Aurélio B. B. Ferreira

Correlation of Total Phenolic and Flavonoid Contents of Brazilian Honeys with Colour and Antioxidant Capacity

Sixty Brazilian honey samples were analysed for their total phenolic content with the Folin-Denis reagent, total flavonoid content by aluminium chloride method, and antioxidant activity by reaction with 2,2-diphenyl-1-picrylhydrazyl radical. Colour was also classified according to visual analysis and Pfund scale. Linear relationships were observed between colour and flavonoid content, total phenolics and antioxidant capacity, and total flavonoid and phenolic contents. The white-coloured Citrus honey showed the lowest antioxidant activity, while the light ambar Verbenaceae honey showed the highest total phenolics and antioxidant activity. Dark-coloured and polyfloral honeys, though less popularized among consumers, showed average to high antioxidant capacity.

Fluorescence and Docking Studies of the Interaction between Human Serum Albumin and Pheophytin.

In the North of Brazil (Pará and Amazonas states) the leaves of the plant Talinum triangulare (popular: cariru) replace spinach as food. From a phytochemical point of view, they are rich in compounds of the group of pheophytins. These substances, related to chlorophyll, have photophysical properties that give them potential application in photodynamic therapy. Human serum albumin (HSA) is one of the main endogenous vehicles for biodistribution of molecules by blood plasma. Association constants and thermodynamic parameters for the interaction of HSA with pheophytin from Talinum triangulare were studied by UV-Vis absorption, fluorescence techniques, and molecular modeling (docking). Fluorescence quenching of the HSA’s internal fluorophore (tryptophan) at temperatures 296 K, 303 K, and 310 K, resulted in values for the association constants of the order of 104 L∙mol−1, indicating a moderate interaction between the compound and the albumin. The negative values of ΔG° indicate a spontaneous process; ΔH° = 15.5 kJ∙mol−1 indicates an endothermic process of association and ΔS° = 0.145 kJ∙mol−1∙K−1 shows that the interaction between HSA and pheophytin occurs mainly by hydrophobic factors. The observed Trp fluorescence quenching is static: there is initial non-fluorescent association, in the ground state, HSA:Pheophytin. Possible solution obtained by a molecular docking study suggests that pheophytin is able to interact with HSA by means of hydrogen bonds with three lysine and one arginine residues, whereas the phytyl group is inserted in a hydrophobic pocket, close to Trp-214.

A Study of the Interaction Between trans-Dehydrocrotonin, a Bioactive Natural 19-nor-Clerodane, and Serum Albumin

The interaction between 19-nor-clerodane trans-dehydrocrotonin (from Croton cajucara Benth.) and bovine serum albumin was studied, applying spectroscopic techniques (fluorescence and circular dichroism), combined with molecular modeling. Fluorescence quenching of albumin by the nor-clerodane (kq ca. 1011 mol L-1 s-1 and Stern-Volmer, KSV, increase with temperature) indicates a combination of static and dynamic quenching mechanism. The binding constant (Kb ca. 103 mol L-1) and circular dichroism data suggest that this association is weak and causes only a moderate change in the α-helix content of the protein. Thermodynamic parameters indicate a spontaneous (Gibbs free energy, ΔGo, ca. -21.28 kJ mol-1 at 310 K) and probably entropy-driven (ΔSo = 0.072 kJ mol-1 K-1) association, typical of hydrophobic interactions. The number of binding sites (n ca. 1) indicates one main binding site and molecular modeling suggests subdomain IIIA (Sudlows site II) as the main binding site to the nor-clerodane, which is able to make hydrophobic interactions with leucine (Leu)-24, phenylalanine (Phe)-36, valine (Val)-40 and tryptophan (Trp)-134 residues.

The (α-1,6) glycosidic bond of isomaltose: a tricky system for theoretical conformational studies

Stable conformations of beta-isomaltose (alpha-D-glucopyranosyl-(1-->6)-beta-D-glucose) in gas-phase and aqueous solution are investigated in this study using quantum mechanical calculations. Conformational maps are calculated at HF/6-31G(d,p) level and lower energy structures are sampled in the most stable regions. Entropic and thermal corrections are considered and the Boltzmann population is obtained for conformers that are representative of the 18 most stable regions found on the potential energy surface. B3LYP/6-31+G(d,p) and B3LYP/6-311+G(2d,2p) calculations are used in conformational samplings. Solvation effects are considered through the polarizable continuum model approach. Hydroxymethyl group orientations are investigated for the most stable conformers. The influence of electronic correlation and solvation on the glycosidic linkage preference (TG, GT, and GG) and hydroxymethyl group orientation (tg, gt, and gg) are discussed. Heteronuclear spin coupling constants ((3)J(C,H)) along the glycosidic linkage are calculated and comparison with other theoretical results and experiments is used to validate the obtained structures.

Determination of lapachol in the presence of other naphthoquinones using 3MPA-CdTe quantum dots fluorescent probe

3MPA-CdTe QDs in aqueous dispersion was used as a fluorescent probe for the determination of lapachol, a natural naphthoquinone found in plants of the Bignoniaceae family genus Tabebuia. Working QDs dispersions (4.5×10(-8) mol L(-1) of QDs) was prepared in aqueous media containing Tris-HCl buffer pH 7.4 and methanol (10% in volume). The excitation was made at 380 nm with signal measurement at 540 nm. To establish a relationship between fluorescence (corrected to inner filter effect) and concentration of lapachol, a Stern-Volmer model was used. The linear range obtained was from 1.0×10(-5) to 1.0×10(-4) mol L(-1). The limit of detection (x(b)-3s(b)) was 8.0×10(-6) mol L(-1). The 3MPA-CdTe QDs probe was tested in the determination of lapachol in urine, previously cleansed in an acrylic polymer. The average recovery was satisfactory.

Photosensitizing Properties of Triplet β‐Lapachones in Acetonitrile Solution

The photochemical reactivity of β‐lapachone (1), nor‐β‐lapachone (2) and 1,2‐naphthoquinone (3) towards amino acids and nucleobases or nucleosides has been examined employing the nanosecond laser flash photolysis technique. Excitation (λ = 355 nm) of degassed solutions of 1–3, in acetonitrile, resulted in the formation of their corresponding triplet excited states. These transients were efficiently quenched by l‐tryptophan, l‐tryptophan methyl ester, l‐tyrosine, l‐tyrosine methyl ester and l‐cysteine (kq ≈ 109 L mol−1 s−1). For l‐tryptophan, l‐tyrosine and their methyl esters new transients were formed in the quenching process, which were assigned to the corresponding radical pair resulting from an initial electron transfer from the amino acids or their esters to the excited quinone, followed by a fast proton transfer. No measurable quenching rate constants could be observed in the presence of thymine and thymidine. On the other hand, efficient rate constants were obtained when 1–3 were quenched by 2′‐deoxyguanosine (kq ≈ 109 L mol−1 s−1). The quantum efficiency of singlet oxygen (1O2) formation from 1 to 3 was determined employing time‐resolved near‐IR emission studies upon laser excitation and showed considerably high values in all cases (ΦΔ = 0.6), which are fully in accord with the ππ* character of these triplets in acetonitrile.

Atividade biológica do lapachol e de alguns derivados sobre o desenvolvimento fúngico e em germinação de sementes

The natural quinones lapachol, α-lapachone and β-lapachone, and the synthetic derivative β-lapachone-3-sulfonic-acid were assayed for inhibition of fungal growth (Fusarium oxysporum) and germination of lettuce seeds (Lactuca sativa L.). β-Lapachone has the strongest activity as a germination inhibitor and lapachol shows no effect. β-Lapachone, followed by lapachol, are the most active in reducing fungal growth.

Studies of the Interaction between BSA and a Plumeran Indole Alkaloid Isolated from the Stem Bark of Aspidosperma cylindrocarpon (Apocynaceae)

Binding between bovine serum albumin (BSA) and a plumeran indole alkaloid (PIA) isolated from the stem bark of Aspidosperma cylindrocarpon (Apocynaceae) was studied by spectroscopic techniques (UV-Vis absorption, circular dichroism, steady state and time-resolved fluorescence), combined with molecular docking. Steady state and time resolved fluorescence data revealed that PIA can quench the BSA fluorescence via a static mechanism: energy transfer from BSA to PIA occurs with high probability. The binding is strong (Kb ca. 10-10 L mol), spontaneous (ΔG° ca. –35.7 kJ mol at 310 K) and entropy-driven (ΔS° = 0.146 kJ mol K). There is just one main binding site (n ca. 1) for the BSA:PIA interaction and the α-helix content of the albumin does not suffer significant perturbation upon PIA binding. Molecular docking results suggest site I as the main binding site to PIA, which is able to interact with the Trp-212, Arg-217, Val-342 and Pro-446 residues.

Synthesis of imidazole derivatives from β-lapachone and related compounds using microwave and supported reagents

New naphthoimidazoles were prepared by reaction, activated by microwave irradiation, between paraformaldehyde, β-lapachone (or related o-quinones) and ammonium acetate supported on montmorillonite k-10, or on basic alumina. Use of the basic support gave the best results, with yields above 80%. This was confirmed for the reaction with β-lapachone and piperonal.

Crystal structure and hydrogen bonding study of (10E)-2,2-dimethyl-3,4-dihydro-2H-benzo[g]chromene-5,10-dione 10-oxime derived from α-lapachone.

The compound (10E)-2,2-dimethyl-3,4-dihydro-2H-benzo[g]chromene-5,10-dione-10-oxime (1) was synthesized from α-lapachone and hydroxylamine chloride in alkaline medium. Single-crystals suitable for X-ray diffraction measurements were grown from an ethanol solution, and the crystal structure of the title molecule is reported for the first time. The title molecule was also characterized by 1H- and 13C-NMR in CDCl3 solution, FTIR and MS. The crystal structure of 1 shows an E stereochemistry and dimers formed through classical hydrogen bonds.