Electron-phonon superconductivity in C-doped topological nodal-line semimetal Zr5Pt3: a muon spin rotation and relaxation (μSR) study

In the present work, we demonstrate that C-doped Zr₅Pt₃is an electron-phonon superconductor (with critical temperatureT_C= 3.8 K) with a nonsymmorphic topological Dirac nodal-line semimetal state, which we report here for the first time. The superconducting properties of Zr₅Pt₃C_0.5have been investigated by means of magnetization, resistivity, specific heat, and muon spin rotation and relaxation (μSR) measurements. We find that at low temperatures, the depolarization rate is almost constant and it can be well described by a single-bands-wave model with a superconducting gap of 2Δ(0)/k_BT_C= 3.84, somewhat higher than the value of BCS theory. From the transverse field μSR analysis, we estimate the London penetration depthλ_L= 469 nm, superconducting carrier densityn_s= 1.83 × 10²⁶ m^-3, and effective massm* = 1.428m_e. The zero field μSR confirms the absence of any spontaneous magnetic field in the superconducting ground state. In order to gain additional insights into the electronic ground state of C-doped Zr₅Pt₃, we also performed first-principles calculations within the framework of density functional theory (DFT). The observed homogenous electronic character of the Fermi surface as well as the mutual decrease ofT_Cand density of states at the Fermi level are consistent with the experimental findings of this study. However, the band structure reveals the presence of robust, gapless fourfold-degenerate nodal lines protected by 6₃screw rotations and glide mirror planes. Therefore, Zr₅Pt₃represents a novel, unprecedented condensed matter system to investigate the intricate interplay between superconductivity and topology.

Investigation of superconducting gap structure in HfIrSi using muon spin relaxation/rotation

We have investigated the superconducting state of HfIrSi using magnetization, specific heat, muon spin rotation and relaxation ([Formula: see text]SR) measurements. Superconductivity was observed at [Formula: see text] K in both specific heat and magnetization measurements. From an analysis of the transverse-field [Formula: see text]SR data, it is clear that the temperature variation of superfluid density is well fitted by an isotropic Bardeen-Cooper-Schrieffer (BCS) type s-wave gap structure. The superconducting carrier density [Formula: see text] m^-3, the magnetic penetration depth, [Formula: see text] nm, and the effective mass, [Formula: see text], were calculated from the TF-[Formula: see text]SR data. Zero-field [Formula: see text]SR data for HfIrSi reveal the absence of any spontaneous magnetic moments below [Formula: see text], indicating that time-reversal symmetry (TRS) is preserved in the superconducting state of HfIrSi. Theoretical investigations suggest that the Hf and Ir atoms hybridize strongly along the c-axis, and that this is responsible for the strong three-dimensionality of this system which screens the Coulomb interaction. As a result, despite the presence of d-electrons in HfIrSi, these correlation effects are weakened, making the electron-phonon coupling more important.

Evidence of a Nodal Line in the Superconducting Gap Symmetry of Noncentrosymmetric ThCoC_{2}

The newly discovered noncentrosymmetric superconductor ThCoC_{2} exhibits numerous types of unconventional behavior in the field dependent heat capacity data. Here we present the first measurement of the gap symmetry of ThCoC_{2} by muon spin rotation and relaxation (μSR) measurements. The temperature dependence of the magnetic penetration depth measured using the transverse field μSR experiment reveals the evidence of a nodal pairing symmetry. To understand this finding, we carry out calculations of the superconducting pairing eigenvalue and eigenfunction (pairing symmetry) due to the spin-fluctuation mechanism by directly implementing the ab initio band structures. We find that the system possesses a single Fermi surface with considerable three dimensionality and a strong nesting along the k_{z} direction. Such nesting promotes a superconducting state with a cosk_{z}-like pairing symmetry with a prominent nodal line on the k_{z}=±π/2 plane. The result agrees well with the experimental data.

An Efficient Highly Regioselective Synthesis of 2,3,4-Trisubstituted Pyrroles by Cycloaddition of Polarized Ketene S,S- and N,S-Acetals with Activated Methylene Isocyanides

An efficient route for regioselective synthesis of 2,3,4- substituted pyrroles allowing precise control over the introduction of a number of substituents and functionalities (tosyl, carbalkoxy, aryl, cyano, nitro, acetyl, benzoyl, cyclic amines, etc.) at the three positions of the pyrrole ring has been developed via 1,3-dipolar cycloaddition of readily accessible polarized ketene S,S- and N,S-acetals with carbanions derived from activated methylene isocyanides.

Cigarette smoke-induced protein oxidation and proteolysis is exclusively caused by its tar phase: prevention by vitamin C

We have reported before that whole phase cigarette smoke (CS) contains stable oxidants that cause oxidative damage and increased proteolysis of proteins [Free Radic. Biol. Med. 27 (1999) 1064]. Here, we demonstrate that these oxidants are exclusively present in the tar phase of the CS and not its gas phase and can almost wholly account for the observed whole phase CS-induced oxidation of human plasma proteins as well as extensive oxidative proteolysis of guinea pig lung and heart microsomal proteins in vitro. The mechanism of the tar phase CS-induced proteolysis of microsomal proteins involves two-steps: (i) initial oxidation of the proteins by oxidants present in the tar extract followed by (ii) rapid proteolytic degradation of the oxidized proteins by proteases present in the microsomes. Like the whole phase CS, the oxidative damage of proteins caused by the tar phase CS, as evidenced by the formation of protein carbonyl and bityrosine as well as loss of tryptophan residues and thiol groups, is also almost completely prevented by ascorbic acid and only partially by glutathione. Other antioxidants, including superoxide dismutase, catalase, vitamin E, beta-carotene and mannitol are ineffective. This again leads us to suggest that adequate intake of vitamin C may help smokers to evade the CS-induced degenerative diseases associated with oxidative damage. The revelation of the acute toxicity of the tar phase with respect to CS-induced oxidative damage also urges the necessity of trapping it more effectively by suitable cigarette filters to reduce the health damage caused to smokers.

Calmodulin activates intersubunit electron transfer in the neuronal nitric-oxide synthase dimer

Neuronal nitric oxide synthase (nNOS) is composed of an oxygenase domain that binds heme, (6R)-tetrahydrobiopterin, and Arg, coupled to a reductase domain that binds FAD, FMN, and NADPH. Activity requires dimeric interaction between two oxygenase domains and calmodulin binding between the reductase and oxygenase domains, which triggers electron transfer between flavin and heme groups. We constructed four different nNOS heterodimers to determine the path of calmodulin-induced electron transfer in a nNOS dimer. A predominantly monomeric mutant of rat nNOS (G671A) and its Arg binding mutant (G671A/E592A) were used as full-length subunits, along with oxygenase domain partners that either did or did not contain the E592A mutation. The E592A mutation prevented Arg binding to the oxygenase domain in which it was present. It also prevented NO synthesis when it was located in the oxygenase domain adjacent to the full-length subunit. However, it had no effect when present in the full-length subunit (i.e. the subunit containing the reductase domain). The active heterodimer (G671A/E592A full-length subunit plus wild type oxygenase domain subunit) showed remarkable similarity with wild type homodimeric nNOS in its catalytic responses to five different forms and chimeras of calmodulin. This reveals an active involvement of calmodulin in supporting transelectron transfer between flavin and heme groups on adjacent subunits in nNOS. In summary, we propose that calmodulin functions to properly align adjacent reductase and the oxygenase domains in a nNOS dimer for electron transfer between them, leading to NO synthesis by the heme.

Vitamin C prevents cigarette smoke-induced oxidative damage in vivo

Our recent in vitro results [4] indicate that cigarette smoke induces oxidation of human plasma proteins and extensive oxidative degradation of the guinea pig lung, heart, and liver microsomal proteins, which is almost completely prevented by ascorbic acid. In this paper, we substantiate the in vitro results with in vivo observations. We demonstrate that exposure of subclinical or marginal vitamin C-deficient guinea pigs to cigarette smoke causes oxidation of plasma proteins as well as extensive oxidative degradation of the lung microsomal proteins. Cigarette smoke exposure also results in some discernible damage of the heart microsomal proteins. The oxidative damage has been manifested by SDS-PAGE, accumulation of carbonyl and bityrosine, as well as loss of tryptophan and protein thiols. Cigarette smoke exposure also induces peroxidation of microsomal lipids as evidenced by the formation of conjugated dienes, malondialdehyde, and fluorescent pigment. Cigarette smoke-induced oxidative damage of proteins and peroxidation of lipids are accompanied by marked drop in the tissue ascorbate levels. Protein damage and lipid peroxidation are also observed in cigarette smoke-exposed pair-fed guinea pigs receiving 5 mg vitamin C/animal/day. However, complete protection against protein damage and lipid peroxidation occurs when the guinea pigs are fed 15 mg vitamin C/animal/day. Also, the cigarette smoke-induced oxidative damage of proteins and lipid is reversed after discontinuation of cigarette smoke exposure accompanied by ascorbate therapy. The results, if extrapolated to humans, indicate that comparatively large doses of vitamin C may protect the smokers from cigarette smoke-induced oxidative damage and associated degenerative diseases.

Vitamin C prevents cigarette smoke induced oxidative damage of proteins and increased proteolysis

Aqueous extract of cigarette smoke (CS) contains some stable oxidants, which oxidize human plasma proteins, bovine serum albumin, amino acid homopolymers, and also cause extensive oxidative degradation of microsomal proteins. Similar observations are made when the aqueous extract of cigarette smoke is replaced by whole phase CS solution or whole phase cigarette smoke. CS-induced microsomal protein degradation is a two step process: (i) oxidation of proteins by the oxidants present in the CS and (ii) rapid proteolytic degradation of the oxidized proteins by proteases present in the microsomes. Using aqueous extract of CS equivalent to that produced from one-twentieth of a cigarette, the observed initial and postcigarette smoke treated values of different parameters of oxidative damage per milligram of microsomal proteins are respectively: 0.24 and 1.74 nmoles for carbonyl formation, 125.4 and 62.8 fluorescence units for tryptophan loss, 10.2 and 33.4 fluorescence units for bityrosine formation, and 58.3 and 12.2 nmoles for loss of protein thiols. When compared with sodium dodecyl sulphate polyacrylamide gel electrophoresis profiles of untreated microsomal proteins, the extent of microsomal protein degradation after treatment with whole phase CS solution or aqueous extract of CS is above 90%. Ascorbate (100 microM) almost completely prevents cigarette smoke-induced protein oxidation and thereby protects the microsomes from subsequent proteolytic degradation. Glutathione is partially effective, but other antioxidants including superoxide dismutase, catalase, vitamin E, probucol, beta-carotene, mannitol, thiourea, and histidine are ineffective. The gas phase cigarette smoke contains unstable reactive oxygen species such as superoxide (O2*-) and hydrogen peroxide (H2O2) that can cause substantial oxidation of pure protein like albumin but is unable to produce significant oxidative damage of microsomal proteins. Gas phase cigarette smoke-induced albumin oxidation is not only inhibited by ascorbate and glutathione but also by superoxide dismutase, catalase and mannitol. The stable oxidants in the cigarette smoke are not present in the tobacco and are apparently produced by the interaction of O2*-/H2O2/OH* of the gas phase with some components of the tar phase during/following the burning of tobacco.