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Longhi G.,University of Brescia | Longhi G.,Consorzio Interuniversitario Science Fisiche della Materia | Fornili S.L.,University of Milan | Turco Liveri V.,University of Palermo
Physical Chemistry Chemical Physics | Year: 2015

Experimental investigations using mass spectrometry have established that surfactant molecules are able to form aggregates in the gas phase. However, there is no general consensus on the organization of these aggregates and how it depends on the aggregation number and surfactant molecular structure. In the present paper we investigate the structural organization of some surfactants in vacuo by molecular dynamics and well-tempered metadynamics simulations to widely explore the space of their possible conformations in vacuo. To study how the specific molecular features of such compounds affect their organization, we have considered as paradigmatic surfactants, the anionic single-chain sodium dodecyl sulfate (SDS), the anionic double-chain sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and the zwitterionic single-chain dodecyl phosphatidyl choline (DPC) within a wide aggregation number range (from 5 to 100). We observe that for low aggregation numbers the aggregates show in vacuo the typical structure of reverse micelles, while for large aggregation numbers a variety of globular aggregates occur that are characterized by the coexistence of interlaced domains formed by the polar or ionic heads and by the alkyl chains of the surfactants. Well-tempered metadynamics simulations allows us to confirm that the structural organizations obtained after 50 ns of molecular dynamics simulations are practically the equilibrium ones. Similarities and differences of surfactant aggregates in vacuo and in apolar media are also discussed. This journal is © the Owner Societies. Source

Castellucci N.,University of Bologna | Falini G.,University of Bologna | Milli L.,University of Bologna | Monari M.,University of Bologna | And 7 more authors.
ChemPlusChem | Year: 2014

Upon slow evaporation of a 1:1 diastereoisomeric mixture of Boc-(l-Phe-l-Oxd)2-OBn (1; Boc=tert-butyloxycarbonyl; l- Oxd=trans-(4S,5R)-4- carboxy 5-methyloxazolidin-2-one, Bn= benzyloxycarbonyl) and Boc-l-Phe-l-Oxd-d-Phe-l-Oxd-OBn (2) in methyl tert-butyl ether, single crystals suitable for an X-ray diffraction study were obtained. In contrast, the two pure oligomers lead to the formation of amorphous solids under any crystallization conditions. The preferential conformation of both oligomers was fully elucidated in the solid phase and compared with the known conformation of Boc-(l-Phe-d- Oxd)2-OBn (3). The preferred conformation of 1 ranges from a polyproline II (PPII) helix to b strands and we can gather that longer and more structured oligomers will form PPII helices. In contrast, compound 3 forms infinite antiparallel b-sheet structures; thus showing the strong effect of the reversal of the absolute configuration of the Oxd moieties on the secondary structure of these hybrid foldamers. The same outcome was retained in solution, as demonstrated by vibrational circular dichroism analysis. Finally, we have demonstrated that a 1:1 mixture of 1 and 2 leads to the formation of new materials with interesting properties that are missing from the two pure compounds, such as the tendency to form crystals, fibers, and globules, depending on the solvent. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

Longhi G.,University of Brescia | Longhi G.,Consorzio Interuniversitario Science Fisiche della Materia | Ceselli A.,University of Milan | Fornili S.L.,University of Milan | And 5 more authors.
Journal of Mass Spectrometry | Year: 2013

The behavior of aqueous solutions of sodium bis(2-ethylhexyl)sulfosuccinate (AOTNa) subject to electrospray ionization (ESI) has been investigated by molecular dynamics (MD) simulations at three temperatures (350, 500 and 800 K). We consider several types of water nanodroplets containing AOTNa molecules and composed of a fixed number of water molecules (1000), N0 AOT AOT- anions (N0 AOT = 0, 5, 10) and N0 Na sodium ions (N0 Na = 0, 5, 10, 15, 20): in a short time scale (less than 1 ns), the AOTNa molecules, initially forming direct micelles in the interior of the water nanodroplets, are observed in all cases to diffuse nearby the nanodroplet surface, so that the hydrophilic heads and sodium ions become surrounded by water molecules, whereas the alkyl chains lay at the droplet surface. Meanwhile, evaporation of water molecules and of solvated sodium ions occurs, leading to a decrease of the droplet size and charge. At 350 K, no ejection of neutral or charged surfactant molecules is observed, whereas at 500 K, some fragmentation occurs, and at 800 K, this event becomes more frequent. The interplay of all these processes, which depend on the values of temperature, N0 AOT and N 0 Na eventually leads to anhydrous charged surfactant aggregates with prevalence of monocharged ones, in agreement with experimental results of ESI mass spectrometry. The quantitative analysis of the MD trajectories allows to evidence molecular details potentially useful in designing future ESI experimental conditions. Copyright © 2013 John Wiley & Sons, Ltd. Copyright © 2013 John Wiley & Sons, Ltd. Source

Longhi G.,University of Brescia | Longhi G.,Consorzio Interuniversitario Science Fisiche della Materia | Abbate S.,University of Brescia | Abbate S.,Consorzio Interuniversitario Science Fisiche della Materia | And 4 more authors.
Physical Chemistry Chemical Physics | Year: 2011

Positively charged supramolecular aggregates formed in vacuo by n AOTNa (sodium bis(2-ethylhexyl)sulfosuccinate) molecules and n c additional sodium ions, i.e. [AOT nNa n+nc]n c, have been investigated by molecular dynamics (MD) simulations for n = 1-20 and n c = 0-5. Statistical analysis of physical quantities like gyration radii, atomic B-factors and moment of inertia tensors provides detailed information on their structural and dynamical properties. Even for n c = 5, all stable aggregates show a reverse micelle-like structure with an internal solid-like core including sodium counterions and surfactant polar heads surrounded by an external layer consisting of the surfactant alkyl chains. Moreover, the aggregate shapes may be approximated by rather flat and elongated ellipsoids whose longer axis increases with n and n c. The fragmentation patterns of a number of these aggregates have also been examined and have been found to markedly depend on the aggregate charge state. In one particular case, for which experimental findings are available in the literature, a good agreement is found with the present fragmentation data. © 2011 the Owner Societies. Source

Abbate S.,University of Brescia | Abbate S.,Consorzio Interuniversitario Science Fisiche della Materia | Lebon F.,University of Brescia | Lebon F.,Consorzio Interuniversitario Science Fisiche della Materia | And 5 more authors.
RSC Advances | Year: 2012

The absolute configuration of 1-and 2-tetralol (1a and 2a), of 5-hydroxy- and 8-hydroxy-2-tetralol (2b and 2c), and of 1-aminotetralin (1b) has been established by comparison of experimental infrared vibrational absorption (VA) and vibrational circular dichroism (VCD) spectra and density functional theory (DFT) calculations. DFT calculations using B3PW91 functional with TZ2P basis set were carried out to predict the VA and VCD spectra. Electronic circular dichroism (ECD) spectra were also measured and compared to time-dependent DFT calculations. The VCD data have been found to be more specific than ECD.© 2012 The Royal Society of Chemistry. Source

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