Helix-coil transition of poly-L-ornithine in solution

Ethanol induced conformational changes of the peptide ligands for the opioid receptors and their relevance to receptor interaction

The FT-IR (Fourier Transform Infrared) Spectrum of [Met 5]-enkephalinamide in aqueous solution shows the presence of both the beta-turn and beta-sheet conformations. The beta-turn and beta-sheet conformations of enkephalins have been proposed to play a role in receptor selectivity. Addition of ethanol alters these secondary structural features and hence the effect of ethanol on ligand-receptor interaction may be mediated primarily through conformational changes of the ligand rather than those of the receptor.

Investigation of polylysine-dipalmitoylphosphatidylglycerol interactions in model membranes

The effect of poly(L-lysine) on dipalmitoylphosphatidylglycerol bilayers has been studied by Raman and infrared spectroscopies, small-angle X-ray diffraction, and carboxyfluorescein escape experiments. The polypeptide is shown to induce a stabilization of the bilayer detected by the increase of interchain vibrational coupling and a slight decrease of the overall disorder. In addition, long polylysine (Mr 150,000) induces a positive shift of the gel to fluid transition temperature and, at lipid to lysine molar ratios greater than 1, a lateral phase separation within the bilayer. Raman and infrared spectra indicate modifications at the head group level. In contrast, short polylysine (Mr 4,000) leads to a decrease of the lipid thermotropic transition temperature, and no modification of the polar head group and no phase separation could be observed. These differences between short and long polypeptides are correlated with the conformation the polypeptide adopts upon binding to the lipid, which favors the formation of alpha-helices in the case of long polypeptides (Mr greater than or equal to 14,000). The X-ray data suggest that the basic polypeptide acts as a bridge between neighboring bilayers, thus causing their aggregation and dehydration.

Alcohol induced conformational transitions of proteins and polypeptides. Thermodynamic studies of some model compounds

Integral enthalpies of solution of diglycine in tert.-butyl alcohol + water and of diglycine and beta-alanine in ethanol + water mixtures were measured at 298.15 K as a function of alcohol concentration. Enthalpies of transfer of the solutes from water to aqueous alcohol mixtures were evaluated from these data. Entropies of transfer of a peptide backbone unit (CH2CONH) and peptide group (CONH) from water to aqueous ethanol solutions were derived from the enthalpy of transfer data and the free energies of transfer of glycine, alpha-alanine, beta-alanine and diglycine. The thermodynamic transfer functions are discussed in terms of "water structure" mediated solute-solute interactions. The observed trends in the thermodynamic transfer functions have also been utilized to rationalize the effect of alcohols on the conformational stability of proteins and polypeptides in aqueous solutions.

Ordered conformations of cationic polypeptides in anionic surfactant solutions

Ionic poly(L-ornithine) at 45 degrees adopts a helical conformation in the presence of sodium dodecyl sulfonate or phosphate just as it is in sodium dodecyl sulfate solution as evidenced from circular dichroic spectra. But the polypeptide remains unordered in sodium tridecanate. Under similar conditions ionic poly(L-lysine) assumes the beta-form in all four anionic surfactant solutions. In sodium dodecyl phosphate solution at 45 degrees (Lys)n also undergoes a beta-to-helix transition in alkaline solution with a midpoint at pH 9.2, noting that dodecyl phosphoric acid has two dissociation constants with pKa of about 3 and 8. Raising the pH of the solution above 11 at 45 degrees converts the helical conformation back to the beta-form. This contrasts with a single beta-to-helix transition at pH 10.9 at 45 degrees for (Lys)n in sodium dodecyl sulfate solution.

The role of the reactive disulfide bond in the interaction of cholera-toxin functional regions

The chemical reactivity of disulfide bonds towards reducing agents, in the absence of denaturing conditions, in cholera toxin has been studied. Treatment of the toxin with dithiothreitol or other mercaptans gave selective reduction of one of the six disulfide bonds of the protein. This reactive disulfide links two distinct functional regions of the toxin, fragment alpha, which activates adenylate cyclase, and fragment gammabeta5, which recognizes the cell surface receptors. Upon reduction, the two fragments remain bound together and the secondary structure of the protein is retained. The two functional regions have been separated and purified only by methods based on charge differences. When mixed together, purified alpha and purified gammabeta5 fragments spontaneously and rapidly re-form the disulfide bond. However, reduction of the disulfide bond is an absolute requirement for freeing the catalytic site of the alpha functional region. Thus, while other non-covalent binding regions are involved in maintaining cholera toxin molecular structure, the reactive disulfide bond may play a role in the mechanism of cell intoxication.

Formation and spectral characterization of Cu(II)-poly(L-ornithine) complexes

Cu(II)-Poly-(1-ornithine) complexes in aqueous solution have been studied using potentiometric titration and absorption and circular dichroism spectra. As in the case of Cu(II)-poly(L-arginine) complexes studied previously, two types of compounds have been detected, labeled complexes I and II. Complex I contains two amine nitrogens and two water molecules coordinated to the copper. Complex II, two amine and two amide nitrogens. Amide nitrogen coordination confers optical activity to the copper d-d transitions. Furthermore, amine and amide nitrogen coordination to the copper are characterized by charge transfer transitions at 250 and 320 nm respectively which were already identified in Cu(II)-poly(L-arginine) systems.