DFT study of the fragmentation mechanism of uracil RNA base

The fragmentation process of the uracil RNA base has been investigated via DFT calculations in order to assign fragments to the ionisation mass spectrum obtained after dissociation induced by collision experiments. The analysis of the electronic distribution and geometry parameters of the cation allows selection of several bonds that may be cleaved and lead to the formation of various fragments. Differences are observed in the electronic behaviour of the bond breaking as well as the energy required for the cleavage. It is reported that N-3-C-4 and N-1-C-2 bonds are more easily cleaved than the C-5-C-6 bond, since the corresponding energy barriers amount to Delta G = +1.627, + 1.710, + 5.459 eV, respectively, which makes the C-5-C-6 bond cleavage almost prohibited. Among all possible formed fragments, the formation of the OCN+ fragment for the peak at m/z = 42 Da is excluded because of an intermediate that was not observed experimentally and too a large free energy barrier. Based on the required free energy, it is observed that two fragment derivatives: C2H4N+ and C2H2O center dot+ may be formed, with a small preference for C2H4N+. This latter product is not formed through a retro Diels Alder reaction in contrast to C2H2O center dot+. The following sequence is proposed for the peak at 42 Da: C2H4N+ (from N-1-C-2, C-4-C-5 cleavages) > C2H2O center dot+ (from N-3-C-4, N-1-C-2 and C-5-C-6 cleavages) > C2H4N+ (from N-1-C-2, N-3-C-4 and C-4-C-5) > C2H2O center dot+ (from C-5-C-6, N-1-C-2 and N-3-C-4 cleavages) > NCO+ (from N-1-C-2, C-4-C-5 and N-3-C-4 cleavages). Finally the peak at 28 Da is assigned to CNH2+ derivatives that can be formed through two different paths, the easiest one requiring 5.4 eV.