Theab-plane optical conductivity of high-Tc superconductors

There is anisotropy in theab-plane optical properties of the high-temperature superconductors, both in the normal state and in the superconducting state. In both states, two components appear in the optical conductivity: a free carrier part and a midinfrared component. BelowT _c , the free carriers form the superconducting condensate. In YBa₂Cu₃O_7–, the anisotropy of the penetration depth shows that the chains contribute strongly to this superfluid. In Bi₂Sr₂CaCu₂O₈, where chains are absent, there is stillab plane anisotropy. BelowT _c a finite absorption parallelb remains at frequencies as small as 20 meV. This anisotropy could be due to anisotropy either of the superconducting gap or the midinfrared component.     

Evaluation of antiarrhythmic efficacy of sotalol in various doses in patients with ventricular tachyarrhythmias

Antiarrhythmic efficacy of sotalol--noncardioselective beta-adrenergic blocking agent with class III antiarrhythmic action was evaluated in 34 patients [pts] (mean age 55 +/- 11) with chronic ventricular arrhythmias and coronary artery disease, 38% with previous myocardial infarction. Two schedules of dosing were tested: 3 x 80 mg and 2 x 160 mg during 28 days of therapy. Pts with Lown class II and IV arrhythmia derived from 24-hours Holter recording were assigned. Ventricular premature complexes [VPCs] and couplets reduction by 80% and total elimination of runs defined antiarrhythmic efficacy. Proarrhythmia was defined by four times increase in VPCs, ten times increase in couplets and runs or sustained VT episodes. RESULTS: Antiarrhythmic efficacy of two doses of sotalol according to study criterion was: 31% for lower dose (3 x 80 mg) and 24% for higher dose (2 x 160 mg). Overall efficacy for both doses was 55%. According to Morganroth criterion, lower dose was effective in 29% pts and both doses, lower and higher, in 41% pts. According to other commonly used criterion: 70% VPCs reduction, 90% couplets reduction and total elimination of runs, lower dose of sotalol was effective in 32% pts and both doses in 47% pts. Significant reduction of heart rate and prolongation of QT and QTc were observed. In 3 pts QT was prolonged over 500 ms. Proarrhythmia according to Velebit criterion was suspected in one patient after one week of 3 x 80 mg teratment which caused premature cessation of therapy. No significant abnormalities in laboratory values were observed. CONCLUSIONS: Antiarrhythmic efficacy of sotalol was comparable to other studies. Its value in pts with malignant ventricular tachyarrhythmias: sustained ventricular tachycardia and ventricular fibrillation requires further studies with higher number of patients.     

Synthesis and characterization of supramolecular protein aggregates: self-assembled, molecularly-ordered, tubes from electrostatic complementation of glutamine synthetase dodecamers

Dodecameric Escherichia coli glutamine synthetase (GS) is formed from identical subunits arranged in face-to-face hexameric rings. In the presence of Zn2+ and other transition metal ions the individual dodecamers 'stack' to form protein tubes. Previous results have suggested that six binuclear intermolecular metal binding sites are generated at each dodecamer-dodecamer interface by juxtaposition of the N-terminal helices of each subunit adjacent to an analogous helix from a docked dodecamer. In principle, replacement of one of the metal binding sites within each pair of helices with charged amino acids could generate electrostatic interactions that would provide the basis for heterospecific protein-protein interactions. In turn, this would allow for ordered assembly of protein tubes with alternating, chemically distinguishable, components. This hypothesis was tested by replacement of one of the metalligating histidines (His12) with aspartic acid, arginine or cysteine. The H12C mutant was further elaborated by selective thiol modification, with either of the charged reagents 2-iodo-acetic acid or 2-chloro-acetamidine, which yield glutamate (H12C-IA) or arginine (H12C-CA) mimics at position 12. Light scattering and electron microscopy were used to monitor the 'stacking ability' of these variants in the presence of Zn2+. No, or few, GS 'tubes' were observed in solutions containing only H12D, H12R, H12C-CA or H12C-IA, in the presence or absence of Zn2+. In contrast, in mixtures containing H12C-CA and either H12D or H12C-IA, the complementary GS variants stack in the presence of 100 microM Zn2+, with apparent second order rate constants that are comparable to the wild type dodecamers. Fluorescence energy transfer experiments with fluorescein-labeled H12C-IA (donor) and rhodamine-labeled H12C-CA (acceptor) were performed and compared with the energy transfer efficiency with mixtures containing variable ratios of acceptor-labeled and donor-labeled wild type GS; the wild type mixtures provide a benchmark for the extent of energy transfer expected in random linear arrangements of donor and acceptor. The efficiency of metal-dependent energy transfer in mixtures containing the acceptor-labeled H12C-CA and the donor-labeled H12C-IA was 3.2-fold greater than expected for a random distribution of charged variants. Together, the results indicate that the charged variants provide a mechanism for heterospecific interaction between chemically distinguishable dodecamers that align in an ordered one-dimensional array.     

Synthesis and characterization of supramolecular protein aggregates: self-assembled, molecularly-ordered, tubes from electrostatic complementation of glutamine synthetase dodecamers

Dodecameric Escherichia coli glutamine synthetase (GS) is formed from identical subunits arranged in face-to-face hexameric rings. In the presence of Zn2+ and other transition metal ions the individual dodecamers 'stack' to form protein tubes. Previous results have suggested that six binuclear intermolecular metal binding sites are generated at each dodecamer-dodecamer interface by juxtaposition of the N-terminal helices of each subunit adjacent to an analogous helix from a docked dodecamer. In principle, replacement of one of the metal binding sites within each pair of helices with charged amino acids could generate electrostatic interactions that would provide the basis for heterospecific protein-protein interactions. In turn, this would allow for ordered assembly of protein tubes with alternating, chemically distinguishable, components. This hypothesis was tested by replacement of one of the metalligating histidines (His12) with aspartic acid, arginine or cysteine. The H12C mutant was further elaborated by selective thiol modification, with either of the charged reagents 2-iodo-acetic acid or 2-chloro-acetamidine, which yield glutamate (H12C-IA) or arginine (H12C-CA) mimics at position 12. Light scattering and electron microscopy were used to monitor the 'stacking ability' of these variants in the presence of Zn2+. No, or few, GS 'tubes' were observed in solutions containing only H12D, H12R, H12C-CA or H12C-IA, in the presence or absence of Zn2+. In contrast, in mixtures containing H12C-CA and either H12D or H12C-IA, the complementary GS variants stack in the presence of 100 microM Zn2+, with apparent second order rate constants that are comparable to the wild type dodecamers. Fluorescence energy transfer experiments with fluorescein-labeled H12C-IA (donor) and rhodamine-labeled H12C-CA (acceptor) were performed and compared with the energy transfer efficiency with mixtures containing variable ratios of acceptor-labeled and donor-labeled wild type GS; the wild type mixtures provide a benchmark for the extent of energy transfer expected in random linear arrangements of donor and acceptor. The efficiency of metal-dependent energy transfer in mixtures containing the acceptor-labeled H12C-CA and the donor-labeled H12C-IA was 3.2-fold greater than expected for a random distribution of charged variants. Together, the results indicate that the charged variants provide a mechanism for heterospecific interaction between chemically distinguishable dodecamers that align in an ordered one-dimensional array.