resulting adiabatic reaction profile is in good agreement with experiment, in contrast to that obtained without scaling the charges. Received: 5 October 2001 / Accepted: 6 September 2002 / Published online: 28 February 2003 Contribution to the Proceedings of the Symposium on Combined QM/MM Methods at the 222nd National Meeting of the American Chemical Society, 2001 Correspondence to: M. Karplus e-mail: marci@tammy.harvard.edu