ACTIVATION-ENERGIES FOR DISSOCIATION OF DOUBLE-STRAND OLIGONUCLEOTIDEANIONS - EVIDENCE FOR WATSON-CRICK BASE-PAIRING IN-VACUO

The dissociation kinetics of a series of complementary and noncompleme ntary DNA duplexes, (TGCA)(2)(3-), (CCGG)(2)(3-), (AATTAAT)(2)(3-), (C CGGCCG)(2)(3-), A(7). T-7(3-), A(7). A(7)(3-), T-7. T-7(3-), and A(7). C-7(3-) were investigated using blackbody infrared radiative dissocia tion in a Fourier transform mass spectrometer. From the temperature de pendence of the unimolecular dissociation rate constants, Arrhenius ac tivation parameters in the zero-pressure limit are obtained. Activatio n energies range from 1.2 to 1.7 eV, and preexponential factors range from 10(13) to 10(19) s(-1). Dissociation of the duplexes results in c leavage of the noncovalent bonds and/or cleavage of covalent bonds lea ding to loss of a neutral nucleobase followed by backbone cleavage pro ducing sequence-specific (a - base) and w ions. Four pieces of evidenc e are presented which indicate that Watson-Crick (WC) base pairing is preserved in complementary DNA duplexes in the gas phase: i. the activ ation energy for dissociation of the complementary dimer, A(7). T-7(3- ), to the single strands is significantly higher than that for the rel ated noncomplementary A(7). A(7)(3-) and T-7. T-7(3-) dimers, indicati ng a stronger interaction between strands with a specific base sequenc e, ii. extensive loss of neutral adenine occurs for A(7). A(7)(3-) and A(7). C-7(3-) but not for A(7). T-7(3-) consistent with this process being shut down by WC hydrogen bonding, nl. a correlation is observed between the measured activation energy for dissociation to single stra nds and the dimerization enthalpy (-Delta H-d) in solution, and iv. mo lecular dynamics carried out at 300 and 400 K indicate that WC base pa iring is preserved for A(7). T-7(3-) duplex, although the helical stru cture is essentially lost. In combination, these results provide stron g evidence that WC base pairing can exist in the complete absence of s olvent.