Probing Fluorine Interactions in a Polyhydroxylated Environment: Conservation of a C-F center dot center dot center dot H-C Recognition Motif in Presence of O-H center dot center dot center dot O Hydrogen Bonds

Three conformationally locked fluorinated polycyclitols have been specially crafted on a rigid trans-decalin backbone, employing a surprisingly facile pyridine-poly(hydrogen fluoride)-mediated stereospecific epoxide ring opening as the key reaction. Molecula design of the three fluorinated probes under study focused on providing an efficient platform for (a) evaluating the ability of covalently bonded fluorine, vis-a-vis the isosteric hydroxy group, to act as a H-bond acceptor and (b) examining the possibility for an organic fluorine moiety, placed suitably in a spatially invariant position, to engage an 1,3-diaxial OH functionality in a purported intramolecular O-H center dot center dot center dot F hydrogen bond. The present endeavour reveals that C(sp(3))-F center dot center dot center dot H-C(sp(3)) hydrogen bonds, though weak and lesser investigated, can indeed be observed and supramolecular recognition motifs, involving such interactions, can be conserved even in crystal structures laden with stronger O-H center dot center dot center dot O hydrogen bonds.

Plasticity in the primary binding site of galactose/N-acetylgalactosamine-specific lectins - Implication of the C-H center dot center dot center dot O hydrogen bond at the specificity-determining C-4 locus of the saccharide in 4-methoxygalactose recognition by jacalin and winged bean (basic) agglutinin I

It is currently believed that an unsubstituted axial hydroxyl at the specificity-determining C-4 locus of galactose is indispensable for recognition by galactose/N-acetylgalactosamine-specific lectins. Titration calorimetry demonstrates that 4-methoxygalactose retains binding allegiance to the Moraceae lectin jacalin and the Leguminosae lectin, winged bean (basic) agglutinin (WBA I). The binding reactions were driven by dominant favorable enthalpic contributions and exhibited significant enthalpy-entropy compensation. Proton NMR titration of C-methoxygalactose with jacalin and WBA I resulted in broadening of the sugar resonances without any change in chemical shift. The alpha-and beta-anomers of 4-methoxygalactose were found to be in slow exchange with free and lectin-bound states. Both the anomers experience magnetically equivalent environments at the respective binding sites. The binding constants derived from the dependence of NMR line widths on 4-methoxygalactose concentration agreed well with those obtained from titration calorimetry. The results unequivocally demonstrate that the loci corresponding to the axially oriented C-4 hydroxyl group of galactose within the primary binding site of these lectins exhibit plasticity. These analyses suggest, for the first time, the existence of C-H ... O-type hydrogen-bond(s) in protein-carbohydrate interactions in general and between the C-4 locus of galactose derivative and the lectins jacalin and WBA I in particular.

An unusual C-H center dot center dot center dot O hydrogen bond mediated reversal of polypeptide chain direction in a synthetic peptide helix

An unusual C-terminal conformation has been detected in a synthetic decapeptide designed to analyze the stereochemistry of helix termination in polypeptides. The crystal structure of the decapeptide Boc-Leu-Aib-Val-Ala-Leu-Aib-Val-(D)Ala-(D)Leu-Aib-OMe reveals a helical segment spanning residues 1-7 and helix termination by formation of a Schellman motif, generated by (D)Ala(8) adopting the left-handed helical (alpha(L)) conformation. The extended conformation at (D)Leu(9) results in a compact folded structure, stabilized by a potentially strong C-H ... O hydrogen bond between Ala(4) (CH)-H-alpha and (D)Leu(9)CO. The parameters for C-H ... O interaction are Ala(4) (CH)-H-alpha .. O=C (D)Leu(9) distance 3.27 Angstrom C-alpha-H .. O angle 176 degrees, and O .. H-alpha distance 2.29 Angstrom. This structure suggests that insertion of contiguous D-residues may provide a handle for the generation of designed structures containing more than one helical segment folded in a compact manner. (C) 2000 Academic Press.

Conformations of three heterocyclicperhydropyrrolobenzofurans and polymeric assembly via co-operative inter-molecular C-H center dot center dot center dot O hydrogen bonds

In 1-cyclo-hexyl-6,6,8a-trimethyl-3a,6,7,8a-tetra-hydro-1H-1-benzofuro[2,3-b]pyrrole-2,4(3H,5H)-dione, C19H27NO3, (I), and the isomorphous compounds 6,6,8a-trimethyl-1-phenyl-3a,6,7,8a-tetra-hydro-1H-1-benzofuro[2,3-b]pyrrole-2,4(3H,5H)-dione, C19H21NO3, (II), and 6,6,8a-trimethyl-1-(3-pyridyl)-3a,6,7,8a-tetra-hydro-1H-1-benzofuro[2,3-b]pyrrole-2,4(3H,5H)-dione, C18H20N2O3, (III), the tetra-hydro-benzo-dihydro-furo-pyrrolidine ring systems are folded at the cis junction of the five-membered rings, giving rise to a non-planar shape of the tricyclic cores. The dihydro-furan and pyrrolidine rings in (I) are puckered and adopt an envelope conformation. The cyclo-hexene rings adopt a half-chair conformation in all the mol-ecules, while the substituent N-cyclo-hexyl ring in (I) assumes a chair form. Short intra-molecular C-HcO contacts form S(5) and S(6) motifs. The isomorphous compounds (II) and (III) are effectively isostructural, and aggregate into chains via inter-molecular C-HcO hydrogen bonds.

Rb2Cd3(SO4)(3)(OH)(2)center dot H2O: structural stability at 500 K

The title compound, dirubidium tricadmium tris(sulfate) dihydroxide dihydrate, consists of sheets of CdO6 octahedra and sulfate tetrahedra propagating in the (100) plane, with Rb+ ions in the interlayer positions. It is isostructural with K2Co3(SO4)(3)(OH)(2)(.)2H(2)O.

Competing magnetic interactions in a dinuclear Ni(II) complex: Antiferromagnetic O-H center dot center dot center dot O moiety and ferromagnetic N-3(-) ligand

Synthesis, structural characteristics, magnetic studies and DFT calculations in Ni(II) dinuclear complexes containing two bridging N-3(-) and an O-(HO)-O-... linkage reveal the existence of ferromagnetic interactions between Ni(II) centers via N-3(-) ligands and antiferromagnetic interactions through the H-bonded moiety. The overall magnetic behavior of the system depends on the delicate balance between these two competing interactions.

Conformations of three heterocyclic perhydropyrrolobenzofurans and polymeric assembly via co-operative inter-molecular C-H center dot center dot center dot O hydrogen bonds

In 1-cyclo-hexyl-6,6,8a-trimethyl-3a,6,7,8a-tetra-hydro-1H-1-benzofuro2, 3-b]pyrrole-2,4(3H,5H)-dione, C19H27NO3, (I), and the isomorphous compounds 6,6,8a-trimethyl-1-phenyl-3a,6,7,8a-tetra-hydro-1H-1-benzofuro2,3-b]p yrrole-2,4(3H,5H)-dione, C19H21NO3, (II), and 6,6,8a-trimethyl-1-(3-pyridyl)-3a,6,7,8a-tetra-hydro-1H-1-benzofuro2, 3-b]pyrrole-2,4(3H,5H)-dione, C18H20N2O3, (III), the tetra-hydro-benzo-dihydro-furo-pyrrolidine ring systems are folded at the cis junction of the five-membered rings, giving rise to a non-planar shape of the tricyclic cores. The dihydro-furan and pyrrolidine rings in (I) are puckered and adopt an envelope conformation. The cyclo-hexene rings adopt a half-chair conformation in all the mol-ecules, while the substituent N-cyclo-hexyl ring in (I) assumes a chair form. Short intra-molecular C-HcO contacts form S(5) and S(6) motifs. The isomorphous compounds (II) and (III) are effectively isostructural, and aggregate into chains via inter-molecular C-HcO hydrogen bonds.

Synthesis, structure and magnetic properties of the polyoxovanadate cluster Zn-2(NH2(CH2)(2)NH2)(5)]{Zn(NH2(CH2)(2)NH2)(2)}(2){V18O42(H2O)\ ]center dot xH(2)O (x similar to 12), possessing a layered structure

A hydrothermal reaction of a mixture of ZnCl2, V2O5, ethylenediamine and water gave rise to a layered poly oxovanadate material. clusters. These clusters, with all the vanadium ions in the +4 state, are connected together through Zn(NH2(CH2)(2)NH2)(2) linkers forming a two-dimensional structure. The layers are also separated by distorted trigonal bipyramidal [Zn-2(NH2(CH2)(2)NH2)(5)] complexes. The Structure, thus, presents a dual role for the Zn-ethylenediamine complex. The magnetic susceptibility studies indicate that the interactions between the V centres in I are predominantly antiferromagnetic in nature and the compound shows highly frustrated behaviour. The magnetic properties are compared to the theoretical calculations based oil the Heisenberg model, in addition to correlating to the structure. Crystal data for the complexes are presented.

Proton-bifurcated C-H center dot center dot center dot(O,O) hydrogen bonds in 2,3-dichloro-6-nitrobenzylaminium chloride

In the crystal structure of the title salt, C7H7Cl2N2O2+ center dot Cl-, the chloride anions participate in extensive hydrogen bonding with the aminium cations and indirectly link the molecules through multiple N+-H center dot center dot center dot Cl- salt bridges. There are two independent molecules in the asymmetric unit, related by a pseudo-inversion center. The direct intermolecular coupling is established by C-H center dot center dot center dot O, C-H center dot center dot center dot Cl and C-Cl center dot center dot center dot Cl- interactions. A rare three-center (donor bifurcated) C-H center dot center dot center dot (O,O) hydrogen bond is observed between the methylene and nitro groups, with a side-on intramolecular component of closed-ring type and a head-on intermolecular component.

Directing role of functional groups in selective generation of C-H center dot center dot center dot pi interactions: In situ cryo-crystallographic studies on benzyl derivatives

The in situ cryo-crystallization study of benzyl derivatives reveals that the molecular packing in these compounds is either through methylene (sp(3)) C-H center dot center dot center dot pi or aromatic (sp(2)) C-H center dot center dot center dot pi interactions depending on the level of acidity of the benzyl proton. These studies of low melting compounds bring out the subtle features of such weak interactions and point to the directional preferences depending on the nature (electron withdrawing, polarizability) of the neighbouring functional group.

N-H center dot center dot center dot F hydrogen bonds in fluorinated benzanilides: NMR and DFT study

Using F-19 and H-1-NMR (with N-14 decoupling) spectroscopic techniques together with density functional theoretical (DFT) calculations, we have investigated weak molecular interactions in isomeric fluorinated benzanilides. Simultaneous presence of through space nuclear spin-spin couplings ((1h)J(N-H center dot center dot center dot F)) of diverse strengths and feeble structural fluctuations are detected as a function of site specific substitution of fluorine atoms within the basic identical molecular framework. The transfer of hydrogen bonding interaction energies through space is established by perturbing their strengths and monitoring the effect on NMR parameters. Multiple quantum (MQ) excitation, up to the highest possible MQ orders of coupled protons, is utilized as a tool for accurate H-1 assignments. Results of NMR studies and DFT calculations are compared with the relevant structural parameters taken from single crystal X-ray diffraction studies.

Analysis of Cl center dot center dot center dot Cl and C-H center dot center dot center dot Cl intermolecular interactions involving chlorine in substituted 2-chloroquinoline derivatives

Six crystal structures of substituted 2-chloroquinoline derivatives have been analysed to evaluate the role of Cl atom as a self recognizing unit resulting in the formation of Cl center dot center dot center dot Cl and C-H center dot center dot center dot Cl interactions to generate supramolecular assembly in the solid state. The features of Type I and Type II geometries associated with Cl center dot center dot center dot Cl interactions have been analysed to show directional preferences leading to differences in the packing motifs in these crystal structures. C-H center dot center dot center dot Cl interactions are generated exclusively in structures depicting Type II Cl center dot center dot center dot Cl interaction have been observed in these structures.

Effect of a QSO on the interstellar medium of its host galaxy

We consider the hydrodynamic evolution of gas in the interstellar medium of the host galaxy of a quasar due to Compton heating by the QSO radiation. We show that a Lagrangean formulation of the problem is necessary. It is found that the "hydrodynamic time scale" becomes important compared to the Compton heating time scale. We also relax the "single fluid" approximation by considering the existence of clouds and taking into account the mass loss from stars. The results predict star burst activity, and thus we explain the blue colors of the active galaxies.