Dihedral angle

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

Product rotational polarization in photo-initiated bimolecular reactions A+BC: Dependence on the character of the potential energy surface for different mass combinations

Abstract: In cases of light-heavy-light (LHL), light-light-heavy (LLH), and light-heavy-heavy (LHH) mass combinations, the product rotational polarization is reported using the quasiclassical trajectory method in the photoinitiated bimolecular reaction A + BC --> AB + C on attractive, mixed, and repulsive surfaces. Four polarization-dependent differential cross sections (PDDCS) which are sensitive to many photoinitiated bimolecular reaction experiments are presented. Furthermore, the dihedral angle distribution P(phi(r)) characterizing the K-K'-J' correlation and the distribution of angle between K and J' P(theta(r)) are discussed. Finally, the angular distributions P(theta(r),phi(r)) of product rotational vectors in the form of polar plots in theta(r) and phi(r) are shown.

Structure of an Iron(II) Dioxygen Complex; A Model for Oxygen Carrying Hemeproteins

Abstract: The preliminary structural characterization of a reversible ferrous dioxygen complex is reported. Mono(N-methyl imidazole) (dioxygen) meso-tetra (α,α,α,α-o-pivalamidephenyl) porphinatorino(II), [Fe(O2)-(N-Me imid) (α,α,α,α-TpivPP)], 1, isolated from toluene solution, crystallizes in the monoclinic system with four molecules in a unit cell of dimensions a = 18.690 (3), b = 19.514 (3), c = 18.638 (3) A, and β = 91.00 (1)°. R = 0.15 for 841 reflections having F2 > 3σ (F2). The complex 1 has four pivalamido groups on one side of the porphyrin forming a hydrophobic pocket of 5.4-A depth which encloses coordinated dioxygen. The dioxygen is coordinated “end-on,” with a bent Fe-O-O bond. The Fe-O-O plane bisects an N-Fe-N right angle of the equatorial iron porphyrin plane and is four way statistically disordered. In addition there is a crystallographic 2-fold axis through iron, coordinated oxygen, and nitrogen of the axially bound N-methyl imidazole. Thus there are two types of coordinated dioxygen with the Fe-O-O plane either parallel or perpendicular to the trans axial imidazole plane. Corresponding values for the Fe-O-O bond angles are 135-(4)° and 137(4)° and for the O-O bond lengths are 1.23 (0.08) and 1.26 (0.08) A, with a dihedral angle of 90° between alternative orientations of the Fe-O-O plane. The Fe-O distance is 1.75 (0.02) A and Fe-N (imidazole) is 2.07 (0.02) A, suggesting multiple bond character in the Fe-O moiety. The similarity of the Mossbauer spectrum of the model complex, 1, with oxyhemoglobin indicates that 1 may be a good model for oxygen binding in the oxygen transport hemeproteins.

The beta-peptide hairpin in solution: conformational study of a beta-hexapeptide in methanol by NMR spectroscopy and MD simulation.

Abstract: The structural and thermodn. properties of a 6-residue β-peptide I that was designed to form a hairpin conformation have been studied by NMR spectroscopy and mol. dynamics simulation in methanol soln. The predicted hairpin would be characterized by a 10-membered hydrogen-bonded turn involving residues 3 and 4, and two extended antiparallel strands. The interproton distances and backbone torsional dihedral angles derived from the NMR expts. at room temp. are in general terms compatible with the hairpin conformation. Two trajectories of system configurations from 100-ns mol.-dynamics simulations of the peptide in soln. at 298 and 340 K have been analyzed. In both simulations, reversible folding to the hairpin conformation is obsd. Interestingly, there is a significant conformational overlap between the unfolded state of the peptide at each of the temps. As already obsd. in previous studies of peptide folding, the unfolded state is composed of a (relatively) small no. of predominant conformers and in this case lacks any type of secondary-structure element. The trajectories provide an excellent ground for the interpretation of the NMR-derived data in terms of ensemble avs. and distributions as opposed to single-conformation interpretations. From this perspective, a relative population of the hairpin conformation of 20% to 30% would suffice to explain the NMR-derived data. Surprisingly, however, the ensemble of structures from the simulation at 340 K reproduces more accurately the NMR-derived data than the ensemble from the simulation at 298 K, and this point needs further investigation.

Twist Angles and Rotational Energy Barriers of Biphenyl and Substituted Biphenyls

Abstract: Using B3LYP/6-311+G* and other methods, twist angles and torsional energies were obtained for biphenyl (BP), 2-halogen BPs, 2,2‘-dihalogen BPs, and 3,3‘-dihalogen BPs, the halogens ranging from F to I. The results were compared with available gas phase and X-ray data. 2,2‘-difluoro BP has a rotational double minimum, at 57.9 and 128.9°, whereas the other 2,2‘-dihalogen BPs have a single minimum at dihedral angles ranging from 84.9 to 94.8°. All 3,3‘-dihalogen BPs have a double minimum at about 45 and 135°. Optimized twist angles and energy barriers were also calculated for 2,2‘-dimethyl BP and for perfluoro as well as perchloro BP. Most structures are accounted for by steric effects. For 2,2‘-dihalogen BPs, however, attractive forces also appear to play a role, as evidenced by the dihedral angle of 2,2‘-dichloro BP lying well below 90°.