Generation of indistinguishable and pure heralded single photons with tunable bandwidth

We describe a new scheme to fully control the joint spectrum of paired photons generated in spontaneous parametric downconversion. We show the capability of this method to generate frequency-uncorrelated photon pairs that are pure and indistinguishable and whose bandwidth can be readily tuned. Importantly, the scheme we propose can be implemented in any nonlinear crystal and frequency band of interest.     

Antibacterial activity of agave fructans against salmonella typhimurium

Abstract

The objective of this study was to evaluate the antimicrobial effect of Agave fructans against the Salmonella Typhimurium in “in vitro” experiments. The result of the antimicrobial activity was 263.89?±?0, 414.95?±?12.83, 494.54?±?13.88, 522.29?±?0, 580.41?±?14.92?AU for 10, 20, 30, 40 and 50% of Agave fructans (AF) respectively. In addition, there is a significant effect on the growth of the bacteria with all the percentages of AF evaluated (p?<?0.001, R2?=?0.859) with respect to the control. The growth rate of Salmonella with 25% AF was statistically significant compared to the control (?0.7353?±?0.586, 0.0079?±?0.002?D.O./h, respectively; p?>?0.01). Agave fructans could be an alternative to prevent the infections caused by Salmonella.     

The marinopyrroles, antibiotics of an unprecedented structure class from a marine Streptomyces sp

Cultivation of an obligate marine Streptomyces strain has furnished the marinopyrroles A and B, densely halogenated, axially chiral metabolites that contain an uncommon bispyrrole structure. X-ray analysis of marinopyrrole B showed that the natural product exists as an atropo-enantiomer with the M-configuration. Though configurationally stable at room temperature, M-(-)-marinopyrrole A can be racemized at elevated temperatures to yield the non-natural P-(+)-atropo-enantiomer. The marinopyrroles possess potent antibiotic activities against methicillin-resistant Staphylococcus aureus.     

The energy landscape of a selective tumor-homing pentapeptide

Recently, a potentially powerful strategy based on phage-display libraries has been presented to target tumors via homing peptides attached to nanoparticles. The Cys-Arg-Glu-Lys-Ala (CREKA) peptide sequence has been identified as a tumor-homing peptide that binds to clotted plasmas proteins present in tumor vessels and interstitium. The aim of this work consists of mapping the conformational profile of CREKA to identify the bioactive conformation. For this purpose, a conformational search procedure based on modified simulated annealing combined with molecular dynamics was applied to three systems that mimic the experimentally used conditions: (i) the free peptide; (ii) the peptide attached to a nanoparticle; and (iii) the peptide inserted in a phage display protein. In addition, the free peptide was simulated in an ionized aqueous solution environment, which mimics the ionic strength of the physiological medium. Accessible minima of all simulated systems reveal a multiple interaction pattern involving the ionized side chains of Arg, Glu, and Lys, which induces a beta-turn motif in the backbone observed in all simulated CREKA systems.     

A new quantitative and low-cost determination method of nitrate in vegetables, based on deconvolution of UV spectra

A new UV-spectrophotometric method for the determination of nitrate in vegetables is presented. The method is based on the spectral deconvolution: UV spectrum of a sample is considered as a linear combination of absorption spectra, named reference spectra. The combination of a small number of spectra of reference allows to reconstitute the shape of UV spectrum of an unknown sample. There have been several fresh vegetables (lettuce, curly lettuce, oak-leaf lettuce), as well as frozen spinaches that have been tested. The results obtained were comparable to those obtained with the reference HPLC method (official European reference method for the determination of nitrate in foodstuffs). The nitrate content varied from 377 to 3240 mg kg⁻¹ of fresh vegetables, and 545 to 1190 mg kg⁻¹ of frozen spinach. The recovery of added nitrate ranged from 91 to 99%. The results were obtained with a laboratory spectrophotometer and also with a dedicated field-type spectrophotometer. This method does not require almost any consumable, is quantitative and very fast reading with easy and low maintenance.     

Computational study of the thermal decomposition of 2-methylbutyraldehyde and 2-pentanone through retro-ene reactions

In this work, a theoretical study at the MP2/6-31G(d) level of the thermal decomposition retro-ene reaction of 2-methylbutyraldehyde was carried out at a pressure of 1.5 atm. and temperatures ranging from 1110 to 1190 K. The progress of the reaction has been followed by means of the Wiberg bond indices which in turn allowed the calculation of the reaction synchronicity. Transition state theory was used to calculate the theoretical rate constant at 1150 K which was compared with the previously reported experimental value at the same conditions. We found that both values show a close agreement. The obtained computational evidence allowed us to support a reaction mechanism which proceeds in two steps: the first one with the formation of ethylene and 1-propenol via a six-membered cyclic transition state and the second one involving keto-enol equilibrium of 1-propenol to propionaldehyde via a four-membered cyclic transition state. It was found that the reaction is a highly synchronous and concerted process. The results obtained for the thermolysis of 2-methylbutyraldehyde were compared with those obtained for the thermolysis of 2-pentanone. A comparison of our results with those reported for their corresponding β-hydroxy counterparts, 3-hydroxy-2-methylpropionaldehyde and 4-hydroxy-2-butanone has also been made. A study of the thermochemistry of the compounds involved in the reactions studied has been carried out at the G3 level.     

Electrochemical gating of single osmium molecules tethered to Au surfaces

The electrochemical study of electron transport between Au electrodes and the redox molecule Os[(bpy)₂(PyCH₂ NH₂CO-]ClO₄ tethered to molecular linkers of different length (1.3 to 2.9 nm) to Au surfaces has shown an exponential decay of the rate constant k _ET ⁰ with a slope β = 0.53 consistent with through bond tunneling to the redox center. Electrochemical gating of single osmium molecules in an asymmetric tunneling nano-gap between a Au(111) substrate electrode modified with the redox molecules and a Pt-Ir tip of a scanning tunneling microscope was achieved by independent control of the reference electrode potential in the electrolyte, E _ref ? E _s, and the tip-substrate bias potential, E _bias. Enhanced tunneling current at the osmium complex redox potential was observed as compared to the off resonance set point tunneling current with a linear dependence of the overpotential at maximum current vs. the E _bias. This corresponds to a sequential two-step electron transfer with partial vibration relaxation from the substrate Au(111) to the redox molecule in the nano-gap and from this redox state to the Pt-Ir tip according to the model of Kuznetsov and Ulstrup (J Phys Chem A 104: 11531, 2000). Comparison of short and long linkers of the osmium complex has shown the same two-step ET (electron transfer) behavior due to the long time scale in the complete reduction-oxidation cycle in the electrochemical tunneling spectroscopy (EC-STS) experiment as compared to the time constants for electron transfer for all linker distances, k _ET ⁰.     

IPy2BF4-mediated transformation of n-pentenyl glycosides to glycosyl fluorides: a new pair of semiorthogonal glycosyl donors

Bis(pyridinium) iodonium(I) tetrafluoroborate (IPy2BF4), a solid and stable reagent, can be used to transform n-pentenyl orthoesters (NPOEs) and n-pentenyl glycosides (NPGs) into glycosyl fluorides. The latter pair constitutes a new set of semiorthogonal glycosyl donors that can be used in glycosylation strategies, alone or in combination with NPOEs.     

Production of artifact methylmercury during the analysis of certified reference sediments: use of ionic exchange in the sample treatment step to minimise the problem

Production of artifact methylmercury (MeHg⁺) during the analysis of two certified reference sediments, CRM-580 and IAEA-405, was investigated. Leaching of the analyte from the solid sample was achieved by ultrasound assisted acidic extraction. The aqueous leachate was either ethylated (NaBEt₄) or phenylated (NaBPh₄) using acetic/acetate or citric/citrate to buffer the solution. Preconcentration of the volatile compounds was carried out by extraction with an organic solvent (n-hexane) or solid phase microextraction (SPME). MeHg⁺ was finally separated and detected by gas chromatography with atomic emission or mass spectrometry detection (GC-MIP-AED or GC-MS). In all the cases the concentrations obtained for MeHg⁺ in the CRM-580 were significantly higher than the certified value. For the IAEA-405, however, the MeHg⁺ concentration found was always statistically indistinguishable from the certified value. Experiments were also conducted with synthetic samples, such as aqueous mixtures of MeHg⁺ and inorganic mercury (Hg²⁺) or silica-gel spiked with both compounds. The methylation rates found (defined as the percentage of Hg²⁺ present in the sample which methylates to give artifact MeHg⁺) ranged from not observable (in certain synthetic aqueous mixtures) to 0.57% (analysis of CRM-580 under certain conditions). As the amount of Hg²⁺ available in the sample seems to be the main factor controlling the magnitude of the artifact, several experiments were conducted using an ionic exchange resin (Dowex M-41) in order to minimise the concentration of this chemical in the reaction medium. First, a hydrochloric leachate of the sample was passed through a microcolumn packed with the exchanger. Second, the resin was mixed with the sample prior to extraction with HCl. In both cases, the predominant Hg²⁺ species, HgCl₄²⁻, was adsorbed on the resin, whereas MeHg⁺, mainly as MeHgCl, remained in solution. Following the second option, a new method to analyse MeHg⁺ in conflictive matrices like certain sediments was proposed. This approach produced better results for the CRM-580, but a MeHg⁺ concentration slightly, but statistically significant, higher than the reference value was still obtained.     

Continuous nondestructive monitoring of <Emphasis Type="Italic">Bordetella pertussis</Emphasis> biofilms by Fourier transform infrared spectroscopy and other corroborative techniques

This work describes the application of several analytical techniques to characterize the development of Bordetella pertussis biofilms and to examine, in particular, the contribution of virulence factors in this development. Growth of surface-attached virulent and avirulent B. pertussis strains was monitored in continuous-flow chambers by techniques such as the crystal violet method, and nondestructive methodologies like fluorescence microscopy and Fourier transform (FT) IR spectroscopy. Additionally, B. pertussis virulent and avirulent strains expressing green fluorescent protein were grown adhered to the base of a glass chamber of 1-μm thickness. Three-dimensional images of mature biofilms, acquired by confocal laser scanning microscopy, were quantitatively analysed by means of the computer program COMSTAT. Our results indicate that only the virulent (Bvg⁺) phase of B. pertussis is able to attach to surfaces and develop a mature biofilm. In the virulent phase these bacteria are capable of producing a biofilm consisting of microcolonies of approximately 200 μm in diameter and 24 μm in depth. FTIR spectroscopy allowed us not only to follow the dynamics of biofilm growth through specific biomass and biofilm marker absorption bands, but also to monitor the maturation of the biofilm by means of the increase of the carbohydrate-to-protein ratio.