MALDI‐MS of flavonoids: a systematic investigation of ionization and in‐source dissociation mechanisms

Matrix assisted laser desorption ionization (MALDI) is a technique widely employed in the analysis of proteins and peptides, and nowadays it has also been applied to small molecules. There is little significant information regarding the in‐source dissociation processes on MALDI for natural products. Twenty‐six flavonoids (flavanones, flavones and flavonols) were analyzed by MALDI using different methods (with different matrices) and without matrix to comprehend the in‐source reactions and establish good analysis methods for these compounds. Depending on the class, structure and the laser intensity applied, methoxylated flavonoid aglycones can eliminate methyl radicals (˙CH₃) in the source, such as flavonols, but lithium 2,4‐dihydroxybenzoate matrix suppresses the ˙CH₃ eliminations and retro‐Diels–Alder cleavages in the source. All of the flavonoid O‐glycosides evaluated herein eliminated the sugar in source, even in the presence of the matrix, and its product radical ions ([M‐H‐sugar]^?˙) were observed in the negative mode. The flavone C‐glycosides suffered intense dissociation, which was reduced by the addition of a matrix and the application of low laser intensity, mainly in the negative mode. Depending on the hydroxyl substituents, the [M‐H‐H]^?˙ ion was observed with variable relative intensity in the spectra. Copyright © 2015 John Wiley & Sons, Ltd.     

Radical Formation in the Gas‐Phase Ozonolysis of Deprotonated Cysteine

Although the deleterious effects of ozone on the human respiratory system are well‐known, many of the precise chemical mechanisms that both cause damage and afford protection in the pulmonary epithelial lining fluid are poorly understood. As a key first step to elucidating the intrinsic reactivity of ozone with proteins, its reactions with deprotonated cysteine [Cys?H]^? are examined in the gas phase. Reaction proceeds at near the collision limit to give a rich set of products including 1) sequential oxygen atom abstraction reactions to yield cysteine sulfenate, sulfinate and sulfonate anions, and significantly 2) sulfenate radical anions formed by ejection of a hydroperoxy radical. The free‐radical pathway occurs only when both thiol and carboxylate moieties are available, implicating electron‐transfer as a key step in this reaction. This novel and facile reaction is also observed in small cys‐containing peptides indicating a possible role for this chemistry in protein ozonolysis.     

Harmonic non-linear response of beck's column to a lateral excitation

The non-linear response of a column with a follower force (Beck's column) subjected to a distributed periodic lateral excitation, or to a support excitation, is determined. An analytical solution for the response amplitude in terms of the loading and system parameters is obtained by a perturbation analysis of the differential equations of motion. Non-linear inertia and non-linear curvature terms are taken into account in the formulation of the differential equations.     

Non-linear flexural-flexural-torsional-extensional dynamics of beams—I. Formulation

The non-linear differential equations of motion, and boundary conditions, for Euler-Bernoulli beams able to experience flexure along two principal directions (and, thus, flexure in any direction in space), torsion and extension are formulated. The beam's material is assumed to be Hookean but its properties may vary along its span. The nonlinearities present in the differential equations include contributions from the curvature expression and from inertia terms. A set of differential equations with polynomial nonlinearities to cubic order, suitable for a perturbation analysis of the motion, is also developed and the validity of the inextensional approximation is assessed. The equations developed here reduce to those for an inextensional beam. In Part II of this paper, a specific example of application is analyzed and the results obtained are compared with those available in the literature where several non-linear terms have been neglected a priori.     

Identification of Nonlinear Aeroelastic Systems Based on the Volterra Theory: Progress and Opportunities

The identification of nonlinear aeroelastic systems based on the Volterra theory of nonlinear systems is presented. Recent applications of the theory to problems in computational and experimental aeroelasticity are reviewed. Computational results include the development of computationally efficient reduced-order models (ROMs) using an Euler/Navier–Stokes flow solver and the analytical derivation of Volterra kernels for a nonlinear aeroelastic system. Experimental results include the identification of aerodynamic impulse responses, the application of higher-order spectra (HOS) to wind-tunnel flutter data, and the identification of nonlinear aeroelastic phenomena from flight flutter test data of the active aeroelastic wing (AAW) aircraft.     

A Non-Linear SGS Model Based On The Spatial Velocity Increment

A new subgrid scale (SGS) modelling concept for large-eddy simulation (LES) of incompressible flow is proposed based on the three-dimensional spatial velocity increment δ u _i . The new model is inspired by the structure function formulation developed by Métais and Lesieur [39] and applied in the context of the scale similarity type formulation. First, the similarity between the SGS stress tensor τ _ij and the velocity increment tensor Q _ij = δ u _i δ u _j is analyzed analytically and numerically using a priori tests of fully developed pipe flow at Re _τ = 180. Both forward and backward energy transfers between resolved and unresolved scales of the flow are well predicted with a SGS model based on Q _ij . Secondly, a posteriori tests are performed for two families of turbulent shear flows. LES of fully developed pipe flow up to Re _τ = 520 and LES of round turbulent jet at Re _D = 25000 carried out with a dynamic version of the model provide promising results that confirm the power of this approach for wall-bounded and free shear flows.     

Off-axial acoustic scattering of a high-order Bessel vortex beam by a rigid sphere

In this paper, the off-axial acoustic scattering of a high-order Bessel vortex beam by a rigid immovable (fixed) sphere is investigated. It is shown here that shifting the sphere off the axis of wave propagation induces a dependence of the scattering on the azimuthal angle. Theoretical expressions for the incident and scattered field from a rigid immovable sphere are derived. The near- and far-field acoustic scattering fields are expressed using partial wave series involving the spherical harmonics, the scattering coefficients of the sphere, the half-conical angle of the wave number components of the beam, its order and the beam-shape coefficients. The scattering coefficients of the sphere and the 3D scattering directivity plots in the near- and far-field regions are evaluated using a numerical integration procedure. The calculations indicate that the scattering directivity patterns near the sphere and in the far-field are strongly dependent upon the position of the sphere facing the incident high-order Bessel vortex beam.