Vincenza Viti

PMR studies on the structures of water-ethyl alcohol mixtures

Abstract The PMR signals of each individual proton species present in water-ethanol mixtures were analyzed throughout the entire range of composition. Chemical shifts and line shapes were studied as a function of ethanol mole fraction ( f ), temperature, and working frequency, from −50 to +80°C, at 60, 100 and 220 MHz. The experimental results are discussed in terms of possible mechanisms of molecular aggregation in the various regions of ethanol concentration. There is evidence that addition of small quantities of ethanol to water promotes H-bonding association among water molecules (at t = 20°C, f ≲ 0.08). At intermediate ethanol concentrations (at t = 20°C, 0.25 ≲ f ≲ 0.75), the linear dependence of f exhibited the chemical shift of the water signal evidences that the water structures are progressively disrupted by increasing alcohol concentration. At the higher ethanol concentration (at 20°C, f ≳ 0.8) the substantial independence of both ethanol and water O H chemical shifts from the concentration suggests that water molecules either coordinate or/and are incorporated into the linear aggregates peculiar to pure ethanol. The width of the f range in which the hydroxyl signals coalesce is progressively reduced with decreasing temperature. We qualitatively tested the applicability of an adiabatic two-site exchange model to the hydroxyl protons of these mixtures. This model is correctly applicable to solutions at both extremes of the concentration range, but it cannot describe the linewidth behaviour of the mixtures at intermediate concentrations.

Conformational studies of ethylene glycol and its two methyl ether derivatives

As a theoretical analysis of the conformational equilibria of ethylene glycol, methoxyethanol and dimethoxyethane, the energy of each stable conformational isomer (rotamer) of these molecules was calculated for various temperatures and solvent dielectric constants. Classical semi-empirical potential functions were used. Besides intrinsic potentials for rotation about single bonds, intramolecular dispersion and repulsive interaction, dipole-dipole interaction and hydrogen bonding energies were included. Interaction with the solvent was considered only in terms of a continuous dielectric medium interacting with the local dipoles and quadrupoles of the molecule. For each rotamer, the dihedral angles giving the lowest energy were determined. From the energies of each rotamer, Boltzmann distributions of populations were obtained, and total concentrations were calculated in various physically distinguishable states, e.g. those with and without internal hydrogen bonds, or those in which the central C-C bond take...

RNA-lipid complexes released from the plasma membrane of human colon carcinoma cells

Cultured cells from human colon adenocarcinoma spontaneously release structures which display an intense 31P NMR signal from RNA and mobile phospholipids. Furthermore, the DPH probe in the cell supernatant shows an intense fluorescence, thus indicating its insertion in lipid vesicles. The total membranes, prepared from the same cells, also release similar structures. The fatty acid chain signals from the mobile lipids, observable in the H NMR spectrum, and the fluorescence polarization of the DPH probe are strongly affected by RNAase digestion, thus indicating an association between RNA molecules and lipids. The enzymatic marker cytochrome c reductase was assayed to rule out possible contamination from endoplasmatic reticulum. A high alkaline phosphatase activity was instead found in the supernatant samples, thus indicating that the shed material is released by the plasma membrane.

1H-MRS lipid signal modulation and morphological and ultrastructural changes related to tumor cell proliferation

Changes in signal intensity of 1H‐MRS mobile lipids, mostly triglycerides, have been observed in cultured HeLa cells during exponential growth. Signals were intense in the first days after seeding and almost disappeared after a few days in culture. Choline‐based metabolites were not affected by growth. Treatment with the antitumor drug lonidamine, which blocks cell proliferation and cell progression through cycle, increased lipid signal intensity. Morphological changes in the organization of the cell surface were detected by scanning electron microscopy during exponential growth, and were confirmed by freeze fracture analysis. The observed metabolic and structural modifications during cell growth were correlated to cell cycle progression of HeLa cells, as high‐intensity lipid signals were typical of cells with a high percentage of S‐ and G2 + M‐phases, while cells with a high frequency in G1‐phase were characterized by mobile lipid signals of very low intensity. Magn Reson Med 42:248–257, 1999.

Gamma-Irradiation Effects on Phosphatidylcholine Multilayer Liposomes: Calorimetric, NMR, and Spectrofluorimetric Studies

Chemical and structural modifications in multilayer liposomes of synthetic phosphatidylcholine induced by gamma irradiation are investigated with different techniques. Fluorescence anisotropy of the DPH probe and differential scanning calorimetry reveal a broadening of the main lipid transition and the disappearance of pretransition. Fluorescence anisotropy is shown to be higher in the irradiated sample and particularly so at low temperatures. NMR and TLC results show that lysolecithin and palmitic acid are formed with a consequent change in bilayer organization. The possibility that these modifications may account for the permeability variations observed in irradiated natural membranes is discussed.

VARIATIONS IN CIRCULAR DICHROISM AND PROTON-NMR RELAXATION PROPERTIES OF MELITTIN UPON INTERACTION WITH PHOSPHOLIPIDS

Melittin, a 26 amino acid cationic peptide extracted from bee venom [ 11, is well known for its ability to interact with phospholipid structures, dramatically increasing the permeability to aqueous solutes of either natural or artificial membranes [2]. Upon interaction with melittin, the aliphatic chains of the phospholipid molecules seem to undergo a pronounced immobilization, as evidenced by infrared and spin label studies [3,4]. As for the melittin molecule, its only tryptophan residue moves, upon interaction with phospholipids, from an aqueous environment to a hydrophobic one, as judged from the blue shift of its fluorescence maximum [5,6]; the changes in fluorescence characteristics are however dependent on the phospholipid used [6]. No change in the infrared absorption or infrared dichroism spectra of the peptide has been evidenced 141. We describe here how the intrinsic circular dichroism of melittin and the NMR relaxation properties of its tryptophan protons are modified upon interaction with phospholipids.

Dielectric properties of 2-methoxyethanol and 1,2-dimethoxyethane: Comparison with ethylene glycol

Abstract Low frequency dielectric constants ϵ of 2-methoxyethanol (MOE) and 1,2-dimethoxythane (DME) were measured as a function of tenperature (−20 ÷ +50°C), and the best fittings are given in an exponential form. Dipole moments μ of the same molecules were measured at different temperatures (DME at 25°C, MOE at 25, 39, 55°C) in diluted solution of apolar solvents (CCl4 and C6H6. The values of ϵ and μ are compared with those previously published. Also the density of ϱ of DME was measured as a function of temperature (−10 ÷ +40°C) and the best fitting is given in a linear form The correlation factor g, introduced by Kirkwood, is calculated for the two molecules and is compared with that of ethylene glycol (EG) calculated from the available data. The g values are discussed in the light of previous conformational studies, in order to give further insight into the association networks formed by intermolecular H bond. The results support the idea of the existence of three-dimensional branched structures in EG.

Maximum entropy spectral analysis of 31P NMR signals from human cells

Abstract Two autoregressive methods, namely Burg and Yule-Walker algorithms, have been applied to 31 P NMR free induction decays of packed human tumor cells where the signalto-noise ratio was limited by short duration records. The comparison between FFT and MEM power spectral density points out some advantages of the latter analysis technique. In particular, very good resolution and sensitivity can be reached even with very short data records. The choice of the autoregression order is discussed.

The use of the maximum entropy method in NMR spectroscopy

Abstract The maximum entropy method can be used to process noisy NMR signals with great advantages in sensitivity and resolution. This method, though employed extensively in spectral analysis, demands some care in its use mainly with respect to the correct choice of the model order. In this paper, we indicate the procedure for its application to NMR data, choosing different data points, orders of model and signal filterings. The signals analyzed are from chemical and biological samples and are characterized by a fairly good signal-to-noise ratio.

Characterization of vesicles, containing an acylated oligopeptide, released by human colon adenocarcinoma cells: NMR and biochemical studies

RNA‐containing vesicles, recovered from the supernatant of high‐density cell samples of human colon carcinoma, produce a high‐resolution 1H NMR spectrum of lipids characterized by isotropic tumbling; these vesicles contain large amounts of triglycerides and cholesterol esters. Both findings have strict analogies to what is displayed by the proteolipid complexes isolated from the sera of tumor‐bearing patients [(1985) Proc. Natl. Acad. Sci. USA 82, 3455–3459; (1986) FEBS Lett. 203, 164–168]. Lipid analysis and enzymatic tests indicate that these vesicles are selected micromaps of plasma membranes, analogous to those that can be recovered from culture media in which tumor cells are grown [(1985) Dev. Biol. 3, 33–57]. Peculiar lipids, an acylated oligopeptide and a modified phospholipid, are also present in the vesicles.