Iron-sulfur proteins involved in electron transfer reactions have finely tuned redox potentials, which allow them to be highly efficient and specific. Factors such as metal center solvent exposure, interaction with charged residues, or hydrogen bonds between the ligand residues and amide backbone groups have all been pointed out to cause such specific redox potentials. Here, we derived parameters compatible with the AMBER force field for the metal centers of iron-sulfur proteins and applied them in the molecular dynamics simulations of three iron-sulfur proteins. We used density-functional theory (DFT) calculations and Seminario's method for the parameterization. Parameter validation was obtained by matching structures and normal frequencies at the quantum mechanics and molecular mechanics levels of theory. Having guaranteed a correct representation of the protein coordination spheres, the amide H-bonds and the water exposure to the ligands were analyzed. Our results for the pattern of interactions with the metal centers are consistent to those obtained by nuclear magnetic resonance spectroscopy (NMR) experiments and DFT calculations, allowing the application of molecular dynamics to the study of those proteins.

O trabalho consiste na re-investigação dos frutos de Iryanthera juruensis e Virola pavonis e das folhas de V. michelii, bem como no estudo dos frutos de V. mollissima. A partir de I. juruensis foram isoladas uma neolignana ariltetralínica (1) e uma neolignana tetraidrofurânica (2), enquanto que de Virola pavonis foram isoladas neolignanas benzofurânicas (6-9), tetraidrofurânicas (2, 11-13, 17), bifenílica (10), diastereoisômeros de uma hidróxi-neolignana 8.O.4'(14-15) e outras. V. mollissima acumula a neolignana ariltetralônica 4 ou o seu derivado seco (5). As folhas de V. michelii apresentaram a ocorrência de lignanas furofurânicas (18)(19). Os lignóides 1 e 2 obtidos dos arilos de I. juruensis apresentaram atividade leishmanicida contra a forma promastigota de Leishmania amazonensis.This work revisits the fruits of Iryanthera juruensis and Virola pavonis and the leaves from V. michelii, as well as describing a study of the fruits of V. mollissima. In I. juruensis aryltetraline neolignan (1) and tetrahydrofuran neolignan (2), were found while from V. pavonis neolignans of benzofuran type (6-9), the tetrahydrofuran type (2, 11-13, 17) and the biphenyl type (10), in addition to diastereoisomers of the 8.O.4'-oxyneolignan type (14 and 15) and others were isolated. The V. mollissima accumulates the aryltetralone neolignan 4 and its seco derivative (5). The lignoids 1 and 2 obtained from I. juruensis arils possess antileishmanial activity against the promastigote form of Leishmania amazonensis.

Virola venosa, popularly known in Brazil as ucuuba-da-mata, occurs naturally in the Amazon region and has potential to provide useful natural compounds, as already known for other Virola species. Therefore, the objective of this study was to determine the chemical composition of bark and leaf extracts of V. venosa, and to test the antioxidant capacity and α-glucosidase inhibition potential of their compounds. Polar extracts showed to be more active in both assays, therefore a bioactivity-guided fractionation was performed to identify the compounds that were responsible for the recorded activities. Using a combination of LC-MS/MS analysis and isolation with NMR identification, eight phenolic compounds were identified. Assays with pure compounds of the active fraction revealed that ferulic acid was the main contributor compound to the observed bioactivity in the crude extracts.

Recebido em 12/2/03; aceito em 10/11/03 NMR INVESTIGATION AND THEORETICAL CALCULATIONS OF THE EFFECT OF SOLVENT ON THE CONFORMA-TIONAL ANALYSIS OF 4',7-DI-HYDROXY-8-PRENYLFLAVAN. The NMR conformational study of 4',7-di-hydroxy-8-prenylflavan 1 was carried out in acetone-d6, DMSO-d6 and CDCl 3 which enabled the proposition of three conformations, namely 1a, 1b and 1c, differing in the position of the prenyl group. Geometry optimizations performed using AM1 method showed that 1a (∆H f = -86.2 kcal/mol) is as stable as 1b (∆H f = -85.1 kcal/mol) and 1c (∆H f = -85.4 kcal/mol). When the solvent was included, the calculations showed that the solute-solvent interactions could be explained either in the light of the electronic intermolecular delocalization or the electrostatic character between solute and solvent. Theoretical calculations (HF/6-31G*, DFT/BLYP/6-31G*, and DFT/B3LYP/6-31G*) showed that the combination of these types of interactions present in each solute-solvent system, dependent on the chemical properties of the solvent, lead to different spatial arrangements of the prenyl group, which in turn determined the conformation of 1.

A new flavan, identified as 4′,7‐dihydroxy‐8‐prenylflavan, was isolated from the ethanolic extract of the stem bark of Brosimum acutifolium. Its structure was elucidated by NMR methods. Copyright © 2000 John Wiley & Sons, Ltd.

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