Isabella L. Karle

Internal Motion and Molecular Structure Studies by Electron Diffraction

A procedure has been developed for the determination of molecular structure by electron diffraction which yields accurate intensity data and obviates the necessity for visual examination of the diffraction photographs. The theory for computing radial distribution curves has been extended to permit accurate curves to be obtained from scattering data covering only a restricted range of angle. From this method, it is possible to obtain not only equilibrium distances but also the probability distributions for the vibrational motion between pairs of atoms in a molecule. The procedure has been applied to CCl4 and CO2 and when comparisons may be made with spectroscopic results, satisfactory agreement is obtained.

Internal Motion and Molecular Structure Studies by Electron Diffraction. II. Interpretation and Method

The application of a recently developed objective procedure to the analysis of electron diffraction photographs from more complex molecules is discussed in terms of the further development of the details of some of the theoretical and experimental aspects of the procedure. The topics discussed are the physical significance of measured vibrational amplitudes, the method for drawing a background line, the calibration of photographic plates, and the computation of intensity curves by means of IBM machines.

Internal Motion and Molecular Structure Studies by Electron Diffraction. III. Structure of CH2CF2 and CF2CF2

The interatomic distances and vibrational amplitudes projected on the lines connecting pairs of atoms at equilibrium have been determined for CH2CF2 and CF2CF2 by an objective procedure for analyzing electron diffraction photographs. Distances involving hydrogen atoms have been evaluated and the procedure for resolving radial distribution peaks involving more than one distance is illustrated. The effects on the accuracy of the method of various characteristics such as sample spread and multiple scattering have been analyzed.

An Electron Diffraction Investigation of Cyclooctatetraene and Benzene

The interatomic distances and their vibrational amplitudes have been determined for C8H8 and C6H6 by quantitative analysis of electron diffraction sector photographs. The D2d tub configuration, with the oblique C–C bonds longer than the horizontal ones, has been confirmed for C8H8. The interatomic distances involving hydrogen atoms have been evaluated.

[Leu5]enkephalin: four cocrystallizing conformers with extended backbones that form an antiparallel β-sheet

[LeuS]enkephalin (Tyr-Gly-Gly-Phe-Leu) grown from N,N-dimethylformamide(DMFA)/water crystallizes with four quite different conformers side-by-side in the asymmetric unit. The four conformers with extended backbones form an infinite antiparallel fl-sheet. ]]-sheets related by the twofold screw axis are separated by a 12/~ spacing. Side groups protrude above and below the fl-sheets and are entirely immersed in a thick layer of solvent occupying the volume between fl-sheets. The crystal, stable only in contact with its mother liquor, appears to be a hybrid consisting of rather rigid sheets of peptide molecules separated by spaces filled with mobile solvent, thus having some resemblance to molecules in solution. Many solvent molecules are completely disordered and may be fluid. The space group is P2~ with a = 18.720(4), b = 24.732 (6), c = 20.311 (5 )A , / ?= 115.86 (1) °, V = 8462A 3. Composition of the asymmetric unit includes four enkephalin molecules, eight water molecules, eight DMFA molecules, plus an unknown number of disordered solvent molecules: 4C28H37NsO 7. 8H20.8C3H7NO. X. The R factor from a restrained least-squares re0108-7681/83/050625-13501.50 finement (with six-parameter thermal factors) is 11.9% for 8155 data with IFol > O. The procedure used for phase determination and structure analysis is described. Parameters for an extensive antiparallel fl-sheet are presented.

Flexibility in peptide molecules and restraints imposed by hydrogen bonds, the AIB residue, and core inserts

The first segments of this article review some of the early crystal structure determinations of beta-bends, gamma-bends, large conformational changes in cyclic peptides upon complexation with metal ions, and both the stability and drastic change in conformation as a function of the solvent used for growing crystals. More recent structure analyses have concerned helical peptides containing the Aib and/or Dpg residues and associated conformational problems such as 310-/alpha-helix transitions, helix aberrations, reversals, severe curving of the helix, unfolding, and hydration of backbone. Ion channels occurring in three crystal forms of zervamicin and possible ion transport and gating mechanisms are described. Finally, hydrogen bonding patterns are presented in supramolecular assemblies of retropeptides with core inserts consisting of oxalyl, adipoyl, suberoyl and polymethylene moieties.

Structure and Internal Motion of C2F6 and Si2Cl6 Vapors

The molecular structure and internal motion of C2F6 vapor and of Si2Cl6 vapor have been investigated by means of the sector‐microphotometer method of electron diffraction. Both molecules exhibit hindered internal rotation, with equilibrium in the trans position. The interdependence of the potential barrier and the modes of vibration other than the torsional is presented graphically. The values for the bonded distances are: C – F = 1.32±0.01 A, C – C = 1.56±0.03 A with ∠ CCF = 109½°±1½° and Si – Cl = 2.01±0.01 A, Si – Si = 2.24±0.06 A with ∠ SiSiCl = 109½°±1°.

Comparison of helix-stabilizing effects of α,α-dialkyl glycines with linear and cycloalkyl side chains

The ability of \alpha, \alpha -di-n-alkyl glycines with linear and cyclic alkyl side chains to stabilize helical conformations has been compared using a model heptapeptide sequence. The conformations of five synthetic heptapeptides (Boc–Val–Ala–Leu–Xxx–Val–Ala–Leu–OMe,

Folding, Aggregation and Molecular Recognition in Peptides

Xxx = Ac_8c, Ac_7c, Aib, Dpg ,and Deg, where Ac_8c

Alkaloids from dendrobatid poison frogs: trans-decahydroquinolines and indolizidines

= 1-aminocyclooctane-1-carboxylic acid,