Mirella Ferrugia

Interactions of melatonin with membrane models : Portioning of melatonin in AOT and lecithin reversed micelles

Abstract: The interaction of melatonin with water containing either sodium bis (2‐ethylhexyl) sulfosuccinate (AOT) or soybean phosphatidylcholine (lecithin) reversed micelles has been investigated by UV absorption spectroscopy, at a molar ratio of melatonin: surfactant 1: 800 for AOT and 1: 400 for lecithin reversed micelles, and by varying the water:surfactant molar ratio (R). Our results suggest that in the presence of domains from apolar organic solvent to surfactant and to water, melatonin positions itself in the micellar phase, with a preferential location in the surfactant polar head group domain, independent of the nature of the surfactant and the amount of water encapsulated into the micellar core. Effects are due to the hydrophilic and lipophilic moieties of melatonin. The effectiveness of melatonin as an electron donor and free radical scavenger has been recently recognized. While supporting the hypothesis that melatonin may provide antioxidant protection without the benefit of receptors, present findings may suggest that the molecule could easily scavenge aqueous as well as lipophilic radicals.

Localization and interactions of melatonin in dry cholesterol/lecithin mixed reversed micelles used as cell membrane models

Abstract:  The state of melatonin confined in dry cholesterol/lecithin mixed reversed micelles dispersed in CCl4 was investigated using 1H‐NMR and FT‐IR spectroscopies as a function of the melatonin to lecithin molar ratio (RMLT) and of the cholesterol to lecithin molar ratio (RCHL). An analysis of experimental results leads to the hypothesis that, independent of RMLT and as a consequence of anisotropic melatonin/lecithin, melatonin/cholesterol and cholesterol/lecithin interactions, melatonin is totally solubilized in reversed micelles. Melatonin is mainly located in and oriented in the nanodomain constituted by the hydrophilic groups of cholesterol and lecithin. A competition of melatonin and cholesterol for the hydrophilic binding sites of the reversed micelles was observed by changing the RCHL. Some possible biological implications of the specific interactions governing the solubilization process, the preferential location and the peculiar properties of melatonin confined in cholesterol/lecithin mixed reversed micelles are discussed.

Effects of the net charge on abundance and stability of supramolecular surfactant aggregates in gas phase.

Self-assembling of amphiphilic molecules under electrospray ionization (ESI) conditions is characterized by quite unexpected phenomenology. The noticeable differences with respect to the condensed phase are attributable to the absence of the surfactant-solvent interactions, the presence of net charge in the aggregates, and the strong deviation from equilibrium conditions. Aiming to investigate the effects of the net charge on abundance and stability of supramolecular surfactant aggregates, positively and negatively charged aggregates of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) and sodium methane sulfonate (MetS), butane sulfonate (ButS) and octane sulfonate (OctS) have been studied by ESI mass spectrometry, energy resolved mass spectrometry and density functional theory calculations. The negatively charged aggregates are found to be less stable than their positive counterparts. The results are consistent with a self-assembling pattern dominated by electrostatic interactions involving the counterions and head groups of the investigated amphiphilic compounds while the alkyl chains point outwards, protecting the aggregates from unlimited growth processes.

Spectrophotometric investigation of the binding of vitamin E to water-containing reversed micelles.

The distribution constants of vitamin E partitioned between apolar organic phase and water-containing reversed micelles of sodium bis (2-ethylhexyl) sulfosuccinate (AOT), didodecyldimethylammonium bromide (DDAB), soybean phosphatidylcholine (lecithin) and tetraethylene glycol monododecyl ether (C12E4) have been evaluated by a spectrophotometric method. The results suggest that in the presence of domains from apolar organic solvent to surfactant and to water, vitamin E is partitioned between the micellar palisade layer and the organic solvent and also that its binding strength to reversed micelles depends mainly by specific interactions between the head group of vitamin E and that of the surfactant. Moreover, in addition to the advantageous interactions between vitamin E and water, the dependence of the distribution constants upon the molar ratio R (R=[water]/[surfactant]) indicates a competition between water and vitamin E for the binding sites at the water/surfactant interface. The biological implications of the preferential location and confinement of vitamin E in water-containing reversed micelles are discussed.

A new general fragmentation reaction in mass spectrometry: The hydrogen-carbon, carbon-carbon double rearrangement of 2 heteroalkyl substituted diphenylmethyl cations

Diphenylmethyl cations formed by benzylic cleavage of the molecular ions of ortho heteroalkyl substituted 1,1-diphenylalkanes undergo the double rearrangement process (H to C followed by C to C) previously reported for ortho-methoxy derivatives. Hence the formation of substituted benzyl (or tropylium) ions allowing this double rearrangement process constitutes an interesting type of fragmentation reaction characteristic for 1,1-diphenylalkanes bearing ortho substituents (OMe, OEt, OiPr, SMe, NHMe, NMe2) which are able to transfer a hydride to the charged benzyl carbon of diphenylmethyl cations formed by benzylic cleavage of the molecular ion.

Studies in Organic Mass Spectrometry; Part 26. Unimolecular Decomposition of Ortho-Methoxy-Substituted Diphenylmethyl Cations

A double rearrangement (hydrogen migration followed by carbon–carbon displacement) constitutes the main fragmentation process of the 2-methoxydiphenylmethyl cations formed by benzylic cleavage of the molecular ion of ortho-methoxy-substituted 1,1-diphenylalkanes with a wide range of internal energy. This has been demonstrated by recording mass analysed ion kinetic energy (MIKE) spectra, calculation of approximate activation energies (obtained by appearance energy difference) and calculation of a degrees-of-freedom effect. A geometry of the transition state, which accounts for the observed substituent effects, is proposed.

Studies in organic mass spectrometry. Part 23. a Role of the aroyl group on the competitive fragmentation reactions of the molecular ion of aroylanilides

The 70 eV and mass-analysed ion kinetic energy (MIKE) spectra of some thiophenecarboxanilides and benzoylanilides (1–10) have been compared in order to investigate the role of the aroyl (or heteroaroyl) moiety on the abundance of the competitive fragmentation reactions occurring in their molecular ions (amide–bond cleavage and phenol radical ion formation). It has been shown that the electron ionisation induced decompositions with high (70 eV) and low (MIKE) internal energy excess are qualitatively similar, but remarkable quantitative differences have been observed that can be accounted for in terms of the different effectiveness in the transmission of electronic effects of substituents in the thiophene and benzene rings.

Hydrogencarbon, carboncarbon double rearrangement induced by proximity effect in alkyldiaryl- and triarylmethyl cations

The double rearrangement process affording benzyl (or tropylium) ions, which constitutes the main fragmentation reaction for 2-alkylheterodiphenylmethyl cations, is also observed in the presence of a further substituent (alkyl or aryl) linked to the benzylic carbon. This process is still responsible for important peaks in the mass spectra.

Gas-phase ion chemistry of protonated melatonin.

The gas-phase ion chemistry of protonated melatonin has been investigated by mass spectrometric (MS) techniques involving chemical ionisation, fast atom bombardment and electrospray ionisation. Either naturally-occurring or collision-induced decomposition experiments have been carried out using side chain tetra-deuterium-labelled derivatives, as well as the corresponding N–D derivatives obtained by exchange with D2O. The analysis of experimental results allows definite pathways for the formation of the ion at m/z 174 to be assigned and sheds some more light on the overall fragmentation pathways. Experiments on labelled derivatives evidenced H–D scrambling processes during fragmentation.

Studies in Organic Mass Spectrometry. Part 20: A Hidden Ortho Effect in the Electron Ionisation Mass Spectra of Some 2′-Alkyl Substituted 2- and 3-Thiophenecarboxanilides

The electron-ionisation-induced amide-bond cleavage of some 2′-methyl- and 2′-ethyl-substituted 2- and 3-thiophenecarbox-anilides, which yields formally anilylium ions having relative intensities apparently in contrast with the Stevenson–Audier rule, has been investigated by mass-analysed ion kinetic energy (MIKE) spectrometry and compared to that of the 3′- and 4′-isomers. It has been shown that, in the case of the 2′-methyl and 2′-ethyl derivatives, the amide-bond cleavage is anchimerically assisted through the hidden migration of a benzyl hydrogen to the nitrogen. Analysis of the MIKE and collision-induced decomposition (CID) MIKE spectra of model compounds indicates that this cryptic ortho effect produces a stable ortho quinoide or aminotropylium structure.