Proposed molecular models of gramicidin S and other polypeptides

Intermolecular anti-parallel beta sheet: Comparison of predicted and observed conformations of gramicidin S

A recently determined x-ray structure of the hydrated gramicidin S-urea complex is compared with a structure predicted by conformational energy minimization. It is shown that the two structures are in good general agreement, including the prediction of a hydrogen bond between the side-chain amino group of ornithine and the backbone carbonyl of phenylalanine. This agreement demonstrates the power of empirical potential energy methods in conformational analysis and illustrates one method for solution of the multiple-minimum problem for small peptides. It is noted that, in the crystal, gramicidin S is a dimer that forms an intermolecular antiparallel four-stranded beta sheet and that differences between the predicted and x-ray structures can be explained by this intermolecular interaction. The residual conformational asymmetry of the x-ray structure is shown to be due to the formation of the complex with urea.

Intracellular and subcellular oedema and dehydration

Changes in intracellular water content appear to be common abnormalities induced by a wide variety of pathogenic mechanisms. Such changes in cell water produce changes in the water in various subcellular organelles bound by semipermeable membranes. Cell and subcell functions then alter in their turn. In isolated alveolar macrophages (rabbit), intracellular and intramitochondrial oedema reduces mitochondrial O2 utilization. Metabolic control is maintained because lactate production reverses (Pasteur effect). On reconstitution, O2 utilization and lactate production return towards normal, indicating reversibility. Cellular and intramitochondrial dehydration also reduces mitochondrial O2 utilization but metabolic control is lost because lactate production also decreases. Osmotic reconstitution does not reverse the abnormality. Exposure to hypotonic media leads to release of lysosomal enzymes (beta-glucuronidase, EC 3.2.1.31) to the extracellular phase of isolated alveolar macrophages. Some of this release is caused by exocytosis although, at low osmotic concentrations, intralysosomal oedema ultimately ruptures lysosomes, with extensive discharge of enzyme. In turn, lysosomal enzymes may injure more normal cells. Impairment of energy metabolism caused by hypoxia leads to intracellular oedema, because Na+ accumulates in the cells when ATP is no longer available for the sodium pump. Continued studies of the disorders in cell physiology caused by changes in cell and subcell water should provide important new insights into a wide variety of disease states (including pulmonary oedema).

Preliminary X-ray crystallographic and Raman studies on gramicidin S.2HCl

The cyclic decapeptide gramicidin S.2HCl has been crystallized from an aqueous solution containing about 33% (v/v) ethanol. The crystals are suitable for X-ray diffraction studies and are obtained reproducibly. Preliminary X-ray crystallographic studies reveal the crystal to have the orthorhombic space group C222(1), with a = 25.76, b = 35.79, and c = 38.13 A. The asymmetric unit consists of two molecules of gramicidin S.2HCl and several molecules of solvent. This result corresponds closely to that reported by Schmidt et al. (1957, Biochem. J. 65, 744-750). A Raman spectrum of the crystal has been recorded. A strong amide I band at 1670 cm-1 and an amide III band at 1240 cm-1 indicate that gramicidin S has a beta-sheet conformation in its crystal structure. An enhancement of Raman signals from gramicidin S.2HCl adsorbed on Ag sol has been observed.

Mise en evidence d'une conformation stable d'un peptidolipide

The Peptidolipide J (CH(3)-(CH(2))(n)-CO-D Phe-L Ile-L Ile-L Phe-L Ala-O-CH(3)) is obtained also in acidic form by a chromatographic artefact. In chloroformic solutions the ester and the acid behave in a very different way: the acid has a stable conformation, as shown by the infra-red spectra and the optical rotary dispersion; this conformation is stabilized by hydrogen bonding between two peptide chains associated as a carboxylic dimer.