PHOTOCHEMICAL TOOLS FOR THE MANIPULATION OF BIOLOGICAL MACROMOLECULES
The reactive intermediate produced upon photolysis of 8-azidoadenosine was studied by chemical trapping studies, laser flash photolysis with UV-Vis and IR detection, and modern computational chemistry. It is concluded that photolysis of 8-azidoadenosine in aqueous solution releases the corresponding singlet nitrene which rapidly tautomerizes to form a closed adenosine diazaquinodimethane in less than 400 fs. The singlet nitrene is too short lived to be observed, and thus, to relax to the lowest triplet state or to become covalently attach to targeted biological macromolecules. The pivotal closed adenosine diazaquinodimethane, the product of nitrene tautomerization, has a lifetime of several minutes, ca. a minute or longer in water and in HEPES buffer at ambient temperature. However, this intermediate reacts rapidly with good nucleophiles such as amines, thiols, and phenolates, and significantly more slowly with weak nucleophile such as alcohols and water. On the basis of these studies, it is clear that the closed adenosine diazaquinondimethane is the pivotal reactive intermediate involved in photolabeling and cross-linking studies using the 8-azidoadenosine family of photoaffinity labeling reagents.Pyrene dihydrodioxins have been synthesized and shown to be effective photochemical blocking groups for pyrene-4,5-dione. The mechanism of quinone release proceeds through the formation of a remarkably stable radical cation. Direct evidence is provided that this radical cation is not only thermally labile, but also is photochemically labile, and that both pathways lead to quinone extrusion. Once initiated with UV light, the pyrene quinone product serves as an electron transfer photosensitizer for the further release of quinone with visible light. The water soluble pyrene dihydrodioxin has been shown to bind to duplex DNA by intercalation (Ka = 7 x 10 ^5 ) and cleave the ?X 174 supercoiled plasmid upon irradiation with UV light (364 nm). This compound also exhibits cytotoxic activity at the micromolar range in a number of human cancer cell lines. Extending these ideas, a pyrene dihydrodioxin incorporating a pyridinium salt as a possible internal electron trap has been shown to release quinone in the presence of dsDNA. The quinone release from an acenaphthene keto-oxetane compound has been briefly investigated.
School:University of Cincinnati
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:photochemistry 8 azidoadenosine dihydrodioxin radical cation
Date of Publication:01/01/2005