The exposure of human skin to continuous UV sunlight can cause a variety of pathological effects such as: sunburn, certain forms of skin cancer, aging and disorders in the immune system. At the molecular level these effects have been explained in terms of photoinduced base lesions in the nucleic acids. There have been reports of purine photoproducts in UV irradiated DNA which could also lead to these deleterious effects. Stable products of the purine moieties of DNA or reactive intermediates participating in these photochemical processes have not been studied in detail. In this continuation proposal our main objectives are to: 1. Continue with the study and characterization of excited states and reactive intermediates participating in the photochemical and photodestruction reactions of purine dinucleosides, oligo, and polynucleotides used as DNA model compounds. These will be characterized from their absorption spectra and kinetic behavior obtained with laser flash photolysis techniques. 2. Study the reactions of purine radical cations with biologically important molecules such as amino acids, sugars and with oxygen using laser flash techniques. 3. Identify and characterize the major products in the photodestruction mechanism of purine nucleosides, nucleotides and dinucleosides and correlate these products to their short lived intermediate precursors. These goals will be accomplished by performing quantum yield measurements and using spectroanalytical techniques. 4. Identify the intermediates participating in the purine photoinitiated cross-linking reactions with molecules considered to be models for the side chains of amino acids and with the amino acids.