Dihedral angle

About: Dihedral angle is a research topic. Over the lifetime, 15718 publications have been published within this topic receiving 174904 citations.

Functionalised oligoenes with unusual topologies: Synthesis, electrochemistry and structural studies on redox-active [3]- and [4]-dendralenes

Abstract: New [3]- and [4]-dendralenes bearing electron-donor 1,3-dithiole and ferrocene substituents have been synthesised. Compounds 8, 15 and 17 have been characterised by single-crystal X-ray diffraction. Two of the dithiole rings of 8 are conjugated (dihedral angle 9 degrees), while the third dithiole ring is almost orthogonal to this plane, and hence its pi-electron system is isolated. For the dendralene precursor molecule 15, the substituted cyclopentadienyl ring, two C=C bonds and fused dithiole and dithiine rings comprise an extended pi-conjugated system. In molecule 17 the potential conjugation path C(6)C(3) C(4)C(5)-C5H5 is distorted by an 8 degrees twist around the C(3)-C(4) bond and a 7 degrees twist around the C(5)-C(21) bond, and the delocalisation along the chain is insignificant. Solution electrochemical data demonstrate that the dendralenes are strong pi-electron donors, which give rise to dication, radical trication or tetracation species. Spectroelectrochemical studies on compounds 7 and 10 suggest that the radical species are situated within the linear 1,2-ethylenediylidene moieties and that a conformational change may occur at the dication redox stage. UV/Vis spectroscopic data are consistent with poor cross-conjugation in these systems.

Mean field theory as a tool for intramolecular conformational optimization. 1. Tests on terminally-blocked alanine and met-enkephalin

Abstract: A self-consistent multitorsional field (SCMTF) method of global optimization of the conformational energy of an oligopeptide is proposed. The method is based on the idea that the maximum of the square of the ground-state wave function is very often close to the global minimum of the potential energy and may be relatively unaffected by the complexity of the potential, e.g., its many minima. Using a mean field approximation, a set of coupled Schroinger equations for the motion of the nuclei is solved iteratively in a dihedral-angle space, each equation depending on a single dihedral angle

Primary sequence dependence of conformation in oligomannose oligosaccharides.

Abstract: The oligomannose series of oligosaccharides from bovine thyroglobulin (BTG) and the variant surface glycoprotein (VSG) ofTrypanosoma brucei have been isolated and sequenced by1H NMR. The structure of Man9GlcNAc2, the parent molecule of the series, is shown below. Structural isomerism occurs within this series through the removal of residues D1, D2, D3, and C. Using spin-spin coupling and chemical shift data the rotamer distributions about the dihedral angle ω for the Manα1-6Man\ and Manα1-6Manα linkages were determined for each member of the series. It is shown that the dihedral angle ω of the Manα1-6Man\ linkage exhibits low flexibility with a preference for the ω = 180° conformation when residue D2 is present and high flexibility when this residue is absent. Flexibility of ω for the Manα1-6Manα is largely independent of primary sequence and is intermediate between the two Manoα1-6Man\ extremes, again with a preference for the ω = 180° conformation. Open image in new window There are, however, data which indicate that removal of residue D3 may confer additional flexibility upon the dihedral angle ω of the Manα1-6Manα linkage. Molecular graphics modelling, together with chemical and enzymatic modification studies, suggest that the origin of the observed primary sequence dependence of the Manα1-6Man\ linkage arises from steric factors. On the basis of these observations taken together with previous work, it is postulated that recognition of individual oligomannose conformations may play a role in the control of N-linked oligosaccharide biosynthesis.

A tetrahedral zinc(II) complex of N-(R)-1-phenylethylsalicylideneimine. Structural and circular dichroism spectral investigations on stereoselectivity

Abstract: The stereoselectivity of bis[N-(R)-1-phenylethylsalicylideneiminato]zinc(II) has been studied by means of single-crystal X-ray analysis and circular dichroism (c.d.) spectra. The complex crystallizes in space group P212121 with a= 17.738(4), b= 29.632(9), c= 9.968(2)A, and Z= 4. The unit cell is comprised of four units each containing two complex molecules (A and B). Each molecule adopts a nearly tetrahedral structure with two bidentate ligands; the dihedral angle defined by the two chelate rings is 81.7° for molecule A and 84.9° for molecule B. The absolute configuration along the pseudo-C2 axis is Λ for both molecules. Based on the X-ray analysis it is suggested that intramolecular non-covalent interactions between the chiral N-substituent and the adjacent chelate ring give rise to stereoselectivity and the Λ configuration around the metal ion. The c.d. couplet (+and – signs) induced at the azomethine π–π* transition at around 27 000 cm–1 is discussed with respect to the absolute configuration and in comparison with the spectra of related Schiff-base complexes.

INDO Molecular Orbital Calculations of Nitrogen–Proton Spin–Spin Coupling Constants Over Two and Three Bonds. Effects of Lone-pair Orientation, of Dihedral Angles, and of Protonation

Abstract: Molecular orbital theory at the INDO level of approximation is used to calculate the Fermi contact contribution to two-bond and three-bond nitrogen–proton coupling constants for a wide variety of compounds. The dependence of calculated N,H coupling constants upon the spatial orientation of the nitrogen lone-pair is examined for selected molecules. For both saturated and unsaturated compounds the calculated 2J(15N,H) is large and negative when the lone-pair is oriented cis to the proton, and is small and of either sign when the lone-pair lies trans to the proton. Calculated 3J(N,H) depends both on the orientation of the nitrogen lone-pair and on the HCCN dihedral angle. Computed 2J(N,H) and 3J(N,H) generally follow the experimentally known substituent and structural effects. It is suggested that observed phosphorus–proton coupling constants in tervalent phosphorus compounds are dependent on lone-pair orientation in a manner analogous to the corresponding nitrogen–proton coupling constants. The influence of...