CLUSTER BEAM CHEMISTRY - HYDRATION OF NUCLEIC-ACID BASES - IONIZATION-POTENTIALS OF HYDRATED ADENINE AND THYMINE

Gas phase hydration of adenine, cytosine, thymine, and uracil in super sonic molecular beams is examined by mass spectra and ionization poten tial measurements. In the beam source, designed for substances of low volatility that readily decompose on heating, a carrier gas (moist arg on or pure water vapor) flows through a porous filter packed with the solid sample before exiting via a supersonic nozzle. With the electron impact energy below the ionization potential of water, progressions o f cluster ions of the form B-m(+)(H2O)(n) with m = 1-3 and n as high a s 15 are obtained. Cytosine, even with modest heating, loses two hydro gens and also undergoes tautomeric conversion to uracil. From appearan ce potential measurements and calibrations, the ionization potentials for B = adenine or thymine clusters (with m = 1, n = 0-3; m = 1 or 2, n = 0-1 or 2) were determined; relative to the neutral base monomer, h ydration by three water molecules decreases the ionization potential b y about 8%, or 0.7 eV.