Claire Loison

Microsolvation Effects on the Optical Properties of Crystal Violet

We present a joint experimental and theoretical study of the photoabsorption and photodissociation behavior of crystal violet, that is, the tris[p-(dimethylamino)phenyl]methyl cation. The photodissociation spectra of isolated and microsolvated crystal violet have been measured. A single band is observed for the bare cation. This is in good agreement with the calculated vibronic absorption spectrum based on time-dependent density functional theory calculations. The interaction of crystal violet with a single water molecule shifts and broadens the photodissociation spectrum, so that it approaches the spectrum obtained in solution. Theoretical calculations of the structure of the complex suggest that the shift in the absorption spectrum originates from a water molecule bonding with the central carbon atom of crystal violet.

Anomeric memory of the glycosidic bond upon fragmentation and its consequences for carbohydrate sequencing

Deciphering the carbohydrate alphabet is problematic due to its unique complexity among biomolecules. Strikingly, routine sequencing technologies—which are available for proteins and DNA and have revolutionised biology—do not exist for carbohydrates. This lack of structural tools is identified as a crucial bottleneck, limiting the full development of glycosciences and their considerable potential impact for the society. In this context, establishing generic carbohydrate sequencing methods is both a major scientific challenge and a strategic priority. Here we show that a hybrid analytical approach integrating molecular spectroscopy with mass spectrometry provides an adequate metric to resolve carbohydrate isomerisms, i.e the monosaccharide content, anomeric configuration, regiochemistry and stereochemistry of the glycosidic linkage. On the basis of the spectroscopic discrimination of MS fragments, we report the unexpected demonstration of the anomeric memory of the glycosidic bond upon fragmentation. This remarkable property is applied to de novo sequencing of underivatized oligosaccharides.Establishing generic carbohydrate sequencing methods is both a major scientific challenge and a strategic priority. Here the authors show a hybrid analytical approach integrating molecular spectroscopy and mass spectrometry to resolve carbohydrate isomerism, anomeric configuration, regiochemistry and stereochemistry.

Influence of the tyrosine environment on the second harmonic generation of iturinic antimicrobial lipopeptides at the air–water interface

The second harmonic generation (SHG) response at the air-water interface from the tyrosine-containing natural iturinic cyclo-lipopeptides mycosubtilin, iturin A and bacillomycin D is reported. It is shown that this response is dominated by the single tyrosine residue present in these molecules owing to the large first hyperpolarizability arising from the non-centrosymmetric aromatic ring structure of this amino acid. The SHG response of these iturinic antibiotics is also compared to the response of surfactin, a cyclo-lipopeptide with a similar l,d-amino acid sequence but lacking a tyrosine residue, and PalmATA, a synthetic linear lipopeptide possessing a single tyrosine residue but lacking the amino acid sequence structuring the cycle of the iturinic antibiotics. From the light polarization analysis of the SHG response, it is shown that the tyrosine local environment is critical in defining the SHG response of these peptides at the air-water interface. Our results demonstrate that tyrosine, similar to tryptophan, can be used as an endogenous molecular probe of peptides and proteins for SHG at the air-water interface, paving the way for SHG studies of other tyrosine-containing bioactive molecules.

Optical properties of a visible push-pull chromophore covalently bound to carbohydrates: solution and gas-phase spectroscopy combined to theoretical investigations.

The use of visible absorbing and fluorescent tags for sensing and structural analysis of carbohydrates is a promising route in a variety of medical, diagnostic, and therapeutic contexts. Here we report an easy method for covalent attachment of nonfluorescent push-pull chromophores based on the 4-cyano-5-dicyanomethylene-2-oxo-3-pyrroline ring to carbohydrate moieties. The impact of sugar grafting on the optical properties of the push-pull chromophore in the gas phase and in solution was investigated by absorption and action spectroscopy and theoretical methods. The labeled sugars efficiently absorb photons in the visible range, as demonstrated by their intense photodissociation in a quadrupole ion trap. A strong blue shift (-70 nm) of the gas-phase photodissociation intensity maximum is observed upon sugar grafting, whereas no such effect is visible on the solution absorption spectra. Molecular dynamics simulations of labeled maltose in the gas phase describe strong interactions between the sulfonated chromophore and the carbohydrate, which lead to cyclic conformations. These are not observed in the simulations with explicit solvation. Time-dependent density functional theory (TD-DFT) calculations on model molecules permit us to attribute the observed shift to the formation of such cyclic conformations and to the displacement of the negative charge relative to the aromatic moiety of the chromophore.

Interleaflet sliding in lipidic bilayers under shear flow: comparison of the gel and fluid phases using reversed non-equilibrium molecular dynamics simulations

The friction between two rubbing surfaces lubricated by water can be diminished if they are coated with phospholipidic bilayers or brushes of polyelectrolytes. In the case of a coating by lipid membranes, the friction is lower when the lipids are in the gel phase rather than in the liquid phase. We investigated the response of fluid or gel bilayers to a mechanical load or under shear using non-equilibrium molecular dynamics simulations (NEMD) to understand whether this difference could come from intermonolayer sliding. The system is composed of a single fully hydrated bilayer of coarse grained phospholipids under a parallel shear with vorticity parallel to the bilayer. In both the liquid and the gel phases, an intermonolayer slip was measured in the velocity profile. In the liquid phase this slip is proportional to the shear stress. In the tilted gel phase of our model the stress is not systematically linear and relaxes differently when the shear is in the direction of the tilt or perpendicular to it. The impact of surface tension (or load) on the friction is different for the liquid and gel phases, but grossly the slip remains of the same order of magnitude.