Landauʼs necessary density conditions for the Hankel transform

We will prove an analogue of Landau?s necessary conditions [H.J. Landau, Necessary density conditions for sampling and interpolation of certain entire functions, Acta Math. 117 (1967) 37–52] for spaces of functions whose Hankel transform is supported in a measurable subset S of the positive semi-axis. As a special case, necessary density conditions for the existence of Fourier–Bessel frames are obtained.     

Amberlyst®-15: a reusable heterogeneous catalyst for the dehydration of tertiary alcohols

Tertiary alcohols react under mild conditions in the presence of Amberlyst^®-15 (dry) (solid-supported sulfonic acid) to give predominantly the most stable alkene in very good yield. The dehydration of tertiary alcohol functionality occurs without observation of rearrangement and polymerization products, and with outstanding substrate tolerance, which include the NHCBz, NHBoc, OSEM, OTBDMS, OBOM and ethylene ketal functional groups. Amberlyst^®-15 (dry) can be easily recovered from the reaction medium and reused for five cycles, maintaining the catalytic efficiency. In addition, the dehydration can occur under continuous operation.     

Simple transformation of crystalline chiral natural anions to liquid medium and their use to induce chirality

New chiral ionic liquids can be prepared simply by combining the tetra-n-hexyl-dimethylguanidinium cation with readily available chiral anions and used as an asymmetric inducing agent as demonstrated for catalytic Rh(II) carbenoid C-H insertion and Sharpless dihydroxylation.     

Diastereomeric differentiation of peptides with CuII and FeII complexation in an ion trap mass spectrometer

Complexation by transition metal ions (CuII and FeII) was successfully used to differentiate the diastereomeric YAGFL, YDAGFL and Y(D)AGF(D)L pentapeptides by electrospray ionization-ion trap mass spectrometry in the positive ion mode using low-energy collision conditions. This distinction was allowed by the stereochemical effects due to the (D)Leu/(L)Leu and the (D)Ala/(L)Ala residues yielding various steric interactions which direct relative dissociation rate constants of the binary [(M - H) + MeII]+ complexes (Me = Cu or Fe) subjected to low-energy, collision-induced dissociation processes. The interpretation of the collision-induced dissociation spectra obtained from the diastereomeric cationized peptides allowed the location of the deprotonated site(s), leading to the postulation of ion structures and fragmentation pathways for both the [(M - H) + CuII]+ and [(M - H) + FeII]+ complexes, which differed significantly. With CuII, consecutive fragmentations, initiated by the decarboxylation at C-terminus, were favored relative to sequence product ions. On the other hand, with FeII, competitive fragmentations resulting in abundant sequence product ions and significant internal losses were preferred. This could be explained by different localizations of the negative charge, which directs the orientation of both the [(M - H) + CuII]+ and [(M - H) + FeII]+ binary complexes fragmentations. Indeed, the free negative charge of the [(M - H) + CuII]+ ions was mainly located at one oxygen atom: either at the C-terminal carboxylic group or, to a minor extent, at the Tyr phenol group (i.e. zwitterionic forms). On the other hand, the negative charge of the [(M - H) + FeII]+ ions was mainly located at one of the nitrogen atoms of the peptide backbone and coordinated to FeII (i.e. salt non-zwitterionic form).Moreover, this study reveals the particular behavior of CuII reduced to CuI, which promotes radical losses not observed from the peptide-FeII complexes. Finally, this study shows the analytical potentialities of the complexation of transition metal ions with peptides providing structural information complementary to that obtained from low-energy, collision-induced dissociation processes of protonated or deprotonated peptides.     

Dispersive liquid-liquid microextraction versus single-drop microextraction for the determination of several endocrine-disrupting phenols from seawaters

Two liquid-phase microextraction procedures: single-drop microextraction (SDME) and dispersive liquid-liquid microextraction (DLLME), have been developed for the determination of several endocrine-disrupting phenols (EDPs) in seawaters, in combination with high-performance liquid chromatography (HPLC) with UV detection. The EDPs studied were bisphenol-A, 4-cumylphenol, 4-tertbutylphenol, 4-octylphenol and 4-n-nonylphenol. The optimized SDME method used 2.5 μL of decanol suspended at the tip of a micro-syringe immersed in 5 mL of seawater sample, and 60 min for the extraction time. The performance of the SDME is characterized for average relative recoveries of 102 ± 11%, precision values (RSD) < 9.4% (spiked level of 50 ng mL⁻¹), and detection limits between 4 and 9 ng mL⁻¹. The optimized DLLME method used 150 μL of a mixture acetonitrile:decanol (ratio 15.7, v/v), which is quickly added to 5 mL of seawater sample, then subjected to vortex during 4 min and centrifuged at 2000 rpm for another 5 min. The performance of the DLLME is characterized for average relative recoveries of 98.7 ± 3.7%, precision values (RSD) < 7.2% (spiked level of 20 ng mL⁻¹), and detection limits between 0.2 and 1.6 ng mL⁻¹. The efficiencies of both methods have also been compared with spiked real seawater samples. The DLLME method has shown to be a more efficient approach for the determination of EDPs in seawater matrices, presenting enrichment factors ranging from 123 to 275, average relative recoveries of 110 ± 11%, and precision values (RSD) < 14%, when using a real seawaters (spiked level of 3.5 ng mL⁻¹).     

Selective arylation of aldehydes with di-rhodium(II)/NHC catalysts

Here is described the preparation of four new rhodium(II) complexes bearing axial NHC ligands. The presence of electron-withdrawing bridging ligands resulted in an enhanced reactivity in the arylation of aldehydes with boronic acids when compared with the tetraacetate counterparts. Complex 15 (Rh₂tfa₄(IPr)₂) proved to be the most active catalyst for this transformation allowing the selective conversion of aromatic, aliphatic and vinyl aldehydes into the respective alcohols in excellent yields. It was demonstrated that the good group tolerance could be further extended to aromatic and conjugated ketones. DFT calculations carried out on this system showed the complementarily of the bridging ligands and axial ligand in these dinuclear complexes. It was also disclosed that Rh(II)/NHC catalytic system can promote the racemization of 1-phenyl ethanol.     

Stereochemical effects during [M-H]− dissociations of epimeric 11-OH-17β-estradiols and distant electronic effects of substituents at C(11) position on gas phase acidity

The affinity of estradiol derivatives for the estrogen receptor (ER) depends strongly on nature and stereochemistry of substituents in C₍₁₁₎ position of the 17β-estradiol (I). In this work, the stereochemistry effects of the 11α-OH-17β-estradiol (III_α) and 11β-OH-17β-estradiol (III_β) were investigated using CID experiments and gas-phase acidity (ΔH_acid^∘) determination. The CID experiments showed that the steroids decompose via different pathways involving competitive dissociations with rate constants depending upon the α/β C₍₁₁₎ stereochemistry. It was shown that the fragmentations of both deprotonated [III_α-H]⁻ and [III_β-H]⁻ epimers were initiated by the deprotonation of the most acidic site, i.e. the phenolic hydroxyl at C₍₃₎. This view was confirmed by H/D exchange and double resonance experiments. Furthermore, the ΔH_acid^∘ of both epimers (III_α and III_β), 17β-estradiol (I), and 17-desoxyestradiol (II) was determined using the extended Cooks’ kinetic method. The resulting values allowed us to classify steroids as a function of their gas-phase acidity as follows: (III_β)≫(II)>(I)>(III_α). Interestingly, the α/β C₍₁₁₎ stereochemistry appeared to influence strongly the gas-phase acidity. This phenomenon could be explained through stereospecific proton interaction with π-orbital cloud of A ring, which was confirmed by theoretical calculation.     

Copper corrosion in soil: influence of chloride contents, aeration and humidity

The corrosion of copper in a typical Portuguese soil was studied. The original soil was characterised, and modifications were produced by adding chloride, and HClO₄ solutions, or by increasing its relative humidity. The aggressiveness degree of the various soil samples was determined. Copper coupons exposed for 3 months in the original and in the modified soil samples were analysed. The average corrosion rates determined from gravimetric data were in good correlation with the soil aggressiveness. The morphology of the corroded copper surfaces, with and without corrosion products, was analysed by visual observation, optical microscopy and scanning electron microscopy. Energy dispersive spectroscopy was used for the semi-quantitative analysis of the corrosion products and X-ray diffraction spectroscopy to identify the crystalline products. Cuprite has been identified on the copper samples corresponding to the interfaces Cu|S6 and Cu|S8, plus paratacamite on the copper coupon exposed to the soil with higher concentration of chloride (S6). Polarisation curves of copper samples in neutral solutions made by adding different amounts of chloride ions to the soil washing water, under deaereated conditions, were recorded and analysed. The passivity breakdown potential has shown, as expected, a displacement to the cathodic direction as the Cl⁻ ion concentration increases.     

Quaternionic and hyper-Kähler metrics from generalized sigma models

The problem of finding new metrics of interest, in the context of SUGRA, is reduced to two stages: first, solving a generalized BPS sigma model with full quaternionic structure proposed by the authors and, second, constructing the hyper-Kähler metric, or suitable deformations of this condition, taking advantage of the correspondence between the quaternionic left-regular potential and the hyper-Kähler metric of the target space. As illustration, new solutions are obtained using generalized Q-sigma model for Wess–Zumino type superpotentials. Explicit solutions analog to the Berger?s sphere and Abraham–Townsend type are given and generalizations of 4-dimensional quaternionic metrics, product of complex ones, are shown and discussed.     

Gas-phase doubly charged complexes of cyclic peptides with copper in +1, +2 and +3 formal oxidation states: formation, structures and electron capture dissociation

Copper complexes with a cyclic D-His-β-Ala-L-His-L-Lys and all-L-His-β-Ala-His-Lys peptides were generated by electrospray which were doubly charged ions that had different formal oxidation states of Cu(I), Cu(II) and Cu(III) and different protonation states of the peptide ligands. Electron capture dissociation showed no substantial differences between the D-His and L-His complexes. All complexes underwent peptide cross-ring cleavages upon electron capture. The modes of ring cleavage depended on the formal oxidation state of the Cu ion and peptide protonation. Density functional theory (DFT) calculations, using the B3LYP with an effective core potential at Cu and M06-2X functionals, identified several precursor ion structures in which the Cu ion was threecoordinated to pentacoordinated by the His and Lys side-chain groups and the peptide amide or enolimine groups. The electronic structure of the formally Cu(III) complexes pointed to an effective Cu(I) oxidation state with the other charge residing in the peptide ligand. The relative energies of isomeric complexes of the [Cu(c-HAHK + H)](2+) and [Cu(c-HAHK - H)](2+) type with closed electronic shells followed similar orders when treated by the B3LYP and M06-2X functionals. Large differences between relative energies calculated by these methods were obtained for open-shell complexes of the [Cu(c-HAHK)](2+) type. Charge reduction resulted in lowering the coordination numbers for some Cu complexes that depended on the singlet or triplet spin state being formed. For [Cu(c-HAHK - H)](2+) complexes, solution H/D exchange involved only the N-H protons, resulting in the exchange of up to seven protons, as established by ultra-high mass resolution measurements. Contrasting the experiments, DFT calculations found the lowest energy structures for the gas-phase ions that were deprotonated at the peptide C(α) positions.