Adamantyl ferrocene derivatives: Antioxidant abilities and effects on model lipid membranes

A series of homoannularly and heteroannularly substituted adamantyl ferrocene derivatives has been synthesized and their effects on membrane fluidity were investigated using liposomes as the membrane models. The liposome formulations of adamantyl ferrocene derivatives were characterized by using dynamic light scattering, differential scanning calorimetry and fluorescence anisotropy measurements. It was demonstrated that adamantyl ferrocene derivatives incorporated into the liposome significantly affect the structure of the lipid bilayer. The results of the study have revealed that adamantyl ferrocene derivatives, compounds 9 – 12, partition into the hydrophobic/hydrophilic interface of the membrane, causing a significant decrease in membrane fluidity. The antioxidant potential of synthesized compounds was assessed with DPPH method and it was shown that the examined compounds possess certain antioxidant activity.     

Theoretical calculations and experimental data on spectral,kinetic and thermodynamic properties of Se∴N and S∴N three-electron-bonded,structurally stabilized σ<Superscript>2</Superscript>σ* radicals

A complementary quantum mechanical and experimental study has been undertaken on the reactivity, formation and properties of Se∴N and S∴N σ²/σ* three-electron-bonded radical species, generated upon one-electron oxidation of selenomethionine, methionine and structurally related compounds. The quantum chemical calculations were based on density functional theory (DFT) hybrid B3LYP and BHandHLYP methods with basis sets ranging from 6-31G(d) to 6-311+G(d,p). Solvent effects, which play an important role concerning structure and energy of ground and excited states, were taken into account as dielectric continuum as well as explicit water molecules. They fully confirm new and previously obtained experimental results concerning the Vis/near-UV absorptions and thermodynamic stability. Special emphasis was put on a comparison between selenium and sulfur. The calculations clearly confirm the higher thermodynamic stability of the Se∴N radical species relative to the S∴N ones, and also corroborate the observed much higher kinetic stability of the former. Concerning optical absorptions, the calculations predict the Se∴N transients to exhibit a blue-shift by about 20 nm relative to the S-based analogues, confirming the few experimental data available so far. The theoretical study includes a comparison of various calculation levels and the influence of the solvent environment, by comparison with vacuum. New experimental data within the scope of this study have been obtained on intramolecularly-formed S∴N radical cation moieties, structurally stabilized by a rigid norbornane backbone. The methionine-related species, with an endo-2-amino, exo-2-carboxyl, and endo-6 methylthio substitution, for example, exhibits almost identical optical and kinetic stability properties as the corresponding species from free methionine. Its optical absorption depends on the protonation state of the carboxyl group, with λ_max at 410 nm for the carboxylate (zwitterionic) form and at 390 nm for the overall cationic form with the protonated carboxyl group. The fast exponential decays with t _1/2 of 490 ns and 2 μs pertain to the decarboxylation of the respective species. A much longer-lived S∴N species with t _1/2 > 500 μs and second order decay kinetics (λ_max 465 nm) was obtained from an endo-2-cyclohexylamino norbornane analogue which does not carry a carboxyl group. The methionine-based S∴N species is not stable anymore in vacuum and in low polarity solvents. This is explained by a decrease in stabilization energy of the 3-e-bond and a faster electron transfer from the carboxylate into the cationic 3-e-center. In conclusion, selenium enhances the thermodynamic and kinetic stability of its radical transients, relative to the sulfur analogues.     

Gold chloride cluster ions generated by vacuum laser ablation

In this work, a simple way for study the possibility of formation a vapor cluster species of tetrachloroauric acid (HAuCl₄), using the laser ablation in the absence of a buffer or reactive atmosphere, and without a postablation supersonic expansion on a commercial matrix assisted laser desorption/ionization time-of-flight mass spectrometer, is reported. Tetrachloroauric acid is known as precursor for the synthesis of gold nanostructures and the complex salts; therefore it is an important task to discover and quantify the species arising from HAuCl₄, in order to understand their role in the gold assisted reactions. Mass spectrum of HAuCl₄ in a reflector negative-ion mode contains the hydrated mono- and dinuclear gold clusters in the m/z range 286–436, and gold chloride clusters in the m/z range 447–795. In the first part of spectrum, m/z range 286–436, the hydrated gold cluster species of type Au _n ^? (H₂O)_m (n?=?1–2; m?=?1, 2, 5, 7, 8) and [Au_n(OH)_k]^?(H₂O)_m (n?=?1–2; k?=?1–2; m?=?1, 4–8) were found. Besides that, there are gold chloride clusters with general formula [AuH_r(HCl)₂]^?(H₂O)_m (m?=?1–5; 8–9; r?=?0–2) in this part of spectrum. In the second part of spectrum, the m/z range 447–795, only gold chloride clusters were obtained. Their general formulae can be written as [AuCl_t(HCl)_v]^?(H₂O)_m (t?=?1–4; v?=?5–8; m?=?2–4, 6–8) and [Au_n(HCl)_v]^?(H₂O)_m (n?=?1–2, v?=?4–5, m?=?1–2, 5, 7). The analysis of concentration effects on the LDI mass spectra of gold clusters reveals that the relative intensities of signals for the mono- and dinuclear Au clusters increase with decreasing the concentration of water HAuCl₄ solutions.     

Morphological characterization of ex situ prepared bismuth film electrodes and their application in electroanalytical determination of the biomolecules

A study on the preparation and characterization of the potentiostatically prepared bismuth films (BiFs), in order to obtain satisfactory electroanalytical tool, is presented. BiFs formed on glassy carbon were characterized by electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The dependence of the BiFs properties upon electrolytes composition and electrochemical parameters are discussed and diagnostic criteria that allow estimation of the BiF morphology are proposed. Analytical performance data of the formed BiF electrodes were obtained by their application to the determination of glutathione (GSH) and folic acid (FA) using square-wave cathodic stripping voltammetry. The dependence of the analytical performance of the formed BiFs on their specific surface areas, along with their different morphology, is discussed. Adequate method and parameters for the electrochemical formation of optimal BiF, in order to fulfill the analytical requirements, are proposed. The best analytical performance was obtained with films formed from acetic buffer solution spiked with EDTA, as a consequence of the improved surface coverage and most arranged homogenous structure of the film. This electrode displays a linear response range toward GSH with estimated detection limit of 0.005 μM and sensitivity of 3.28 μA μM^?1 for linear range of 0.01 to 0.1 μM. Also, the utilization of the BiF electrode for the determination of FA was demonstrated by direct electroreduction of FA.     

Spin-label W-band EPR with Seven-Loop–Six-Gap Resonator: Application to Lens Membranes Derived from Eyes of a Single Donor

Spin-label W-band (94 GHz) electron paramagnetic resonance (EPR) with a five-loop–four-gap resonator (LGR) was successfully applied to study membrane properties (Mainali et al. J Magn Reson 226:35–44, 2013). In that study, samples were equilibrated with the selected gas mixture outside the resonator in a sample volume ~100 times larger than the sensitive volume of the LGR and transferred to the resonator in a quartz capillary. A seven-loop–six-gap W-band resonator has been developed. This resonator permits measurements on aqueous samples of 150 nL volume positioned in a polytetrafluoroethylene (PTFE) gas permeable sample tube. Samples can be promptly deoxygenated or equilibrated with an air/nitrogen mixture inside the resonator, which is significant in saturation-recovery measurements and in spin-label oximetry. This approach was tested for lens lipid membranes derived from lipids extracted from two porcine lenses (single donor). Profiles of membrane fluidity and the oxygen transport parameter were obtained from saturation-recovery EPR using phospholipid analog spin-labels. Cholesterol analog spin-labels allowed discrimination of the cholesterol bilayer domain and acquisition of oxygen transport parameter profiles across this domain. Results were compared with those obtained previously for membranes derived from a pool of 100 lenses. Results demonstrate that EPR at W-band can be successfully used to study aqueous biological samples of small volume under controlled oxygen concentration.     

Chromatography,mass spectrometry,and molecular modeling studies on ammodytoxins

The ammodytoxins (Atxs) are neurotoxic phospholipases which occur in Vipera ammodytes ammodytes (Vaa) snake venom. There are three Atx isoforms, A, B, and C, which differ in only five amino acid positions at the C-terminus but differ substantially in their toxicity. The objective of this study was to establish an analytical method for unambiguous identification of all three isoforms and to use the method to assess a procedure for purification of the most toxic phospholipase, AtxA, from the venom. Isolation procedure for AtxA consisted of isolation of Atx-cross-reactive material (proteins recognized by anti-Atx antibodies), by use of an affinity column, then cation exchange on CIM (Convective Interaction Media) disks. The purification procedure was monitored by means of reversed-phase chromatography (RPC) and mass spectrometry (MS). Although previous cation exchange of the pure isoforms enabled separate elution of AtxA from B and C, separation of AtxA from Atxs mixture was not accomplished. RPC was not able to separate the Atx isoforms, whereas an MS based approach proved to be more powerful. Peptides resulting from tryptic digestion of Atxs which enable differentiation between the three isoforms were successfully detected and their sequences were confirmed by post-source decay (PSD) fragmentation. Separation of Atx isoforms by ion-exchange chromatography is most presumably prevented by Atxs heterodimer formation. The tendency of Atxs to form homodimers and heterodimers of similar stability was confirmed by molecular modeling.     

Gas-phase conjugation to arginine residues in polypeptide ions via N-hydroxysuccinimide ester-based reagent ions

Gas-phase conjugation to unprotonated arginine side-chains via N-hydroxysuccinimide (NHS) esters is demonstrated through both charge reduction and charge inversion ion/ion reactions. The unprotonated guanidino group of arginine can serve as a strong nucleophile, resulting in the facile displacement of NHS from NHS esters with concomitant covalent modification of the arginine residue. This reactivity is analogous to that observed with unprotonated primary amines such as the N-terminus or ε-amino group of lysine. In solution, however, the arginine residues tend to be protonated at pH values low enough to prevent hydrolysis of NHS esters, which would render them relatively unreactive with NHS esters. This work demonstrates novel means for gas-phase conjugation to arginine side chains in polypeptide ions.     

Thermodynamic characterization of 3-[(3-cholamidopropyl)-dimethylammonium]-1-propanesulfonate (CHAPS) micellization using isothermal titration calorimetry: temperature, salt, and pH dependence

A systematic investigation of the micellization process of a biocompatible zwitterionic surfactant 3-[(3-cholamidopropyl)-dimethylammonium]-1-propanesulfonate (CHAPS) has been carried out by isothermal titration calorimetry (ITC) at temperatures between 278.15 K and 328.15 K in water, aqueous NaCl (0.1, 0.5, and 1 M), and buffer solutions (pH = 3.0, 6.8, and 7.8). The effect of different cations and anions on the micellization of CHAPS surfactant has been also examined in LiCl, CsCl, NaBr, and NaI solutions at 308.15 K. It turned out that the critical micelle concentration, cmc, is only slightly shifted toward lower values in salt solutions, whereas in buffer media it remains similar to its value in water. From the results obtained, it could be assumed that CHAPS behaves as a weakly charged cationic surfactant in salt solutions and as a nonionic surfactant in water and buffer medium. Conventional surfactants alike, CHAPS micellization is endothermic at low and exothermic at high temperatures, but the estimated enthalpy of micellization, ΔHM0, is considerably lower in comparison with that obtained for ionic surfactants in water and NaCl solutions. The standard Gibbs free energy, ΔGM0, and entropy, ΔSM0, of micellization were estimated by fitting the model equation based on the mass action model to the experimental data. The aggregation numbers of CHAPS surfactant around cmc, obtained by the fitting procedure also, are considerably low (nagg ≈ 5 ± 1). Furthermore, some predictions about the hydration of the micelle interior based on the correlation between heat capacity change, Δcp,M0, and changes in solvent-accessible surface upon micelle formation were made. CHAPS molecules are believed to stay in contact with water upon aggregation, which is somehow similar to the micellization process of short alkyl chain cationic surfactants.     

Adsorption of lipid liquid crystalline nanoparticles: effects of particle composition, internal structure, and phase behavior

Controlling the interfacial behavior and properties of lipid liquid crystalline nanoparticles (LCNPs) at surfaces is essential for their application for preparing functional surface coatings as well as understanding some aspects of their properties as drug delivery vehicles. Here we have studied a LCNP system formed by mixing soy phosphatidylcholine (SPC), forming liquid crystalline lamellar structures in excess water, and glycerol dioleate (GDO), forming reversed structures, dispersed into nanoparticle with the surfactant polysorbate 80 (P80) as stabilizer. LCNP particle properties were controlled by using different ratios of the lipid building blocks as well as different concentrations of the surfactant P80. The LCNP size, internal structure, morphology, and charge were characterized by dynamic light scattering (DLS), synchrotron small-ange X-ray scattering (SAXS), cryo-transmission electron microscopy (cryo-TEM), and zeta potential measurements, respectively. With increasing SPC to GDO ratio in the interval from 35:65 to 60:40, the bulk lipid phase structure goes from reversed cubic micellar phase with Fd3m space group to reversed hexagonal phase. Adding P80 results in a successive shift toward more disorganized lamellar type of structures. This is also seen from cryo-TEM images for the LCNPs, where higher P80 ratios results in more extended lamellar layers surrounding the inner, more dense, lipid-rich particle core with nonlamellar structure. When put in contact with a solid silica surface, the LCNPs adsorb to form multilayer structures with a surface excess and thickness values that increase strongly with the content of P80 and decreases with increasing SPC:GDO ratio. This is reflected in both the adsorption rate and steady-state values, indicating that the driving force for adsorption is largely governed by attractive interactions between poly(ethylene oxide) (PEO) units of the P80 stabilizer and the silica surface. On cationic surface, i.e., silica modified with 3-aminopropltriethoxysilane (APTES), the slightly negatively charged LCNPs give rise to a very significant adsorption, which is relatively independent of LCNP composition. Finally, the dynamic thickness measurements indicate that direct adsorption of intact particles occurred on the cationic surface, while a slow buildup of the layer thickness with time is seen for the weakly interacting systems.