RESUMEN
EPR measurements were made at Q-band microwave frequencies on powder samples pressed into pellets and X-irradiated at 4 K. Measurements were made at 4 K after no anneal, then after a 77 K anneal, and then after a 300 K anneal. In poly(dA):poly(T) the free radical distribution is approximately a simple sum of the distributions in the separate homopolymers. In poly(dA-T) the free radical distribution differs from that of poly(dA):poly(T). The clearest difference is that in poly-(dA):poly(T) the concentration of one-electron-reduced thymine (Tre.) is reduced relative to the total radical concentration. On warming the thymine-containing samples from 77 K to room temperature, the Tox. radical disappears and the (T-H5'). radical appears. Also, the Tre. radical disappears and the (T + H6). radical appears. There are three main conclusions. First, little or no transfer of free radicals between strands is needed to explain the data. Second, when A and T are interstacked, either the Tre. radical is less stable against recombination than other radical products or radical transfer occurs to an adjacent adenine. Third, in the poly- and oligonucleotides, the Tox. radical is a likely precursor to the (T-H5'). radical.
Asunto(s)
Radicales Libres , Poli dA-dT/efectos de la radiación , Polidesoxirribonucleótidos/efectos de la radiación , Espectroscopía de Resonancia por Spin del Electrón , Congelación , Rayos XRESUMEN
Dose-response curves for free radical trapping in monomers, dimers, and polymers of dAMP and TMP have been measured. Powder samples pressed into pellets were X-irradiated and observed at 4 K using EPR at Q-band microwave frequencies. The initial free-radical yield (G value) and the destruction rate (k value) are reported for 22 samples. The absolute magnitude and relative changes in G are informative. For example, the duplex of homopolymers, poly(dA):poly(T)(Na), gives a large G of 6 while the closely related duplex of alternating ATs, poly(dA-T)(Na), gives a G value of 3. At G of around 6, the range of the bound electrons (e-) and electron loss centers (holes) is believed to be very limited. Two factors are suggested as important to limiting the range, proton transfer between strands and the low probability of radical transfer between strands. The lower G values are viewed as being due to relatively small increases in the range of e- and/or holes.