Photoaffinity labeling with 8-azidoadenosine and its derivatives: chemistry of closed and opened adenosine diazaquinodimethanes.

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. A perbenzoylated derivative of 8-azidoadenosine cannot undergo this tautomerization, and instead, it fragments upon photolysis to form an opened adenosine diazaquinodimethane. The singlet nitrene is too short-lived to be observed and, thus, to relax to the lowest triplet state or to become covalently attached to targeted biological macromolecules. The pivotal closed adenosine diazaquinodimethane, the product of nitrene tautomerization, has a lifetime of ca. 1 min 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 nucleophiles such as alcohols and water. On the basis of these studies, it is clear that the closed adenosine diazaquinodimethane, and not the singlet or triplet nitrene, is the pivotal reactive intermediate involved in photolabeling and cross-linking studies using the 8-azidoadenosine family of photoaffinity labeling reagents.