Thermal Decomposition of Multiply Charged T-Rich Oligonucleotide Anions in the Gas Phase. Influence of Internal Solvation on the Arrhenius Parameters for Neutral Base Loss

Arrhenius activation parameters (Ea, A) for the loss of neutral nucleobases from a series of T-rich, doubly and triply deprotonated 15- and 20-mer oligodeoxynucleotides (ODN) containing a single reactive base (X = A or C) with the sequence, XT14, XT19 and T19X, have been determined using the blackbody infrared radiative dissociation technique. The A-containing anions are significantly more reactive (≥3000 times) than the C-containing ions over the temperature range investigated. Importantly, the Arrhenius parameters for the loss of AH exhibit a strong dependence on size of the ODN and, to some extent, the charge state; the Arrhenius parameters increase with size and charge (Ea = 29–39 kcal mol−1, A = 1015–1020 s−1). In contrast, the parameters for the loss of CH are much less sensitive to size (Ea = 35–39 kcal mol−1, A = 1014–1017 s−1). The results are consistent with a greater contribution from the internal solvation of the reactive base to the Arrhenius parameters for the loss of A, compared with C, from the 15- and 20-mers. To further probe differences in internal solvation of A and C, hydrogen/deuterium exchange was carried out on AT19−3, T19A−3, CT19−3 and T19C−3 using D2O as the exchange reagent. However, the H/D exchange results did not reveal any differences in internal solvation within the ODN anions. Arrhenius parameters for the dissociation of noncovalent complexes of T20−3 and the neutral nucleobase AH or CH have also been determined. Differences in the parameters indicate differences in the nature of the intermolecular interactions. It is proposed that neutral A-T interactions (i.e., base-base), which originate in solution, dominate in the case of (T20 + AH)−3, while charge solvation, involving CH and a deprotonated phosphate group, is present for (T20 + CH)−3.