Complex Geometry and Dirac Equation

Complex geometry represents a fundamentalingredient in the formulation of the Dirac equation bythe Clifford algebra. The choice of appropriate complexgeometries is strictly related to the geometricinterpretation of the complex imaginary unit . We discuss two possibilities which appearin the multivector algebra approach: the₁₂₃ and ₂₁ complexgeometries. Our formalism provides a set of rules which allows an immediate translation between thecomplex standard Dirac theory and its version withingeometric algebra. The problem concerning a doublegeometric interpretation for the complex imaginary unit is also discussed.     

An expeditious method for the determination of organochlorine pesticides residues in estuarine sediments using microwave assisted pre-extraction and automated headspace solid-phase microextraction coupled to gas chromatography-mass spectrometry

Determination of organochlorine pesticides (OCPs) in sediments implicates extraction of these compounds from the matrix, which is difficult owing to strong interaction among OCPs and different constituents of the sediments, particularly organic content. The method here described is a combination of microwave assisted extraction (MAE), headspace solid-phase microextraction (HS-SPME) and gas chromatography-mass spectrometry (GC-MS), acting in selected-ion storage mode, or GC-electron capture detector (ECD, for routine analysis). Methanol was used as extracting solvent and aliquots of the MAE extracts (after inclusion of a step for sulfur elimination when required) were used to prepare aqueous solutions for HS-SPME. A complete automation of the SPME procedure increases the sample throughput, including standard addition for calibration purpose. The procedure has the advantage of exclude additional clean-up steps and pre-concentration before SPME. Application to reference sediments of different characteristics revealed absence of significant interferences from the matrix for alpha-lindane, gamma-lindane, aldrin, dieldrin, endrin, 4,4'-DDT, 4,4'-DDD, 4,4'-DDE, heptachlor, heptachlor epoxide and good sensitivity. Detection limits ranged from 0.005 to 0.11 ng of OCP per gram of dried sediment using GC-MS and from 0.01 to 0.26 ngg(-1) using GC-ECD. The linear response ranges embraced 5-6 orders of magnitude (up to 1000 ngg(-1)) in GC-MS, being narrower for GC-ECD. The method was successfully applied to sandy and muddy sediments from Portuguese rivers estuaries, enabling quantification of seven OCPs. The method resulted effective, relatively simple and fast, being suitable for routine monitoring of residues of OCPs from sediments of different grain size and organic matter content, which influence concentration, mobility and availability of contaminants.     

Et3N-induced demethylation-annulation of 3-alkynyl-4-methoxy-2-pyridones and structurally related compounds in the synthesis of furan-fused heterocycles

Various 3-iodo-4-methoxypyridin-2-ones and related pyrone and coumarin derivatives have been demonstrated as readily available precursors of 2-substituted furan-fused heterocycles by means of in situ sequential Sonogashira-acetylide coupling, dealkylation, and regioselective furan annulation reactions. A Et3N-induced S(N)2 process has been established that accounts for the dealkylation process.     

Electrospray ionization-ion trap mass spectrometry study of PQAAPro and PQProAA mimetic derivatives of the antimalarial primaquine

Electrospray ionization-ion trap mass spectrometry (ESI-MS) of imidazolidin-4-one peptidomimetic derivatives of the antimalarial drug primaquine (PQ) is reported. These compounds contain the imidazolidin-4-one moiety either at the N- or the C-terminal of a dipeptide backbone, thus respectively mimicking PQ-Amino Acid-Proline (PQAAPro) and PQProAA derivatives of PQ. Both the peptidomimetics and precursors previously developed by us are promising drug candidates, as they were found to be active against rodent Plasmodium berghei malaria and Pneumocystis carinii pneumonia. Collision-induced dissociation (CID) and tandem-mass spectra (MS) of the title compounds, and fragmentation pathways thereof, led to the following findings: (1) CID patterns present some parallelism with the reactivity towards hydrolysis previously found for the same or related compounds; (2) a positional shift of the imidazolidin-4-one ring is reflected on both degree and pathways of fragmentation, which makes tandem-MS a key tool for differentiation of imidazolidin-4-one isomers; (3) the major MS/MS fragmentation of PQProAA mimetics involves release of a neutral diketopiperazine (DKP), in parallel to the “diketopiperazine pathway” described in tandem-MS studies of oligopeptides; (4) the relative abundance of a major fragment in tandem-MS spectra is inversely correlated with the size of the N-terminal AA in PQProAA mimetics. Overall, this work embodies an original and valuable contribution towards a deeper insight into the molecular properties of novel antimalarials, which can be viewed as representative of both the 8-aminoquinoline and, especially, the imidazolidin-4-one structural classes.     

Dark channels in resonant tunneling transport through artificial atoms

We investigate sequential tunneling through a multilevel quantum dot confining multiple electrons in the regime where several channels are available for transport within the bias window. By analyzing solutions to the master equations of the reduced density matrix, we give general conditions on when the presence of a second transport channel in the bias window quenches transport through the quantum dot. These conditions are in terms of distinct tunneling anisotropies which may aid in explaining the occurrence of negative differential conductance in quantum dots in the nonlinear regime.     

Psychoacoustics of in-car switch buttons: From feelings to engineering parameters

The car interior is becoming quieter and other sounds are now exposed to user perception, such as the sound produced by interface buttons when actuated. So, the functional role of the button sound on interface operation and its aesthetic and emotional role on the user experience are now more important than before. However, little research and design effort has been paid to understand how to design buttons that produce a pleasant sound. Moreover, the button’s sound requirements received by interface manufacturers are ill-defined, insufficient or even inexistent, and consequently their conversion into specifications for manufacturing is problematic and leads to long and costly development processes. The purpose of this paper is to contribute to identify relevant acoustic parameters that explain the users sound preferences. Data on preference subjective judgments were collected and buttons acoustic signals were measured allowing the development of preference models based on partial least squares regression and neural networks methods. The former was successful in selecting the relevant parameters to describe the preference ratings of the buttons sound. The later, dealing with the non-linear nature of acoustic perception, was able to predict preferences based on the relevant parameters.     

Easy and stereoselective approach to alpha,beta-unsaturated gamma-lactones fused to pyranoses from furanose scaffolds

The first facile and efficient route to pyranose-fused butenolides from furanose scaffolds, convenient for scaling up production, is described. Wittig olefination of 1,2-O-isopropylidene pentofuranos- or hexofuranos-3-uloses with a resonance-stabilized ylide led to the stereoselective formation of the (Z)-alpha,beta-unsaturated ester. In the presence of acid labile 5-O- or 5,6-di-O-protecting groups, acid hydrolysis of the Wittig product resulted in isomerization to the pyranose form and spontaneous lactonization to give the target molecules in good overall yield.     

Electrochemical and spectroscopic characterisation of amphetamine-like drugs: application to the screening of 3,4-methylenedioxymethamphetamine (MDMA) and its synthetic precursors

A complete physicochemical characterisation of MDMA and its synthetic precursors MDA, 3,4-methylenedioxybenzaldehyde (piperonal) and 3,4-methylenedioxy-β-methyl-β-nitrostyrene was carried out through voltammetric assays and Raman spectroscopy combined with theoretical (DFT) calculations. The former provided important analytical redox data, concluding that the oxidative mechanism of the N-demethylation of MDMA involves the removal of an electron from the amino-nitrogen atom, leading to the formation of a primary amine and an aldehyde. The vibrational spectroscopic experiments enable to afford a rapid and reliable detection of this type of compounds, since they yield characteristic spectral patterns that lead to an unequivocal identification.Moreover, the rational synthesis of the drug of abuse 3,4-methylenedioxymethamphetamine (MDMA or “ecstasy”) from one of its most relevant precursors 3,4-methylene-dioxyamphetamine (MDA), is reported. In addition, several approaches for the N-methylation of MDA, a limiting synthetic step, were attempted and the overall yields compared.