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Usnic acid is a secondary metabolitefound in several species oflichens, organisms resulting from the symbiosis between fungi andalgae. This compound has been extensively studied because of its pharmacologicalproperties. Despite its potential medicinal importance, it exhibitsa high degree of toxicity. The confinement of the usnic acid withinliposomes has been investigated as a possibility to reduce its toxicity.In this work, we characterize the interaction of usnic acid in itsneutral and anionic states (usniate) with DPPC and DOPC by means ofmolecular dynamics simulations. A lamellar to nonlamellar transitionis observed for both membranes upon contact with usniate within 100ns time scale. The transition suggests the formation of a liposome-likestructure encapsulating the metabolyte. Furthermore, such processoccurs at a significantly shorter time frame for DOPC. [ABSTRACT FROM AUTHOR]

Principal component analysis was used in the selection of the B3PW91/6-311+G(3df) method for calculations of As2O3 molecular species. This multivariate analysis compared experimental properties (structure and ionization energy) of As4O6 molecule with several computational methods (Hartree-Fock, second-order Møller-Plesset perturbation theory, and density functional theory) and basis sets. At the selected level of theory, we were able to calculate nine structures of As2O3 with three of them being stable ones (no negative force constants) in the following stability order: D3H (0.0), DIAMOND OUT (32.77 kJ mol−1), and GAUCHE (43.81 kJ mol−1). Several intramolecular conversions were studied with the GAUCHE → DIAMOND OUT transformation being barrierless, whereas the DIAMOND OUT structure is converted into the D3H isomer through a Gibbs free-energy barrier of 40.83 kJ mol−1. Starting from the GAUCHE structure of As2O3 and its enantiomeric form, it was possible to obtain directly (barrierless) the As4O6 species in a spontaneous process (Δr G = −399.92 kJ mol−1 at 298 K and 1 atm) in gas phase. Thus, the As4O6 species is probably the precursor of arsenic hydroxide through hydrolysis reactions. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Abstract: MP2/6-31++G(d,p) and B3LYP/6-31++G(d,p) theoretical calculations have been employed to investigate the hydrogen bonding formation involving the vic-, cis- and trans-C2H2F2 isomers and hydrogen fluoride. Our calculations have revealed for each isomer the preferential existence of two possible hydrogen-bonded complexes: a non-cyclic complex and a cyclic complex. For all the three isomers the binding energies for the non-cyclic and cyclic hydrogen complexes are essentially equal using both the MP2 and B3LYP calculations, being that the cyclic structure is slightly more stable. For instance, the binding energies including BSSE and ZPE corrections for the non-cyclic and cyclic structures of cis-C2H2F···HF are 8.7 and 9.0kJmol−1, respectively, using B3LYP calculations. The cyclic complex formation reduces the polarity, in contrast to what occurs with the non-cyclic complex. This result is more accentuated in vic-C2H2F2···HF. In this latter, Δμ(cyclic) is −3.07D, whereas Δμ(non-cyclic) is +1.92D using B3LYP calculations. Their corresponding MP2 values are +0.44D and −1.89D, respectively. As expected, the complexation produces an Hing frequency downward shift, whereas its IR intensity is enhanced. On the other hand, the vibrational modes of the vic-, cis- and trans-C2H2F2 isomers are little affected by complexation. The new vibrational modes due to hydrogen bonding formation show several interesting features, in particular the HF bending modes which are pure rotations in the free molecule. [Copyright &y& Elsevier]

The molecular electrostatic potential (MEP) of the indole molecule was calculated in a three-dimensional grid in which the molecule was centered at the origin. To evaluate the dependence of MEP on the type of calculation, semiempirical, ab initio, and density functional theory methods with different basis sets were employed. The data matrix generated by these calculations was analyzed by principal component analysis (PCA). The appearance of outliers and the effect of wavefunction modifications such as the introduction of electron correlations and diffuse functions were highlighted by the use of PCA. The spatial localization of such effects around the molecule was possible from the loadings values associated with the graphical analysis of the grid points. [ABSTRACT FROM AUTHOR]

The stabilities of N-ethyl,N-methylformamide and N-ethyl,N-methylacetamide in their syn and anti forms have been calculated using HF/6-311++G**, MP2(FC)/6-311++G**//HF/6-311++G** and B3LYP/6-311++G**//HF/6-311++G** methods. The calculated NMR chemical shifts were evaluated using the last method. The relative position of the methyl part of the N-ethyl group has been found to be almost perpendicular to the plane of the heavy atoms in disagreement with the previous report. The conformational preference is discussed in terms of both steric and hyperconjugative/anomeric effects. The consequences of these effects on the geometry and NMR spectra have been analyzed. [Copyright &y& Elsevier]

RHF and MP2 ab initio molecular orbital calculations using the 4-31G** and 6-311G** basis sets have been employed to obtain Green''s function matrix elements (GD,A) and intermolecular charge transfers (ΔQcorr) of the CnNH…CnNH and CnNH…NCnH (n=1 and 3) hydrogen bonded complexes. Our calculations reveal that these two parameters are linearly correlated. Their values increase by effect of the addition of a C2 group to the proton donor and/or acceptor and when the hydrogen bonding is formed by the NH…C skeleton in comparison to the NH…N one. Our calculations have also revealed that the corresponding increments in GD,A and ΔQcorr due to change from NCH…NCH to CNH…NCH or from NCH…CNH to CNH…CNH are very similar. Finally, both GD,A and ΔQcorr show the same behavior when the hydrogen bonding distance is progressively increased from the equilibrium position until 4.5 A˚, forming two exponential curves practically superposed. Correlations between ΔE and ΔQcorr as well as ΔE and GD,A are also discussed. [Copyright &y& Elsevier]