Search

Peptides with a regular sequence of enantiomeric residues (L and D) along the chain have received considerable attention because of their accessibility to unique conformations and because they are model compounds for the naturally occurring peptide gramicidin A, which shows monovalent cation selective transmembrane transport. The solid-state structure of the linear hexapeptide t-Boc-(D-alle-L-Ile)3-OMe has been determined by X-ray diffraction techniques and refined to a final R factor of 0.068. The molecule shows a bent U-shaped conformation stabilized by three intramolecular H-bonds of the N-H...O = C type: a type II beta-bend (4-->1 bend or C10 ring structure) with L-Ile2 and D-aIle3 at positions 2 and 3 of the bend, an alpha-turn (5-->1 bend or C13 ring structure) and a 1-->5 bend or C17 ring structure. The first two 10-membered and 13-membered bends are enclosed in the latter 17-membered hydrogen-bonded ring structure. This structural motif is common to hepta- and octa-peptide cyclic molecules, showing that ring closure is not required to achieve a particular topology in the molecular design of specific bended conformations.

The general problems presented by the determination of the optical activity of polymers in the crystalline state are considered. The necessity of using unoriented samples, if the polarimetric data are to be compared with analogous data measured in solution, is emphasized. The optical activity in the crystalline state of a number of stereoregular vinyl polymers has been measured by employing either suspensions of fine powders or thin plates with no detectable orientation. The results of optical rotatory dispersion studies in the crystalline state suggest that, at least in the case of the poly-α-olefins investigated, the same chromophores are responsible for the optical activity in the crystalline state and in solution.

A symposium report on the crystal structure and conformation of title peptides cyclo(L-Val-D-Val)3 and cyclo(L-Phe-DPhe)3.

A single‐crystal x‐ray diffraction analysis of Boc‐L‐Ala‐D‐aIle‐L‐Ile‐OMe has been carried out. The analysis has shown (a) that the tripeptide molecules have in part an α‐extended conformation, the torsion angles of the L‐Ala and D‐aIle residues being φ1 = −75.1° and ψ1 = −25.8° and φ2 = 67.3° and ψ2 = 44.1°, respectively, and (b) that the molecules are organized in rippled planes where they occur in relative antiparallel orientation linked together side by side by H bonds. This molecular organization of the tripeptide corresponds closely to that of an antiparallel α‐pleated sheet, and likely constitutes the first example of a structure of this kind for which a characterization at the atomic level has been achieved. A molecular dynamics study has shown that the molecular conformation of the tripeptide in the crystalline state is determined primarily by intermolecular interactions. © 1994 John Wiley & Sons, Inc. Copyright © 1994 John Wiley & Sons, Inc.

In the present paper we describe the single-crystal x-ray analyses of two cyclic hexapeptides containing an equal number of alternating L,D-residues as putative analogues of the metal binding compounds, enniatin and beauvericine. Both the molecules of c(L-Val-D-Val)3 and c(L-Phe-D-Phe)3 retain in the solid state the center of symmetry and crystallize with six and eight trifluoroacetic acid molecules, respectively. The peptides are strongly hydrogen bonded to the solvent molecules. We estimate, on the basis of the molecular geometry and spatial arrangement of the peptide carbonyl groups and in comparison with other metal binding cyclic peptides, the ability of these molecules to interact with metal ions as 1:1 complexes.