Semi-Rigid Nitroxide Spin Label for Long-Range EPR Distance Measurements of Lipid Bilayer Embedded ß-Peptides.

ß-Peptides are an interesting new class of transmembrane model peptides based on their conformationally stable and well-defined secondary structures. Herein, we present the synthesis of the paramagnetic ß-amino acid ß3 -hTOPP (4-(3,3,5,5-tetramethyl-2,6-dioxo-4-oxylpiperazin-1-yl)-d-ß3 -homophenylglycine) that enables investigations of ß-peptides by EPR spectroscopy. This amino acid adds to the, to date, sparse number of ß-peptide spin labels. Its performance was evaluated by investigating the helical turn of a 314 -helical transmembrane model ß-peptide. Nanometer distances between two incorporated ß3 -hTOPP labels in different environments were measured by using pulsed electron/electron double resonance (PELDOR/DEER) spectroscopy. Due to the semi-rigid conformational design, the label delivers reliable distances and sharp (one-peak) distance distributions even in the lipid bilayer. The results indicate that the investigated ß-peptide folds into a 3.2514 helix and maintains this conformation in the lipid bilayer.

Oxovanadium(IV) Coordination Compounds with Kojic Acid Derivatives in Aqueous Solution.

Hydroxypyrone derivatives have a good bioavailability in rats and mice and have been used in drug development. Moreover, they show chelating properties towards vanadyl cation that could be used in insulin-mimetic compound development. In this work, the formation of coordination compounds of oxovanadium(IV) with four kojic acid (5-hydroxy-2-(hydroxymethyl)-4-pyrone) derivatives was studied. The synthetized studied ligands (S2, S3, S4, and SC) have two or three kojic acid units linked through diamines or tris(2-aminoethyl)amine chains, respectively. The chemical systems were studied by potentiometry (25 °C, ionic strength 0.1 mol L-1 with KCl), and UV-visible and EPR spectroscopy. The experimental data were analyzed by a thermodynamic and a chemometric (Multivariate Curve Resolution-Alternating Least Squares) approach. Chemical coordination models were proposed, together with the species formation constants and the pure estimated UV-vis and EPR spectra. In all systems, the coordination of the oxovanadium(IV) starts already under acidic conditions (the cation is totally bound at pH higher than 3-4) and the metal species remain stable even at pH 8. Ligands S3, S4, and SC form three coordination species. Two of them are probably due to the successive insertion of the kojate units in the coordination shell, whereas the third is most likely a hydrolytic species.

Ternary copper(II) complexes with 1,10­phenanthroline and various aminoacidates: A spectroscopic and voltammetric study in aqueous solution.

Ternary copper(II) complexes with 1,10­phenanthroline and the aminoacids l­arginine, l­aspartic acid, l­histidine, l­glutamic acid, l­glutamine, l­leucine, l­lysine, l­methionine, l­phenylalanine, l­tryptophan, l­tyrosine, l­valine, were studied in aqueous solution by means of UV-Vis-NIR spectrophotometry, EPR spectroscopy either at room or at low temperatures, and Square Wave Voltammetry. From the experimental data it is possible to conclude that most of these ternary complexes show a pseudo-octahedral geometry with a CuN3O in plane chromophore and two oxygen atoms coming from water molecules perpendicularly bound to the equatorial plane. An exception to this general behaviour is given by the ternary copper(II) complex with 1,10­phenanthroline and histidine at pH value near the neutrality because of the terdentate nature of histidine when it coordinates by means of its histamine-like mode. In this case, evidence for a probable square-based pyramidal stereochemistry is given in support. At pH values around 5 the histidine behaves as bidentate ligand coordinating by its glycine-like mode, so as the copper(II) ternary complex with 1,10­phenathroline shows the pseudo-octahedral geometry found for all the ternary complexes with the other aminoacids. Moreover the ternary complex species with histidine at pH 5 and 7 are in equilibrium with each other as a function of the aqueous solution pH value and the temperature. In fact, the examination of low temperature EPR spectra at pH near 7 revealed not only a square-based pyramid complex but also products of decomposition. These results were also confirmed by the trend found in the formal redox potentials by the voltammetric measurements on many of these ternary complexes.