High Thermal Parameters Research Articles

X-ray analyses of aspartic proteinases: The three-dimensional structure at 2.1 Å resolution of endothiapepsin

The molecular structure of endothiapepsin (EC 3.4.23.6), the aspartic proteinase from Endothia parasitica, has been refined to a crystallographic R-factor of 0.178 at 2.1 Å resolution. The positions of 2389 protein non-hydrogen atoms have been determined and the present model contains 333 solvent molecules. The structure is bilobal, consisting of two predominantly β-sheet domains that are related by an approximate 2-fold axis. Of approximately 170 residues, 65 are topologically equivalent when one lobe is superimposed on the other. Twenty β-strands are arranged as five β-sheets and are connected by regions involving 29 turns and four helices. A central sheet involves three antiparallel strands from each lobe organized around the dyad axis. Each lobe contains a further local dyad that passes through two sheets arranged as a sandwich and relates two equivalent motifs of four antiparallel strands (a,b,c,d) followed by a helix or an irregular helical region. Sheets 1N and 1C, each contain two interpenetrating ψ structures contributed by strands c,d,d′ and c′,d′,d, which are related by the intralobe dyad. A further sheet, 2N or 2C, is formed from two extended β-hairpins from strands b,c and b′,c′ that fold above the sheets 1N and 1C, respectively, and are hydrogen-bonded around the local intralobe dyad. Asp32 and Asp215 are related by the interlobe dyad and form an intricate hydrogen-bonded network with the neighbouring residues and comprise the most symmetrical part of the structure. The side-chains of the active site aspartate residues are held coplanar and the nearby main chain makes a “fireman's grip” hydrogen-bonding network. Residues 74 to 83 from strands a′N and b′N in the N-terminal lobe form a β-hairpin loop with high thermal parameters. This “flap” projects over the active site cleft and shields the active site from the solvent region. Shells of water molecules are found on the surface of the protein molecule and large solvent channels are observed within the crystal. There are only three regions of intermolecular contacts and the crystal packing is stabilized by many solvent molecules forming a network of hydrogen bonds. The three-dimensional structure of endothiapepsin is found to be similar to two other fungal aspartic proteinases, penicillopepsin and rhizopuspepsin. Even though sequence identities of endothiapepsin with rhizopuspepsin and penicillopepsin are only 41% and 51%, respectively, a superposition of the three-dimensional structures of these three enzymes shows that 237 residues (72%) are within a root-mean-square distance of 1.0 Å.

Multiply-bonded dimetal fluoroalkoxides—II. The disordered structure of Mo 2(OC(CF 3) 3) 4(NMe 2) 2 and its implications as to rearrangement by an internal flip

The existence of the title compound, which was tentatively reported in 1977, has been confirmed and its structure studied crystallographically. It crystallizes in space group P2 1/ n with a = 10.571(3), b = 14.927(3), c = 12.099(3) Å, β = 103.21(3)°, V = 1859(2) Å 3, Z = 2. The centrosymmetric molecules have an anti, staggered rotational conformation, with the C 2NMoMoNC 2 group of atoms being essentially planar. There is a disorder such that 69% of the Mo 2 units lie in one direction and 31% in a nearly perpendicular direction. This is presumed to indicate that there are entire molecules that adopt these two different orientations. Although no more than one of each ligand atom could be discerned, all of them, especially those in the (CF 3) 3C groups refined with unusually high thermal displacement parameters. The two MoMo distances, 2.216(2) Å (69%) and 2.201(3) Å (31%), while different in a statistical sense (0.015(4) Å) are nearly equal and of the appropriate magnitude for a triple bond between the molybdenum atoms. The observed disorder leads to the concept that the Mo 2 unit might undergo internal flips from one orientation to the other within a ligand cage that need relax only slightly from one orientation to the other. This would provide a simple unimolecular mechanism for interchanging proximal and distal R groups on NR 2 ligands in this and related molecules.

Crystal and molecular structure of 15-oxosparteine-N(1)-oxide perchlorate

[C15H25N2O2 +·ClO 4 − ,M r =364.83, is monoclinic:P21,a=7.885(1),b=10.685(1),c=10.658(1) A,β=105.63(1)°,V c =864.8(2) A3,Z=2,D x =1.39(1)g cm−3, λ(CuKα)=1.54178 A,μ(CuKα)=21.4 cm−1,F(000)=388 e,T=292K,R=0.066 for 1205 unique reflections. The piperidine ringsA, B, C, andD have chair, chair, boat, and half-chair conformations, respectively. The quinolizidine system (ringsA/B) has atrans configuration. The cations are interconnected by a hydrogen bond O(N1)⋯O(C15) of 2.552(10) A into chains along [010]. Atoms C(12) and C(13) from ringD of the cation are disordered. Very high thermal parameters of the oxygen atoms of the perchlorate anion may indicate its orientational disorder.

Metal ions doped into caesium cadmium trinitrite: Spectroscopic, crystallographic and thermal decomposition studies

CsCd(NO 2)3 has a disordered cubic structure ( a 0 = 5.466 A ̊ , space group Pm3) rather than the face-centred cubic A 2A′M(NO 2) 6 structure with a 0 = 11 A ̊ . The cadmium atom has a very high thermal parameter ( U = 0.099 A ̊ 3 ) which is consistent with static disordered displacement of the atom from the equilibrium position. R = 0.058 for 100 observed reflections. CsCd(NO 2) 3 acts as host lattice for many metal ions including Mn(II) and Cr(III) which do not form hexanitrite complexes. The compositions and spectra of these doped complexes suggest local ordering of the NO 2 groups. Unlike many nitrite complexes CsCd(NO 2) 3 undergoes a simple thermal decomposition to CdO and CsNO 3.