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M Fardis

Publications and source records attributed to M Fardis.

11 recordsLinked to original sources
PubMed2006

Low temperature charge and orbital textures in La0.875Sr0.125MnO3.

By using nuclear magnetic resonance techniques we show that for T<30 K the La0.875Sr0.125MnO3 compound displays a nonuniform charge distribution, comprised of two interconnected Mn ion subsystems with different spin, orbital, and charge couplings. The NMR results agree very well with the two spin wave stiffness constants observed at small q values in the spin wave dispersion curves [Phys. Rev. B 67, 214430 (2003)]. This picture is probably related to a yet undetermined charge and orbital superstructure occurring in the ferromagnetic insulating state of the La0.875Sr0.125MnO3 compound.

Journal Article↗
PubMed2005

Morphology, ionic diffusion and applicability of novel polymer gel electrolytes with LiI/I2.

Novel polymer gel electrolytes have been prepared by incorporating LiI-I(2) solutions into a polyethylene oxide matrix supported by a TiO(2) filler. The gel electrolytes, based on either acetonitrile or propylene carbonate solvents are compared with liquid standard ones and are examined by (7)Li solid state nuclear magnetic resonance relaxometry and diffusion measurements. In parallel, the triiodide apparent diffusion coefficient has been determined by linear sweep voltammetry. The results are correlated with atomic force microscopic images of the electrolytes and give insight of the dynamic properties of the ions in the constrained polymer medium. Furthermore, the dissociation of the ions is estimated by relating the ionic conductivity to the ionic diffusion. As a prime application, the polymer gel electrolytes were incorporated in dye sensitized solar cells and the measured energy conversion efficiencies were successfully correlated with their morphological, diffusive and conducting properties.

Chemical Phenomena↗
PubMed2003

Orbital domain state and finite size scaling in ferromagnetic insulating manganites.

55Mn and 139La NMR measurements on a high quality single crystal of ferromagnetic (FM) La0.80Ca0.20MnO3 demonstrate the formation of localized Mn(3+,4+) states below 70 K, accompanied by a strong cooling-rate dependent increase of certain FM neutron Bragg peaks. (55,139)(1/T(1)) spin-lattice and (139)(1/T(2)) spin-spin relaxation rates are strongly enhanced on approaching this temperature from below, signaling a genuine phase transition at T(tr) approximately 70 K. The disappearance of the FM metallic signal by applying a weak external magnetic field, the different NMR radio-frequency enhancement of the FM metallic and insulating states, and the observed finite size scaling of T(tr) with Ca (hole) doping, as observed in powder La(1-x)CaxMnO3 samples, are suggestive of freezing into an inhomogeneous FM insulating and orbitally ordered state embodying "metallic" hole-rich walls.

Journal Article↗
PubMed2002

Genetically engineered analogs of ascomycin for nerve regeneration.

The polyketides FK506 (tacrolimus) and FK520 (ascomycin) are potent immunosuppressants that function by inhibiting calcineurin phosphatase through formation of an FKBP12-FK506/520-calcineurin ternary complex. They also have calcineurin-independent neuroregenerative properties in cell culture and animal models of nervous system disorders. Based on the crystal structure of the FKBP12-FK506-calcineurin complex, we deduced that the 13- and 15-methoxy groups of FK506 or FK520 are important for inhibition of calcineurin phosphatase but not for binding to FKBP12. By genetic modification of the FK520 gene cluster, we generated 13- and 15-desmethoxy analogs of FK520 that contain hydrogen, methyl, or ethyl instead of methoxy at one or both of these positions. These analogs bind FKBP12 tightly, have decreased calcineurin phosphatase inhibition and immunosuppressive properties, and enhance neurite outgrowth in cell cultures. A representative compound was also shown to accelerate nerve regeneration and functional recovery in the rat sciatic nerve crush model.

Acetyltransferases↗
PubMed2001

(139)La NMR investigation of quasistatic orbital ordering in La(1-x)Ca(x)MnO(3).

We report (139)La nuclear magnetic resonance in ferromagnetic and insulating (FMI) La(1-x)Ca(x)MnO(3), 0.10< or =x< or =0.20, which at low temperatures shows the formation of Mn octants with enhanced Mn-O wave function overlapping and electron-spin alignment. The rapid increase of the relaxation rates and the "wipeout" of the (139)La NMR signal intensity on heating, imply a quasistatic character for the Mn octant cells in the FMI phase, which freeze below a transition temperature T(f).

Journal Article↗
PubMed2001

Synthesis of 14,15-dehydroerythromycin A ketolides: effects of the 13-substituent on erythromycin tautomerism.

A ketolide was prepared from 14,15-dehydroerythromycin A by two different routes. The first approach involving oxidation of the 3-OH of 3-descladinosyl-14,15-dehydroerythromycin A 2'-O-acetate gave unexpectedly high levels of 3,11-double oxidation. This may be due to greater formation of the 9,12-hemiketal in 14,15-dehydroerythromycin A and concomitant exposure of the 11-OH group for oxidation. NMR studies of 14,15-dehydroerythromycin A support this hypothesis, revealing a 9:1 ratio of 9-ketone to 9,12-hemiketal in CDCl3 and a 1:1 ratio in CD3OD as contrasted with the corresponding tautomer ratios of 30:1 in CDCl3, and 6: 1 in CD3OD with erythromycin A. Alteration of the 13-substituent on the erythronolide A ring from ethyl to vinyl thus favors formation of the 9,12-hemiketal. A second route to the ketolides was developed based on these findings, in which the 11-OH is eliminated prior to oxidation of the 3-OH.

Anti-Bacterial Agents↗
PubMed2000

55Mn NMR investigation of electronic phase separation in La1-xCaxMnO3 for 0.2 < or = x < or = 0.5.

55Mn NMR line shape measurements in La1-xCaxMnO3 for 0.20< or =x< or =0.50 provide experimental evidence about the existence of two distinct regions in the T-x magnetic phase diagram, where the homogeneous ferromagnetic (FM) metallic state is separated into FM metallic and FM insulating regions. These results are in agreement with recent theoretical predictions, which reveal a novel electronic phase separation in two FM states, providing orbital ordering and Jahn-Teller phonons are taken into consideration.

Journal Article↗
PubMed1997

Synthetic strategies in combinatorial chemistry.

Different synthetic strategies are required for the two kinds of libraries being developed for combinatoral chemistry. Preparation of a 'focused' library involves conversion of a known synthetic route to the solid-phase or automated format, whereas generation of a 'prospecting' library is driven more by availability of input materials and considerations of structural novelty. While both types can be identified over the history of combinatorial chemistry, elaborating syntheses of novel, nonoligomeric, 'drug-like' molecules for use in prospecting libraries is a relatively recent development.

Chemistry, Organic↗
PubMedDate not supplied

NMR in porous materials.

Nuclear magnetic resonance (NMR) was used to determine the pore size distribution of hardened Portland cement pastes. The method is based on the well-known freezing point depression of water when confined inside the pore matrix of a material. It is demonstrated how this technique can be applied in cementitious materials to probe the microstructure of the main hydration product: the cement gel.

Cementation↗