Structural studies and T c dependence in La2−x Dy x Ca y Ba2Cu4+y O z type mixed oxide superconductors

A new series of mixed oxide superconductors with the stoichiometric composition La_2−x Dy _x Ca _y Ba₂Cu_4+y O _z (x=0.0 − 0.5, y=2x) has been studied for structural and superconductiong properties. Our earlier studies on La_2−x (Y/Er) _x Ca _y Ba₂Cu_4+y O _z series, show a strong dependence of T _c on hole concentration (p _sh). In the present work, the results of the analysis of the neutron diffraction measurements at room temprerature on x=0.3 and 0.5 samples are reported. It is interesting to know that Ca substitutes for both La and Ba site with concomitant displacement of La onto Ba site. Superconductivity studies show that maximum T _c is obtained for x=0.5, y=1.0 sample (T _c ∼ 75 K), for La_1.5Dy_0.5Ca₁Ba₂Cu₅O _z (La-2125).     

Enzymatic deamination of the epigenetic base N-6-methyladenine

Two enzymes of unknown function from the amidohydrolase superfamily were discovered to catalyze the deamination of N-6-methyladenine to hypoxanthine and methyl amine. The methylation of adenine in bacterial DNA is a common modification for the protection of host DNA against restriction endonucleases. The enzyme from Bacillus halodurans, Bh0637, catalyzes the deamination of N-6-methyladenine with a k(cat) of 185 s(-1) and a k(cat)/K(m) of 2.5 × 10(6) M(-1) s(-1). Bh0637 catalyzes the deamination of N-6-methyladenine 2 orders of magnitude faster than adenine. A comparative model of Bh0637 was computed using the three-dimensional structure of Atu4426 (PDB code: 3NQB) as a structural template and computational docking was used to rationalize the preferential utilization of N-6-methyladenine over adenine. This is the first identification of an N-6-methyladenine deaminase (6-MAD).     

Transmission Raman spectroscopy for quality control in model cocrystal tablets

Transmission Raman spectroscopy is employed for the first time to analyse model formulations comprising tabletted cocrystals. The ability of transmission Raman to differentiate between formulations on the basis of both drug loading and drug chemistry (cocrystal versus separate components) is investigated.     

Magnetism and structural chemistry of the n = 1 Ruddlesden-Popper phases La4LiMnO8 and La3SrLiMnO8.

Polycrystalline samples of La4LiMnO8 and La3SrLiMnO8 have been studied by a combination of X-ray diffraction (XRD), neutron diffraction (ND), 6Li MAS NMR, electron microscopy (EM), and magnetometry. Room-temperature XRD and ND measurements suggest that both compounds have the K2NiF4 structure, with a disordered arrangement of Li and Mn over the six-coordinate sites. However, MAS NMR and EM demonstrate the presence of local 1:1 Li:Mn order on these sites, and EM shows that although cation order is well-developed in each xy sheet of corner-sharing octahedra, the sheets are stacked randomly along z. The structures are best described as paracrystalline, and many of the concepts of conventional crystallography are inapplicable. Magnetometry and low-temperature ND experiments show that, despite their paracrystallinity, the two compounds are ordered antiferromagnetically with susceptibility maxima at 26 and 18 K, respectively, and with ordered magnetic moments of 3.61(6) and 2.3(1) muB per Mn cation at 2 K. Anisotropic peak broadening reveals a 2D character in the magnetic behavior of both compounds, and La3SrLiMnO8 is well-modeled as a quadratic layer S = 3/2 Heisenberg antiferromagnet.     

A modular cross-linking approach for exploring protein interactions

A method is described for the elucidation of protein-protein interactions using novel cross-linking reagents and mass spectrometry. The method incorporates (1) a modular solid-phase synthetic strategy for generating the cross-linking reagents, (2) enrichment and digestion of cross-linked proteins using microconcentrators, (3) mass spectrometric analysis of cross-linked peptides, and (4) comprehensive computational analysis of the cross-linking data. This integrated approach has been applied to the study of cross-linking between the components of the heterodimeric protein complex negative cofactor 2.     

A possible mechanism to explain the synergistic effects exhibited by mixtures of alkyltin mercaptoesters as stabilisers for PVC

The phenomenon of synergism observed with mixtures of mono- and dialkyltin stabilisers for PVC has been recognised and exploited for many years, but remains largely unexplained. Whilst a similar effect in mixed metal (Ba/Cd or Ca/Zn) carboxylate stabilisers has been explained, mechanistic studies of the organotin systems have not appeared. Infra-red spectroscopy has now been used to study the Cl/mercaptoester exchange equilibria of alkyltin mercaptoester (isooctylthioglycollate) chlorine compounds which are undoubtedly produced when the organotin mercaptoesters act as PVC stabilisers. The exchange process is shown to be facile, even at ambient temperatures, and a regenerative mechanism, based on these observations, is proposed to explain the synergistic effect.     

Synthesis and characterization of mono- and bis-methano[60]fullerenyl amino acid derivatives and their reductive ring-opening retro-bingel reactions

The addition of N-(diphenylmethylene)glycinate esters (Ph2C=NCH2CO2R) 3-6 to [60]fullerene under Bingel conditions gives, respectively, the methano[60]fullerenyl iminoesters 7-10. Upon treatment of 7-9 with sodium cyanoborohydride, in the presence of a protic or a Lewis acid, a novel reductive ring-opening reaction occurred to give the corresponding 1,2-dihydro[60]fullerenyl glycine derivatives 11-13. Using tethered bis-N-(diphenylmethylene)glycinate esters 33 and 34derived from m- and p-benzenedimethanol scaffolds, the corresponding bis-methano[60]fullerenyl iminoesters 35-38 were synthesized under double Bingel reaction conditions. The m-benzenedimethanol derivative 33 gave the trans-4 (35) and cis-3 (36) regioisomeric bisadducts in a ratio of 80:20. The analogous para-tethered derivative 34 afforded the trans-3 (37) and trans-4 (38) regioisomers in a 80:20 ratio. The regiochemistry of the major bisadducts 35 and 37 (via the trans-esterified 39) were unequivocally determined using 2D INADEQUATE and C-C TOCSY NMR experiments. The regiochemistry of these bis-additions were unexpected on the basis of literature precedents. These results unequivocally show that the regiochemistry of tethered bis-additions is not solely dependent on the nature of the tether. A mixture of the trans-4 and cis-3 nonsymmetrical bisadducts 45 and 46 was obtained from the double-Bingel cyclopropanation of a bis-N-(diphenylmethylene)glycinate tether based on a 1,3-naphthyldimethanol scaffold. The regiochemistry of these compounds (45 and 46) was identified by correlation with the diethyl esters 40 and 47, prepared by trans-esterification of 35/45 and 36/46, respectively. The INADEQUATE and molecular modeling experiments allowed topological mapping of the fullerene surfaces of the bis-methano[60]fullerenes 38 and 42. Reductive ring-opening reactions on the tethered bis-methano[60]fullerenes 35-37, 45, and 46 gave none of the expected bis-fullerenylglycinates rather the reductive ring-opening-retro-Bingel products, the 1,2-dihydro[60]fullerenylglycinates 48, 49, 52, and 53. These compounds resulted from the reductive ring-opening of one methanoimino ester moiety and a retro-Bingel reaction of the other. Under analogous reductive ring-opening-retro-Bingel conditions, the nontethered bis-methano[60]fullerene 40 afforded the 1,2-dihydro[60]fullerenylglycinate 12. Thus, it was concluded that the tether was not the driving force for the reductive elimination of one of the methano groups.