José Carlos Netto-Ferreira

Reactivity of fluorenylideneketene towards amines. A laser photolysis study with ultraviolet and infrared detection

Abstract The ketene derived from diazoquinone 5 can be readily detected by laser photolysis techniques with IR detection and its behavior agrees with that observed with more conventional UV-visible detection. The ketene reacts with amines (e.g. k ∼7 × 107 M−1 s−1 for diethylamine) to give ylides (yielding amides as final products) that absorb in the 300–340 nm and 1670–1680 cm−1 regions.

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.

Photochemistry of cyclic vicinal tricarbonyl compounds. Photochemical reaction of 1,2,3-Indanetrione with 2,3-dimethyl-2-butene: hydrogen abstraction and photocycloaddition

A irradiacao na regiao do ultravioleta de 1,2,3-indanotriona, em presenca de 2,3-dimetil-2-buteno, em diclorometano, leva a uma mistura complexa de produtos formados a partir de reacoes de abstracao de hidrogenio, fotocicloadicao [2p+2p] (reacao de Paterno-Buchi) e fotocicloadicao [4p+2p]. A distribuicao de produtos e dependente de fatores eletronicos e estericos, com estes ultimos sendo os responsaveis pelo alto rendimento quimico de formacao do produto resultante da cicloadicao fotoquimica [4p+2p].

Photochemistry of cyclic vicinal tricarbonyl compounds.: [2+2] Photocycloaddition of 1,2,3-indanetrione to electron rich olefins

Photolysis of 1,2,3-indanetrione (1) in dichloromethane, in the presence of 2,3-diphenyl-1,4-dioxene, leads to the formation of the corresponding oxetane 6, through a Paterno-Buchi reaction. The irradiation of 1 in the presence of 2-methyl-2-butene gives, in addition to the oxetane 7, products arising from an allylic hydrogen abstraction process, i.e. E-8 and 9. On the other hand, in the presence of 2,4,4-trimethyl-1-pentene, the only product formed, i.e. 12, results from an allylic hydrogen abstraction reaction from the primary center. The regioselectivity found in some of these reactions was associated to strong steric effects in the approach between triketone triplet and the olefin.

Laser flash photolysis study of the phenolic hydrogen abstraction by 1,2-aceanthrylenedione triplet

The nanosecond laser flash photolysis technique has been used to study the reactivity of 1,2-aceanthrylenedione (1) triplet towards phenols. Using acetonitrile as solvent, the quenching rate constant kq ranged from 3.3 x 105 L mol-1s-1 (phenol) to 1.3 x 107 L mol-1s-1 (4hydroxyphenol). Phenols with electron donating substituents were more reactive. The kinetic isotope effect (kH/kD) observed for the quenching of 1 with 4-methoxyphenol was 1.5. The Hammett plot for the reaction of the triplet of 1 with phenols gave a reaction constant r = 1.04 in acetonitrile solution. The low value for the quenching rate constant measured for this triplet with 1,3-cyclohexadiene (kq=2.0 x 104 L mol-1s-1) and the diffusion controlled quenching rate constant obtained for b-carotene suggest that the energy of the lowest triplet state for 1,2-aceanthrylenedione is located between 88 and 219 kJ mol-1.

Scavenging of photoenols by acids and bases

Abstract Photoenols generated by irradiation of ortho -methylacetophenone, ortho -methylbenzophenone and ortho -benzylbenzophenone are readily quenched by acids and bases. The rate constants for these reaction have been employing laser flash photolysis techniques.

Laser flash photolysis study of the reactivity of α-ketenylbenzocyclobutenone with water and alcohols

Abstract Photolysis of 2-diazo-1,3-indandione leads to ketene formation via the Wolff rearrangement. Ketene 2 hydrates to produce enol 3a that rearranges to 4a. Homophthalic acid 5a is formed by photodecomposition of 4a upon prolonged irradiation.

Time-resolved absorption and emission spectra of triplet state β-phenylpropiophenone adsorbed on silicalite

Abstract Time-resolved triplet absorption and emission spectra of the title compound were obtained using the diffuse reflectance laser flash photolysis (DRLFP) technique. The decays were non-exponential, indicating a non-homogeneous distribution of adsorption sites and different conformational restrictions. Dispersive kinetics enabled us to obtain an average rate constant for the decay of k = (6.9 ± 1.3) × 10 5 s −1 and a width of distribution of γ = 3.4.

Singlet oxygen production by pyrano and furano 1,4-naphthoquinones in non-aqueous medium.

The influence of ring size on the photobehaviour of condensed 1,4-naphthoquinone systems, such as pyrano- and furano-derivatives (1 and 2, respectively) has been investigated. The absorption spectra for both families of naphthoquinones reveal clear differences; in the case of 2 they extend to longer wavelengths. A solvatochromic red shift in polar solvents is consistent with the π,π* character of the S(0)→ S(1) electronic transition in all cases. Theoretical (B3LYP) analysis of the HOMO and LUMO Kohn-Sham molecular orbitals of the S(0) state indicates that they are π and π* in nature, consistent with the experimental observation. A systematic study on the efficiency of singlet oxygen generation by these 1,4-naphthoquinones is presented, and values larger than 0.7 were found in every case. In accordance with these results, laser flash photolysis of deoxygenated acetonitrile solutions led to the formation of detectable triplet transient species with absorptions at 390 and 450 nm (1) and at 370 nm (2), with φ(ISC) close to 1. Additionally, the calculated energies for the T(1) states relative to the S(0) states at UB3LYP/6-311++G** are ca. 47 kcal mol(-1) for 1 and 43 kcal mol(-1) for 2. A comparison of the geometrical parameters for the S(0) and T(1) states reveals a marked difference with respect to the arrangement of the exocyclic phenyl ring whilst a comparison of electronic parameters revealed the change from a quinone structure to a di-dehydroquinone diradical structure.