Electron transfer driven decomposition of adenine and selected analogs as probed by experimental and theoretical methods

Abstract

We report on a combined experimental and theoretical study of electron-transfer-induced decomposition of adenine (Ad) and a selection of analog molecules in collisions with potassium (K) atoms. Time-of-flight negative ion mass spectra have been obtained in a wide collision energy range (6-68 eV in the centre-of-mass frame), providing a comprehensive investigation of the fragmentation patterns of purine (Pu), adenine (Ad), 9-methyl adenine (9-mAd), 6-dimethyl adenine (6-dimAd), and 2-D adenine (2-DAd). Following our recent communication about selective hydrogen loss from the transient negative ions (TNIs) produced in these collisions [T. Cunha et al., J. Chem. Phys. 148, 021101 (2018)], this work focuses on the production of smaller fragment anions. In the low-energy part of the present range, several dissociation channels that are accessible in free electron attachment experiments are absent from the present mass spectra, notably NH loss from adenine and 9-methyl adenine. This can be understood in terms of a relatively long transit time of the K cation in the vicinity of the TNI tending to enhance the likelihood of intramolecular electron transfer. In this case, the excess energy can be redistributed through the available degrees of freedom inhibiting fragmentation pathways. Ab initio theoretical calculations were performed for 9-methyl adenine (9-mAd) and adenine (Ad) in the presence of a potassium atom and provided a strong basis for the assignment of the lowest unoccupied molecular orbitals accessed in the collision process.

Electron Transfer Induced Decomposition in Potassium–Nitroimidazoles Collisions: An Experimental and Theoretical Work

“…Formation of C 2 H 2 N − can proceed from the loss of an HCN unit from the dehydrogenated parent anion, whereas CH − may result from the loss of another HCN unit from C 2 H 2 N − . This behavior was also previously observed in the case of K-adenine collisions with the HCN loss mechanism discussed in detail [24]. The BRs in Figure 9 show that C 2 H 2 N − and CH − thresholds of formation are 10 eV (25 eV lab frame) and 24.3 eV (50 eV lab frame), respectively.…”

“…Moreover, in electron transfer experiments with imidazole, the major anionic fragments are the dehydrogenated parent anion (IMI−H) − , CN − , and C2H2N − , the latter related to the loss of an HCN unit from the dehydrogenated parent anion, which also leads to CH − formation (loss of another HCN unit). A similar HCN loss mechanism is also operative in more complex molecules and was already observed for potassium collisions with adenine and its derivative compounds [24] and pyrimidine [25]. The calculated highest occupied and lowest unoccupied whereas the loss of OH is completely quenched, as discussed recently [23].…”

“…This demonstrates that the hydrogenated parent anion may be a precursor in the formation of other anions. This behavior was already shown in electron transfer collisions in neutral potassium atoms with adenine [24], a molecule that is formed by a pyrimidine and an imidazole ring. Formation of (IMI−H) − can occur from the cleavage of an C−H or N−H bond, represented as:…”

“…The electron affinity of the cyanomethyl radical is (1.530 ± 0.010 eV) [28] which may indicate that the imidazole collision induced dissociation process is mainly dictated (>53 eV, see Figure 8) by the excess energy that is deposited in IMI through the available internal degrees of freedom. Similarly to the mechanism described in K collisions with adenine [24], uracil, and thymine [37], and in contrast to K−pyrimidine studies [25], in imidazole, CN − formation may also proceed from the decomposition of the dehydrogenated parent anion.…”

“…Both processes are formed after a complex rearrangement of the transient negative ion. The cyanide anion was already observed in other biomolecules, such as pyrimidine [25], adenine [24], thymine and uracil [37] upon electron transfer with potassium as electron donor. According to the BR results (see Figures 5-7), the CN − threshold of formation is 9 eV for all molecules except for 4(5)NI, which is below 2.4 eV.…”