RESUMO
beta- and gamma-lumicolchicines are photoproducts formed by the cycloisomerization of the tropolone ring of colchicine (COL) alkaloids. The mechanism of the photoconversion, suggested to involve the triplet state, is examined here by studying the effect of the solvent polarity on the lumicolchicine photoisomer ratio. Triplet COL, detected by laser flash photolysis, is quenched by oxygen, but not by transtilbene or 1-methylnaphtalene. Neither the quantum yield of conversion of COL nor the photoproduct ratio was altered by the presence of oxygen. Likewise, energy transfer to COL from triplet acetone produced by either isobutanal/horseradish peroxidase system or tetramethyldioxetane thermolysis failed to provoke photoreaction of COL. Our data argue against the intermediacy of a COL triplet state in the photoisomerization and stress on the role of specific solvent-solute interactions in determining the partitioning of excited singlet state into the beta- and gamma-isomer formation.
Assuntos
Colchicina/química , Solventes/química , Estrutura Molecular , FotoquímicaRESUMO
1. The fluorescence behavior of a non-carcinogenic (pyrene) and a carcinogenic (benzo[a]pyrene) aromatic hydrocarbon was examined in the presence of a phospholipid bilayer membrane in the gel phase. 2. The monomer emission spectrum of pyrene in the membrane is very similar to that in micelles indicating a site near the aqueous surface region. Benzo[a[pyrene monomer spectra exhibit a red shift in the membrane relative to aliphatic hydrocarbon solvents. On the basis of spectral shifts in other homogeneous solvents, it is inferred that the carcinogen is located in the upper portion of the membrane acyl region, a more polarizable environment than the hydrocarbon core of the bilayer. 3. Analysis of the excimer to monomer emission intensity ratio as a function of probe molar ratio indicates that pyrene is much less soluble in the membrane than benzo[a]pyrene. 4. These results complement published EPR data which show that carcinogenic aromatic hydrocarbons cause structural changes in the membrane, while non-carcinogenic ones do not. These differences in membrane solubility and ability to alter membrane structure are discussed in the context of the different carcinogenic potencies of the hydrocarbons.