Another look at the degree‐Kirchhoff index

Let G be an arbitrary graph with vertex set {1,2, …,N} and degrees d_i ≤ D, for fixed D and all i, then for the index R′(G) = ∑_{i < j}d_id_jR_ij we show that We also show that the minimum of R′(G) over all N‐vertex graphs is attained for the star graph and its value is 2N² ? 5N + 3. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Evaluation of the electrophoretic behaviour of opioid peptides Separation by capillary electrophoresis-electrospray ionization mass spectrometry

A general equation established in a previous study was used to model the electrophoretic mobility of a series of opioid peptides as a function of pH of the separation electrolyte. The concordance between the predicted and the experimental electrophoretic mobilities was excellent and the optimum pH for the separation of the modelled compounds could be predicted from a limited amount of experimental data. The equations were also useful for the accurate determination of the ionization constants of the polyprotic analytes. It was also demonstrated that if ionization constant values are known, the CE separations of the studied peptides can easily be predicted taking into account the classical semiempirical relationships between electrophoretic mobility and charge-to-mass ratio (m_e versus q/M^α). The separations simulated considering the accurate charge-to-mass ratios of each peptide at a certain pH value were in good agreement with the experimental results.Once an optimum separation pH value and a running buffer compatible with electrospray mass spectrometry (ESI) detection were selected, a method for the separation and characterization of this series of analytes by capillary electrophoresis-electrospray ionization mass spectrometry (CE-ESI-MS) was established using a commercial sheath-flow interface. Method validation was performed in order to prove the suitability of the proposed method for quantitative analysis. Thus, quality parameters, such as repeatability, reproducibility, limits of detection and linearity were determined.     

Vapor-liquid equilibrium of hexadecapolar fluids from a perturbation-based equation of state

In this work a numerically tractable expression for the interaction potential between two point hexadecapoles with octahedral symmetry and a molecular-based equation of state derived by perturbation theory for hexadecapolar fluids are presented. The polar system is modeled by square-well particles with a point hexadecapole with octahedral symmetry at their centers. This equation of state is analytical in the state variables and in the potential parameters and allows us to study the effects of the hexadecapolar moment strength on the thermodynamic properties and liquid-vapor phase diagram. The equation presented here is applied to the thermodynamics of sulfur hexafluoride and gives very good predictions for the saturation pressures and the vapor-liquid phase diagram.     

Development of a capillary electrophoresis method for the characterization of "palo azul" (Eysenhardtia polystachya)

The tree Eysenhardtia polystachya (Ortega) Sarg. has quite a wide popular use within the traditional Mexican medicine as herbal remedy. Popular practices constitute a relevant enough basis to design optimum analytical methods in order to determine basic principles of diverse medicinal plants. This has become one of the essentials needed to characterize such products, for which it is fundamentally important to develop an efficient and reliable separation method. This work presents the results concerning the development and optimization of a novel CE method for the separation of components from water/etanol (1:1) extracts of E. polystachya, using the following conditions, considered the best obtained: phosphate buffer 10 mM, 20 kV voltage, and pH 8.1 at 214 nm and 50 mM, 12.5 kV voltage with pH 8.1 at 426 nm. The optimization takes into account the parameters associated in the resulting electropherograms, such as number of peaks, migration times, and the Deltat(m) of the neighboring peaks. Under optimal conditions the separation intended was attained within 15 and 20 min for 214 and 426 nm, respectively. The characterization method developed was applied to the analysis of diverse extracts of E. polystachya.     

Inverse hydrogen bonds between XeH2 and hydride and fluoride derivatives of Li, Be, Na and Mg

A theoretical study of the inverse hydrogen bonds complexes formed by the XeH₂ molecule and hydride and fluoride derivatives of Li, Be, Na and Mg has been carried out by means of DFT (B3LYP/DGDZVP) and ab initio [MP2/DGDZVP and MP2/LJ18/6-311++G(2d,2p)] calculations. The complexes obtained present interaction energies up to ?81 kJ/mol. The analysis of the electron density shows electron transfer from the XeH₂ to the electron acceptor molecules. The calculated absolute chemical shieldings show the high sensitivity of the xenon atom upon complexation.     

Primary photophysical properties of moxifloxacin--a fluoroquinolone antibiotic

Abstract The photophysical properties of the fluoroquinolone antibiotic moxifloxacin (MOX) were investigated in aqueous media. MOX in water, at pH 7.4, shows two intense absorption bands at 287 and 338 nm (epsilon = 44 000 and 17 000 dm(3) mol(-1) cm(-1), respectively). The absorption and emission properties of MOX are pH-dependent, pK(a) values for the protonation equilibria of both the ground (6.1 and 9.6) and excited singlet states (6.8 and 9.1) of MOX were determined spectroscopically. MOX fluoresces weakly, the quantum yield for fluorescence emission being maximum (0.07) at pH 8. Phosphorescence from the excited triplet state in frozen ethanol solution has a quantum yield of 0.046. Laser flash photolysis and pulse radiolysis studies have been carried out to characterize the transient species of MOX in aqueous solution. On laser excitation, MOX undergoes monophotonic photoionization with a quantum yield of 0.14. This leads to the formation of a long-lived cation radical whose absorption is maximum at 470 nm (epsilon(470) = 3400 dm(3) mol(-1) cm(-1)). The photoionization process releases hydrated electron which rapidly reacts (k = 2.8 x 10(10) dm(3) mol(-1) s(-1)) with ground state MOX, yielding a long-lived anion radical with maximum absorption at 390 nm (epsilon(390) = 2400 dm(3) mol(-1) cm(-1)). The cation radical of MOX is able to oxidize protein components tryptophan and tyrosine. The bimolecular rate constants for these reactions are 2.3 x 10(8) dm(3) mol(-1) s(-1) and 1.3 x 10(8) dm(3) mol(-1) s(-1), respectively. Singlet oxygen sensitized by the MOX triplet state was also detected only in oxygen-saturated D(2)O solutions, with a quantum yield of 0.075.     

Chemical modification of zinc hydroxide nitrate and Zn-Al-layered double hydroxide with dicarboxylic acids

A zinc hydroxide nitrate (ZHN), Zn5(OH)8(NO3)2.2H2O, and a layered double hydroxide (LDH), Zn/Al-NO3 were doped with 0.2 mol% of Cu2+ during alkaline chemical precipitation. Both compounds were intercalated with adipate ((-)OOC(CH2)4COO(-)), azelate ((-)OOC(CH2)7COO(-)), and benzoate (C6H5COO(-)) ions through ion exchange reactions. Solid state 13C nuclear magnetic resonance spectroscopy showed only one signal of carboxylic carbon for adipate and azelate intercalated into LDH, indicating that the carboxylic ends of both acids were equivalent, whereas the signal split when the intercalation was into the ZHN matrix. The electron paramagnetic resonance (EPR) spectrum of copper in octahedral cation sites of LDH layers showed a Hamiltonian parameter ratio g ||/A ||=170 cm and, after intercalation of adipate, the change was not significant: g ||/A ||=174 cm. This result indicates that the carboxylate ions did not coordinate with copper centers. Nonetheless, the intercalation of azelate increased the ratio to g ||/A ||=194 cm, similar to the spectra of ZHN modified with adipate, g ||/A ||=199 cm, and azelate, g ||/A ||=183 and 190 cm, which are associated with the coordination of copper by weak carboxylate anion ligands. Copper occupies octahedral or tetrahedral sites in ZHN layers, and the EPR spectra indicate that the dicarboxylate anions reacted preferentially with octahedral sites, whereas benzoate reacted with both sites.     

Complexation of morin with three kinds of cyclodextrin. A thermodynamic and reactivity study

Properties of inclusion complexes between morin (M) and beta-cyclodextrin (betaCD), 2-hydroxypropyl-beta-cyclodextrin (HPbetaCD) and Heptakis (2,6-O-di methyl) beta-cyclodextrin (DMbetaCD) such as aqueous solubility and the association constants of this complex have been determined. The water solubility of morin was increased by inclusion with cyclodextrins. The phase-solubility diagrams drawn from UV spectral measurements are of the A(L)-type. Also ORAC(FL) studies were done. An increase in the antioxidant reactivity is observed when morin form inclusion complex with the three cyclodextrin studied. Finally, thermodynamics studies of cyclodextrin complexes indicated that for DMbetaCD the inclusion is primarily enthalpy-driven process meanwhile betaCD and HPbetaCD are entropy-driven processes. This is corroborated by the different inclusion geometries obtained by 2D-NMR.     

Synthesis and characterisation of helical beta-peptide architectures that contain (S)-beta3-HDOPA(Crown Ether) derivatives

A new set of beta-amino acids that carry various crown ether receptors on their side chains of the general formula (S)-beta(3)-HDOPA(crown ether) (HDOPA: homo-3,4-dihydroxyphenylalanine; (crown ether): [15]crown-5 ([15-C-5]), [18]crown-6 ([18-C-6]), [21]crown-7 ([21-C-7]), 1,2-Benzo-[24]crown-8 ([Benzo-24-C-8]) and (R)-Binol-[20]crown-6 ([(R)-Binol-20-C-6])) was prepared. Peptides that are based on these new crowned beta-amino acids combined with (1S,2S)-ACHC (2-aminocyclohexanecarboxylic acid), which is known to be a potent 3(14)-helix inducer, to the hexamer level, with two crowned residues at the i and i+3 positions of the main-chain, were synthesized in solution by stepwise coupling using Boc-N(alpha)-protection (Boc: tert-butoxycarbonyl) and the EDC/HOAt C-activation method. Their conformational analysis was performed by using FTIR absorption, NMR and CD spectroscopy techniques. Our results are in full agreement with a 3(14)-helix conformation.