Do electrospray mass spectra of surfactants mirror their aggregation state in solution?

One important feature in the gas phase chemistry of surfactants is to ascertain whether their aggregates produced by electrospray ionization reflect those formed in the starting solution. With this aim, we have performed ESI-MS, ESI-MS/MS and ER-MS spectra of bis(2-ethylhexyl)sulfosuccinate (AOTNa) solutions in different solvents, i.e. water, water/methanol, methanol and n-hexane. The results clearly indicate that, notwithstanding the strongly different aggregation state in solution (direct micelles in water and in water/methanol, molecular dispersion in methanol and reverse micelles in n-hexane) and marked effects of the solvent polarity on the total ionic current, the surfactant aggregates in gas phase show identical structural features. Analogous conclusions can be drawn analyzing the infrared multiple photon dissociation (IRMPD) spectra of AOTNa solutions in water/methanol and n-hexane. Moreover, according to the idea that gas phase can be considered an apolar environment par excellence, data consistently suggest a reverse micelle-like aggregation. Some peculiarities of the mechanisms leading to aggregate formation through electrospray ionization of surfactant solutions in solvent media with different polarity have been also discussed.

Interactions of alpha-tocopherol with biomembrane models: binding to dry lecithin reversed micelles.

The state of alpha-tocopherol (Vitamin E) in solutions of dry lecithin reversed micelles dispersed in an apolar medium has been investigated as a function of the Vitamin E to surfactant molar ratio (RVE) at fixed surfactant concentration by FT-IR, 1H NMR and SAXS with the aim to emphasize the role played by anisotropic intermolecular interactions and confinement effects as driving forces of its partitioning between apolar bulk solvent and polar nanodomains and of mutual Vitamin E/reversed micelle effects. It has been found that its binding strength to reversed micelles, triggered by steric and orientational constrains, is mainly regulated by specific interactions between the hydrophilic groups both of Vitamin E and surfactant. Moreover, the RVE dependence of the Vitamin E distribution constant and of the micellar size suggest that the inclusion of increasing amounts of Vitamin E in reversed micelles involves substantial changes in the structural and dynamical properties of the micellar aggregates. The occurrence of mutual effects and the partitioning of Vitamin E between hydrophilic/hydrophobic interfaces and apolar domains allow to infer some important biological implications concerning the capacity of Vitamin E to scavenge free radicals arising from hydrophilic and/or hydrophobic domains, possible variations of its local reactivity respect to that observed in bulk as well as its significant influence on the stability of biomembranes.

Surfactant self-assembling in gas phase: electrospray ionization- and matrix-assisted laser desorption/ionization-mass spectrometry of singly charged AOT clusters.

The self-assembling of sodium bis (2-ethylhexyl) sulfosuccinate (AOT) in gas phase has been investigated by electrospray ionization- and matrix-assisted laser desorption/ionization mass spectrometry. Large surfactant clusters with an aggregation number close to that found in apolar media have been observed either as positive or negative ions. Moreover, the marked predominance of singly charged species as well as preliminary theoretical calculations strongly suggest an aggregate structure characterized by an internal hydrophilic core hosting the extra charge surrounded by an apolar shell constituted by the surfactant alkyl chains. This structure is similar to that of the more familiar reversed micelles formed when an appropriate surfactant is solubilized in apolar solvents. Finally, similar trends are observed independently either on the ionization technique or the polarity of the solvent used. This, together with the large dependence of the aggregation number on the flow rates, strongly indicates that self-assembling of the surfactant molecules occurs during the evaporation step.

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.

Guest-to-host proton transfer in melatonin-beta-cyclodextrin inclusion complex by ionspray, fast atom bombardment and tandem mass spectrometry.

Ionspray (IS) and fast atom bombardment (FAB) positive ionization mass spectrometry (MS) of 1 : 1 beta-cyclodextrin (beta-CD)-melatonin (MLT) host-guest complex allowed the detection of gaseous protonated 1 : 1 beta-CD-MLT. Tandem MS collision-induced dissociation (CID) of such protonated 1 : 1 beta-CD-MLT species showed the proton (charge) to be retained to a significant extent by the host and by its cage fragmentation products, in spite of the higher proton affinity of MLT with respect to that of beta-CD. This requires an endothermic guest-to-host proton transfer to occur within the gaseous association. Collisional activation could be accounted for by the promotion of such an endothermic process; however, the proton affinity decrease of the guest determined by the loss of the elements of acetamide, which is a dominant MS dissociation reaction of pure protonated MLT, could also provide a rationale for such an endothermic guest-to-host proton transfer. This proposal parallels the reaction scheme we had previously formulated for the analogous MS and tandem MS behaviour of 1 : 1 beta-CD-5-methoxytryptamine inclusion complex with the protonated 5-methoxytryptamine guest undergoing deamination.

Detection and localisation of disulphide bonds in a synthetic peptide reproducing the sequence 1-30 of Par j 1.0101 by electrospray ionisation mass spectrometry.

The structural characterisation of a synthetic peptide reproducing the sequence 1-30 of Par j 1.0101, a major allergenic protein present in the pollen of Parietaria judaica, by combined use of chemical and enzymatic cleavage, reversed-phase high-performance liquid chromatography and electrospray ionisation mass spectrometry (ESI-MS), is described. Direct ESI-MS of the synthetic peptide after reaction with methyl iodide showed that the product is a mixture of two peptides: one form in which two out of the four cysteine residues present in the sequence are oxidised and a minor amount of another form in which all the cysteines are fully reduced. It was ascertained, using the combined procedure indicated above and without prior separation of the two species, that the disulphide bond in the partially oxidised form is located between cysteines 29 and 30. These results show the usefulness of this approach for characterising synthetic peptides containing multiple cysteine residues in the sequence.

Interactions of tryptophan and serotonin with biomembrane models: binding to reversed micellar systems of ionic and non ionic surfactants.

The interactions of Tryptophan (TRP) and Serotonin (5-HT), with water-containing sodium bis (2-ethylhexyl) sulfosuccinate (AOT), didodecyldimethylammonium bromide (DDAB) and tetraethylene glycol monododecyl ether (C12E4) reversed micelles have been investigated by UV absorption spectroscopy. Our results suggest that independently of the nature of the surfactant and the amount of the water encapsulated into the micellar core, TRP and 5-HT are solubilized in the micellar phase, preferring to be located in a shallow region constituted by the hydrated surfactant head groups. This is due to the amphiphilic nature of TRP and 5-HT and the biological implications are discussed.

Oxidation of melatonin by oxoferryl hemoglobin: a mechanistic study.

Reaction of melatonin with the hypervalent iron centre of oxoferryl hemoglobin, produced in aqueous solution from methemoglobin and H2O2, has been investigated at 37 degrees C and pH 7.4, by absorption spectroscopy. The reaction results in reduction of the oxoferryl moiety with formation of a heme-ferric containing hemoprotein. Stopped-flow spectrophotometric measurements provide evidence that the reduction of oxoferryl-Hb by melatonin is first-order in oxoferryl-Hb and first-order in melatonin. The bimolecular reaction constant at pH 7.4 and 37 degrees C is 112 +/- 1.0 M(-1) s(-1). Two major oxidation products from melatonin have been found by gas chromatography-mass spectroscopy: the cyclic compound 1,2,3,3a,8,8a-hexahydro-1-acetyl-5-methoxy-3a-hydroxypyrrolo[2,3-b]indole (cyclic 3-hydroxy-melatonin), and N-acetyl-N'-formyl 5-methoxykynuramine (AFMK). The percentage yield of the two major products appears dependent on the ratio [oxoferryl-Hb]:[melatonin]--the higher the ratio the higher the yield of AFMK. The observed stoichiometry oxoferryl-Hb(reduced):melatonin(consumed) is 2, when the ratio [oxoferryl-Hb]:[melatonin] is 1:1, but appears >2 at higher molar ratios. The reduction of the hypervalent iron of the oxoferryl moiety may be consistent with an oxidation of melatonin by two one-electron steps.

Studies in organic mass spectrometry. Part 25. Benzyl ion formation in chemical ionisation (methane or isobutane) of some ortho-alkylhetero-substituted diphenylcarbinols

The behaviour of some ortho-alkylhetero-substituted diphenylcarbinols, including deuterium labelled derivatives, under chemical ionisation (methane or isobutane) conditions has been investigated. It has been determined that ortho-alkylhetero diphenylmethyl cations formed by water elimination from the protonated molecule undergo consecutive rearrangements which afford benzyl (or tropylium) ions previously observed for ortho-substituted diphenylcarbenium ions generated by electron ionisation. This reaction also occurs under low-energy collision conditions. Copyright 2000 John Wiley & Sons, Ltd.

Studies in organic mass spectrometry. Part 24dagger electron ionization mass spectra of some aryl(2-nitrobenzo

The main fragmentation routes of eighteen title compounds and of three 5-chloro derivatives have been investigated with the aid of linked scan (B/E = constant) spectrometry, accurate mass measurements and deuterium labelling. Copyright 1999 John Wiley & Sons, Ltd.

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

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.

The structure and the stereochemistry of atractyliretin.

The nor-kaurane diterpene Atractyliretin was obtained by acid hydrolysis of Atractyloside, a toxic substance isolated from ATRACTYLIS GUMMIFERA L (Compositae). On the basis of spectral (IR, (1)H-NMR, (13)C-NMR and MS) analysis and chemical degradation its structure and stereochemistry was identified as 4.