Biomedical subjects

Albérico B F da Silva

Publications and source records attributed to Albérico B F da Silva.

7 recordsLinked to original sources

PubMed2006

Rate coefficient for the reaction SiO + Si2O2 at T = 10-1000 K.

The reaction paths for the formation of Si3O3 molecules have been investigated at high level ab initio quantum chemical calculations by using the QCISD method with the 6-311++G(d,p) basis set. The cis-Si2O2 isomer does not participate in the chemical mechanism for the formation of Si3O3 molecules. Although the SiO + cis-Si2O2 reaction is exothermic and spontaneous, it is not expected to explain the growth mechanism of Si3O3 in the interstellar silicate grains of circumstellar envelopes surrounding M-type giants. The reaction of SiO with cyclic Si2O2 molecules is exothermic, is spontaneous, and has a nonplanar transition state. The Gibbs free energy for the transition state formation, (DeltaG0#), is around 5.5 kcal mol-1 at 298 K. The bimolecular rate coefficient for this reaction, kT, is about 1 x 10-12 cm3 molecule-1 s-1 at 298 K and in the collision limit, 1.5 x 10-10 cm3 molecule-1 s-1, at 500 K. The activation energy, Ea, is about 8 kcal mol-1. The enthalpy of Si3O3 fragmentation is 53.9 kcal mol-1 at 298 K. The SiO + cyclic Si2O2 reaction is expected to be the most prominent reaction path for the Si3O3 formation in interstellar environment and fabrication of silicon nanowires.

Journal Article↗

PubMed2006

The nuclear electric quadrupole moment of antimony from the molecular method.

Relativistic Dirac-Coulomb (DC) Hartree-Fock calculations are employed to obtain the analytic electric field gradient (EFG) on the antimony nucleus in the SbN, SbP, SbF, and SbCl molecules. The electronic correlation contribution to the EFGs is included with the DC-CCSD(T) and DC-CCSD-T approaches, also in the four-component framework, using a finite-difference method. The total EFG results, along with the experimental nuclear quadrupole coupling constants from microwave spectroscopy, allow to derive the nuclear quadrupole moments of (121)Sb and (123)Sb, respectively, as -543(11) and -692(14) mb.

Journal Article↗

PubMed2006

The use of classification methods for modeling the antioxidant activity of flavonoid compounds.

A study using two classification methods (SDA and SIMCA) was carried out in this work with the aim of investigating the relationship between the structure of flavonoid compounds and their free-radical-scavenging ability. In this work, we report the use of chemometric methods (SDA and SIMCA) able to select the most relevant variables (steric, electronic, and topological) responsible for this ability. The results obtained with the SDA and SIMCA methods agree perfectly with our previous model, in which we used other chemometric methods (PCA, HCA and KNN) and are also corroborated with experimental results from the literature. This is a strong indication of how reliable the selection of variables is.

Antioxidants↗

PubMed2005

Deactivation of triplet-excited riboflavin by purine derivatives: important role of uric acid in light-induced oxidation of milk sensitized by riboflavin.

The reactivity of purine derivatives (uric acid, xanthine, hypoxanthine, and purine) toward triplet-excited riboflavin in aqueous solution at pH 6.4 is described on the basis of kinetic (laser flash photolysis), electrochemical (square-wave voltammetry), and theoretical data (density functional theory, DFT). Direct deactivation of triplet-excited riboflavin in aqueous solution, pH 6.4 at 24 degrees C, in the presence of uric acid, xanthine, and hypoxanthine strongly suggests a direct electron transfer from the purine to the triplet-excited riboflavin with k = 2.9 x 10(9) M(-1) s(-1) (DeltaH(++) = 14.7 kJ mol(-1), DeltaS(++) = -15.6 J mol(-1) K(-1)), 1.2 x 10(9) M(-1) s(-1) (DeltaH(++) = 34.3 kJ mol(-1), DeltaS(++) = +45.3 J mol(-1) K(-1)), and 1.7 x10(8) M(-1) s(-1) (DeltaH(++) = 122 kJ mol(-1), DeltaS(++) = +319 J mol(-1) K(-1)), respectively. From the respective one-electron oxidation potentials collected in aqueous solution at pH 6.4 for uric acid (E = +0.686 vs normal hydrogen electrode, NHE), xanthine (E = +1.106 vs NHE), and hypoxanthine (E = +1.654 vs NHE), the overall free energy changes for electron transfer from the quencher to the triplet-excited riboflavin are as follows: uric acid (DeltaG(o) = -114 kJ mol(-1)), xanthine (DeltaG(o) = -73.5 kJ mol(-1)), hypoxanthine (DeltaG(o) = -20.6 kJ mol(-1)), and purine (DeltaG(o) > 0). The inertness observed for purine toward triplet-excited riboflavin corroborates with its electrochemical inactivity in the potential range from 0 up to 2 V vs NHE. These data are in agreement with the DFT results, which show that the energy of the purine highest occupied molecular orbital (HOMO) (-0.2685 arbitrary unit) is lower than the energy of the semioccupied molecular orbital (SOMO) (-0.2557 a.u.) of triplet-excited riboflavin, indicating an endergonic process for the electron-transfer process. The rate-determining step for deactivation by purine derivatives can be assigned to an electron transfer from the purine derivative to the SOMO orbital of the triplet-excited riboflavin. The results show that uric acid may compete with oxygen and other antioxidants to deactivate triplet-excited riboflavin in milk serum and other biological fluids leading to a free radical process.

Animals↗

PubMed2005

A study on the influence of molecular properties in the psychoactivity of cannabinoid compounds.

Several molecular properties are calculated for a set of 26 cannabinoid compounds with the goal of connecting the psychoactivity of the compounds with an appropriate set of calculated properties. For this purpose we used quantum chemical (the AM1 semi-empirical method) and chemometric methods. The AM1 method was employed to calculate the set of quantum chemical molecular properties and the chemometric methods were employed with the aim of selecting the most relevant properties to be correlated with psychoactivity. The chemometric methods used were Principal Component Analysis (PCA), Hierarchical Cluster Analysis (HCA) and the K-Nearest Neighbor (KNN) method. The chemometric analysis showed that an electronic property (energy of LUMO), a hydrophobic property (log P), a steric property (volume of the substituent at the C4 position) and a topological property (Lovasz-Pelikan index) were the most important variables for the separation between the psychoactive and psychoinactive compounds. In order to validate our PCA, HCA and KNN results, eight new cannabinoid compounds (with known psychoactivity) were used in a prediction study and were classified correctly by the methods used in this work, indicating that our PCA, HCA and KNN models are able to predict reliable psychoactivity of cannabinoid compounds.

Animals↗

PubMed2004

A polynomial version of the generator coordinate Dirac-Fock method.

A polynomial version of the Generator Coordinate Dirac-Fock (p-GCDF) method is introduced and applied to develop Adapted Gaussian Basis Sets (AGBS) for helium- and beryllium-like atomic species (He, Ne +8, Ar +16, Sn +48, Be, Ne +6, Ar +14, and Sn +46) and for Kr and Xe atoms. The Dirac-Fock-Coulomb and Dirac-Fock-Breit energies obtained with these basis sets are in excellent agreement with numerical finite-difference calculations. Moreover, the sizes of the AGBS generated here with the p-GCDF method are significantly smaller than the size of previous relativistic Gaussian basis sets.

Journal Article↗

PubMedDate not supplied

A study on the antipicornavirus activity of flavonoid compounds (flavones) by using quantum chemical and chemometric methods.

The AM1 semiempirical method is employed to calculate a set of molecular properties (variables) of 45 flavone compounds with antipicornavirus activity, and 9 new flavone molecules are used for an activity prediction study. Principal Component Analysis (PCA), Hierarchical Cluster Analysis (HCA), Stepwise Discriminant Analysis (SDA), and K-Nearest Neighbor (KNN) are employed in order to reduce dimensionality and investigate which subset of variables should be more effective for classifying the flavone compounds according to their degree of antipicornavirus activity. The PCA, HCA, SDA, and KNN methods showed that the variables MR (molar refractivity), B(9) (bond order between C(9) and C(10) atoms), and B(25) (bond order between C(11) and R(7) atoms) are important properties for the separation between active and inactive flavone compounds, and this fact reveals that electronic and steric effects are relevant when one is trying to understand the interaction between flavone compounds with antipicornavirus activity and the biological receptor. In the activity prediction study, using the PCA, HCA, SDA, and KNN methodologies, three of the 9 new flavone compounds studied were classified as potentially active against picornaviruses.

Antiviral Agents↗