Magnetism of coupled spin tetrahedra in ilinskite-type KCu5_{5}O2_2(SeO3_3)2_2Cl3_3

Synthesis, thermodynamic properties, and microscopic magnetic model of ilinskite-type KCu$_{5}$O$_2$(SeO$_3$)$_2$Cl$_3$ built by corner-sharing Cu$_4$ tetrahedra are reported, and relevant magnetostructural correlations are discussed. Quasi-one-dimensional magnetic behavior with the short-range order around 50\,K and the absence of long-range order down to at least 2\,K is observed experimentally and explained in terms of weakly coupled spin ladders (tubes) with a complex topology formed upon fragmentation of the tetrahedral network. This fragmentation is rooted in the non-trivial effect of the SeO$_3$ groups that render the Cu--O--Cu superexchange strongly ferromagnetic.Comment: 9 pages, 7 figure

Phase relations and crystal structures in the systems (Bi,Ln)2WO6 and (Bi,Ln)2MoO6 (Ln=lanthanide)

Several outstanding aspects of phase behaviour in the systems (Bi,Ln)(2)WO6 and (Bi,Ln)(2)MoO6 (Ln = lanthanide) have been clarified. Detailed crystal structures, from Rietveld refinement of powder neutron diffraction data, are provided for Bi1.8La0.2WO6 (L-Bi2WO6 type) and BiLaWO6, BiNdWO6, Bi0.7Yb1.3WO6 and Bi0.7Yb1.3WO6 (all H-Bi2WO6 type). Phase evolution within the solid solution Bi2-xLaxMoO6 has been re-examined, and a crossover from gamma(H)-Bi2MoO6 type to gamma-R2MoO6 type is observed at x similar to 1.2. A preliminary X-ray Rietveld refinement of the line phase BiNdMoO6 has confirmed the alpha-R2MoO6 type structure, with a possible partial ordering of Bi/Nd over the three crystallographically distinct R sites. (c) 2006 Elsevier Inc. All rights reserved.</p

Dissecting structural basis of the unique substrate selectivity of human enteropeptidase catalytic subunit

<div><p>Enteropeptidase is a key enzyme in the digestion system of higher animals. It initiates enzymatic cascade cleaving trypsinogen activation peptide after a unique sequence DDDDK. Recently, we have found specific activity of human enteropeptidase catalytic subunit (L-HEP) being significantly higher than that of its bovine ortholog (L-BEP). Moreover, we have discovered that L-HEP hydrolyzed several nonspecific peptidic substrates. In this work, we aimed to further characterize species-specific enteropeptidase activities and to reveal their structural basis. First, we compared hydrolysis of peptides and proteins lacking DDDDK sequence by L-HEP and L-BEP. In each case human enzyme was more efficient, with the highest hydrolysis rate observed for substrates with a large hydrophobic residue in P2-position. Computer modeling suggested enzyme exosite residues 96 (Arg in L-HEP, Lys in L-BEP) and 219 (Lys in L-HEP, Gln in L-BEP) to be responsible for these differences in enteropeptidase catalytic activity. Indeed, human-to-bovine mutations Arg96Lys, Lys219Gln shifted catalytic properties of L-HEP toward those of L-BEP. This effect was amplified in case of the double mutation Arg96Lys/Lys219Gln, but still did not cover the full difference in catalytic activities of human and bovine enzymes. To find a missing link, we studied monopeptide benzyl-arginine-β-naphthylamide hydrolysis. L-HEP catalyzed it with an order lower <i>K</i>_m than L-BEP, suggesting the monopeptide-binding S1 site input into catalytic distinction between two enteropeptidase species. Together, our findings suggest structural basis of the unique catalytic properties of human enteropeptidase and instigate further studies of its tentative physiological and pathological roles.</p> </div