We have performed measurements of the microwave surface impedance Zg --Rs + iXg of the organic superconductor r-(BEDT-TTF)2Cu(NCS)2 down to T = 0.8 K. The reactance Xs, and therefore the penetration depth A, exhibits no temperature dependence for T « T,. This result is in full agreement with calculations of the BCS ground state and gives no indication of unconventional pairing. Furthermore, we have evaluated the complex conductivity o = o. z + io. 2 and find a pronounced peak in oq below T which can be identi6ed as a coherence peak.The v phase of the two-dimensional organic conductor Di-(bisethylenedithio-tetrathiafulvalene) dithiocyanocuprate [i.e., (BEDT-TTF)2Cu(NCS)2] is the most prominent representative of the more than twenty superconducting salts of the BEDT-TTF family. Even five years after its discovery, the question of whether the superconducting state is of the conventional BCS type remains controversial. Various experiments examining the superconducting state of the BEDT-TTF materials indicate deviations from what is expected for singlet pairing. The critical field shows an unexpected behavior at low temperatures and the H nuclear spin-lattice relaxation rate was found to have an anomalous temperature dependence. Some measurements of the specific heat exhibit a strange T law, but others find the specific heat in agreement with the BCS predictions. These experiments have been interpreted in terms of triplet pairing, spin density wave transitions below T, and vortex glass transitions as expected for a ground state with higher momentum pairing. While both NMR and specific heat measurements are sensitive also to quasiparticle excitations, the parameters which characterize the electrodynamics, the penetration depth A, and surface resistance Bg, are free from such complications. Consequently, the magnitude and temperature dependence of these parameters may, in principle, distinguish between the various possible symmetries of superconducting states. Low frequency magnetization measurements with H, parallel to the layers led to an unusually large penetration depth along the (bc) plane, and a T dependence suggesting higher momentum pairing. Studies of the reversible magnatization, however, show evidence of conventional Cooper pairing. Prom the scaling behavior of the radio-frequency penetration depth, Sridhar et al. recently argued for a nonconventional Meissner state where the penetration depth cannot be defined in the common way. Direct experiments of the penetration depth employing muon spin rotation (@SR) techniques are also highly controversial, one suggests 8-wave pairing, the other shows important deviations from the BCS behavior below 1.5 K. A similar situation has arisen for meai/2 & 2crac )(2) surements of the microwave surface impedance. The surface reactance Xg, which is proportional to A, was found to be temperature independent for T « T in both configurations parallel and perpendicular to the layers.Recently, however, microwave surface impedance experiments were reported, which show a slight tempe...

. Can. J. Chem. 66,2703Chem. 66, (1988. Scheraga's ECEPP program has been used to determine the relationship between one-point energies and the energies minimized by a quadratic procedure, for different sized (+, +, x ,) dihedral angle grid searches of simple peptides. Based on these trials, a new subroutine, INIT, has been written and incorporated into the program. This subroutine calculates the one-point ECEPP energies of up to 200,000 random permutations of (4, +) = 0, + 90, 180 and (X ,) = -60, 180 with all w = X,] = 180; after each 40,000 permutations, the structures having the 10 lowest energies are ordered and selected for minimization. The ECEPP program has been modified further to provide an MMP2(85) input file as part of its normal output. This allows structures located by INIT to be minimized automatically, and then refined by the Newton-Raphson minimization and MMPEP parameters of MMP2(85).During the development of this protocol, it has been found that MMPEP correctly reproduces the C7 conformational preference of the alanyl dipeptide in nonpolar media, and also the experimentally observed shift to a~ and PII conformations in polar solvents, when the dielectric constant E = 78.5 D. This dielectric constant has, therefore, been selected for a conformational analysis, using INITIMMPEP, of the peptides Gly-Trp-Met-Asp-Phe-NH2 (GLMAP), Gly-D-Trp-Met-Asp-Phe-NH2 (GDMAP), and Gly-Gly-Met-Asp-Phe-NH2 (GGMAP); GLMAP is CCK-5, a cholecystokinin fragment possessing anticonvulsant activity, which is found in the brain. The analogs GDMAP and GGMAP are not active, or much less so than GLMAP. One strongly preferred structure has been found, in each case, for GLMAP (GLMAP3) and GDMAP (GDMAP39). These structures are very similar, in the nature of the edge-to-face stacking of their Trp and Phe aromatic rings, but the C-terminal regions differ. The conformation of the C-terminal tetrapeptide of GLMAP3 is identical to a previously calculated structure of CCK-4. A comparison of the C-terminal regions of GLMAP3 and GDMAP39 with the structure of 5,5-diphenylhydantoin suggests that the mechanisms of anticonvulsant action of phenytoin and CCK-5 are not the same. In the absence of an aromatic-aromatic interaction, no clear-cut conformational preference is found for GGMAP. The structure of GGMAP whose C-terminal region is the same as that of GLMAP3 (GGMAP20) is 1.51 kcal/mol higher in energy than the lowest energy structure, and lacks the hydrophobic wall provided by the Trp residue of GLMAP3. Au cours du dtveloppement de ce protocole, on a trouvt que MMPEP reproduit correctement la preference conformationnelle du C7 du dipeptide de l'alanyle dans des milieux non-polaues ainsi que le dtplacement observt exptrimentalement vers les conformations c i~ et PI1 dans des solvants polaires, lorsque E = 78,5 D. On a donc choisi cette constante ditlectrique pour une analyse conformationnelle, utilisant le programme INITIMMPEP, des peptides Gly-Trp-Met-Asp-Phe-NH2 (GLMAP), Gly-D-Trp-Met-Asp-Phe-NH2 (GDMAP) et Gly-Gly-Met-Asp-Phe-NH2 (...

No abstract

We have performed detailed measurements of the surface impedance in the normal a n d superconducting s t a t e of K-(BEDT-"TF)2Cu(NCS)z i n t h e millimeter wave frequency range (1 cm" to 3 cm-I), and have evaluated the complex conductivity for different crystallographic orientations. Above the transition temperature, t h e material behaves like a metal with a scattering rate of approximately 30 cm-'. In t h e superconducting s t a t e the electrodynamics of both materials is in good agreement with cdculations based on a BCS ground state: the penetration d e p t h is temperature independent for T-+ 0; while the penetration depth and the coherence length are strongly anisotrop, t h e superconducting energy g a p shows no indications of line nodes.

We have performed detailed measurements of the surface impedance in the normal a n d superconducting s t a t e of K-(BEDT-"TF)2Cu(NCS)z i n t h e millimeter wave frequency range (1 cm" to 3 cm-I), and have evaluated the complex conductivity for different crystallographic orientations. Above the transition temperature, t h e material behaves like a metal with a scattering rate of approximately 30 cm-'. In t h e superconducting s t a t e the electrodynamics of both materials is in good agreement with cdculations based on a BCS ground state: the penetration d e p t h is temperature independent for T-+ 0; while the penetration depth and the coherence length are strongly anisotrop, t h e superconducting energy g a p shows no indications of line nodes.