Comparison of the collision-induced dissociation of duplex DNA at different collision regimes: evidence for a multistep dissociation mechanism.

The dissociation mechanism of duplex DNA has been investigated in detail by collision-induced dissociation experiments at different collision regimes. MS/MS experiments were performed either in a quadrupole collision cell (hybrid quadrupole-TOF instrument) or in a quadrupole ion trap with different activation times and energies. In addition to the noncovalent dissociation of the duplex into the single strands, other covalent bond fragmentation channels were observed. Neutral base loss from the duplex is favored by slow activation. In fast activation conditions, however, the major reaction channel is the noncovalent dissociation into single strands, which is highly entropy-favored. Fast activation regimes can favor the entropy-driven noncovalent dissociation, while in slow heating conditions the competition with enthalpy-driven covalent fragmentation can completely hinder the dissociation of the complex. We also evidence that the noncovalent dissociation of DNA duplex is a multistep process involving a progressive unzipping, preferentially at terminal positions. This is proposed to be a general feature for complexes containing a high number of contributing interactions organized at the interface of the ligands. The overall (observed) dissociation kinetics of noncovalent complexes can therefore depend on a complicated mechanism for which a single transition state description of the kinetics is too simplistic.