Analysis and use of fragment-occurrence data in similarity-based virtual screening

Current systems for similarity-based virtual screening use similarity measures in which all the fragments in a fingerprint contribute equally to the calculation of structural similarity. This paper discusses the weighting of fragments on the basis of their frequencies of occurrence in molecules. Extensive experiments with sets of active molecules from the MDL Drug Data Report and the World of Molecular Bioactivity databases, using fingerprints encoding Tripos holograms, Pipeline Pilot ECFC_4 circular substructures and Sunset Molecular keys, demonstrate clearly that frequency-based screening is generally more effective than conventional, unweighted screening. The results suggest that standardising the raw occurrence frequencies by taking the square root of the frequencies will maximise the effectiveness of virtual screening. An upper-bound analysis shows the complex interactions that can take place between representations, weighting schemes and similarity coefficients when similarity measures are computed, and provides a rationalisation of the relative performance of the various weighting schemes.     

Structure of dichloroethanolaminecopper(II) and dibromoethanolaminecopper(II)

CuCl₂(NH₂CH₂CH₂OH) is monoclinic, space group C2/c, witha+11.092(2),b+10.012(2),c+6.401(1) Å, =121.71(1)°, andV=604.8(2)ų withZ+4. The structure was refined to a final value ofR+0.27 for 518 unique observed reflections with |F|>3. The ethanolamine and halide ions coordinate to the copper(II) ion forming a pseudoplanar four-coordinate monomeric CuCl₂L² complex (L²+bidentate ligand). The copper ion also interacts with the halides of neighboring complexes through the formation of semi-coordinate Cu-Cl linkages (2.926(1) Å) between oligomers. The copper thus attains a 4+2 elongated octahedral coordination geometry. In this space group, the ligand is statisticallydisordered The Cu-Cl distance is 2.267(1)Å while the Cu-N/O distances are 2.025(2)Å. The dibromide structure is isomorphous with the dichloride structure, with space group C2/c, anda+11.285(2),b+10.218(2),c+6.715(1) Å, =121.65(1)°, andV=659.2(2)ų.     

A comparison of the crystal structures of (dabcoH2)CuCl4 and (dabcoH2)CuCl4⋅H2O

The crystal structures of the anhydrous and monohydrated (dabcoH₂)CuCl₄nH₂O salts (n = 0,1) have been determined and a comparison of their crystal structures has been reported. The anhydrous salt is monoclinic P2₁/c with a = 9.045(2) Å, b = 6.9270(10) Å, c = 18.767(4) Å and = 90.07(3)° with V = 1175.8(4) ų. The hydrated salt is also monoclinic P2₁/c with a = 9.266(2) Å, b = 9.395(2) Å, c = 14.386(3) Å and = 93.32(3)° with V = 1250.3(5) ų. The structures of the two salts are closely related. Both compounds contain isolated distorted (compressed) tetrahedral CuCl₄ ^2– anions, diprotonated dabco cations, and in the case of the hydrated salt, lattice H₂O molecules. In the anhydrous salt, each of the two N–H⁺ groups of the dabcoH₂ ²⁺ dications form bifurcated hydrogen bonds to chloride ions on adjacent CuCl₄ ^2– anions along the c axis, thus forming chains running parallel to the c axis. In the hydrated salt, a water molecule is inserted into one of the pair of bifurcated hydrogen bonds, forming instead N–H O–H CuCl₄ ^2– linkages. The chains thus formed are cross-linked by O–H Cl hydrogen bonds between the chains. Because of this additional hydrogen bonding, the CuCl₄ ^2– anions in the hydrated salt are distorted further from tetrahedral geometry.     

Chemical valence in SF4 and SF4O

A qualitative molecular orbital treatment is used to correlate the electronic structure with the observed molecular structures of SF₄ and SF₄O.

---

Zusammenfassung Das M. O.-Verfahren wurde auf SF₄ und SF₄O bei Vorgabe der geometrischen Struktur qualitativ angewendet und auf diese Weise Einblick in die Elektronenstruktur gewonnen.

---

Résumé Un traitement qualitatif aux orbitales moléculaires met en corrélation la structure électronique et la géométrie observée de SF₄ et SF₄O.