Syntheses of all singly labeled [ ¹⁵ N]adenines: Mass spectral fragmentation of adenine

Abstract

Syntheses of all five of the singly labeled ['5N]adenines are now provided. The presence or absence of twobond '5N-'H spin couplings in their 1H NMR spectra confirm the location of the isotope in each case. The fragmentation patterns in their mass spectra are indicative ofthe sequential losses ofHCN units and of CH2N2 from adenine upon electron impact.The synthesis of [6-amino-14N]adenine and [9-'5N]adenine provided useful information concerning the -fragmentation of adenine under electron impact (1). We considered that the completion ofthe synthesis ofall ofthe singly-labeled ['5N]adenines would be valuable not only in the determination ofthe total mass spectral fragmentation patterns ofadenine but also in 15N NMR spectroscopic investigations of adenine and adenine-containing molecules (24). Moreover, the additional '5N-labeled adenines could extend the value that has already been demonstrated in the study of mechanisms of heterocyclic ring rearrangements (1, 5, 6) and loci of alkylation (7). [6-amino-`5N]Adenine and [9-1'N]Adenine. These singly labeled adenines were prepared by the methods of Leonard and Henderson (1). MATERIALS AND METHODS[1-_5N]Adenine. Method A. 6-Benzylamino[1-15N]purine (1) (100 mg in 6 ml oft-butyl alcohol) was treated with 2 mol equivalents ofKMnO4 (140 mg in 5 ml ofwater). The reaction mixture was heated gently at 60-70'C for 20-25 min. Additional aqueous 10% (wt/vol) KMnO4 was added, and heating was continued for 10 min. After cooling, unreacted KMnO4 was destroyed by addition of solid NaHSO3. [3-15N]Adenine. A procedure in this laboratory that used 7-benzyladenine as an intermediate (8, 11) was adapted to 15N labeling. The starting material, 4-bromo-5-['5N]nitroimidazole, was prepared as described for the unlabeled compound (12). 4-Bromoimidazole (2.33 g) in absolute ethanol (12 ml) was cooled at 0C and carefully treated with 70% aqueous H15N03 (1 ml).The solution was evaporated in vacuo, and the residue was treated with concentrated sulfuric acid (12 ml) and heated at 90°C for 1 hr. The cooled reaction mixture was diluted carefully with cold water, and the off-white solid that deposited was collected by filtration and dried (2.51 g). Subsequent steps (8)

Electron impact fragmentation of adenine: partial ionization cross sections for positive fragments

“…Sethi et al [15] conclude from studies with isotope labeled adenines that most of the fragmentation pathways exhibit largely random isotopic retention patterns, and that deuterium labeling is inconclusive because of rearrangements of hydrogen prior to expulsion of HCN. The studies of Barrio et al [14] with 15 N labeled adenines combined with the results of Sethi et al [15], show that the formation of 108 u by HCN loss is over 90% site specific for N1 and C2 and 6-8% for C2 and N3. Contrary to this, the populations of M-2(HCN) ions (81 u) and of M-HCN-NH 2 CN ions (66 u) originate from pathways involving initial HCN loss from all the five nitrogen atoms in the molecule.…”

“…Studies with isotope labeled adenines [13][14][15] show that most of the fragmentation pathways exhibit largely random isotopic retention patterns, indicating the presence of multiple fragmentation pathways. Based on this we attribute the rapid rise of these ion yields, with powers p 2, to several onsets very close to each other due to the presence of parallel fragmentation pathways with similar appearance energies.…”

“…Rice and Dudek [12] have measured a mass spectrum of adenine for 70 eV electron impact and have identified three of the main sequential fragmentation pathways. Occolowitz [13], Barrio et al [14] and Sethi et al [15] have measured mass spectra with isotope labeled adenines, showing that most Contribution to the Topical Issue "COST Action Nano-IBCT: Nano-scale Processes Behind Ion-Beam Cancer Therapy", edited by Andrey Solov'yov, Nigel Mason, Gustavo García, Eugene Surdutovich. a e-mail: peter.vanderburgt@nuim.ie of the fragmentation pathways exhibit largely random isotopic retention patterns, and that multiple pathways of successive fragmentations occur in parallel. Minaev et al [16] have measured the absolute total ionization cross sections of adenine for electron energies up to 200 eV as well as partial cross sections for the production of 24 fragment ions at 95 eV.…”

Ammonia elimination from protonated nucleobases and related synthetic substrates

“…The EI study of adenine showed that the major fragmentation path involves successive loss of HCN molecules. Studies of labeled adenines [29,30] demonstrated that the initially eliminated HCN contains the N1 atom and the loss of the NH 2 group was not observed. For guanine, the initial expulsion of neutral cyanamide (H 2 NCN) is the dominant fragmentation (from the pyrimidine's N1-C2-NH 2 moiety), and some NH 2 group elimination was observed as well.…”

Communication: Electron ionization of DNA bases

“…Similar loss of HCN from adenine on fragmentation has been reported using deuterated adenine 36 and using 15 N-labeled adenine. 37 adenine is a pentamer of HCN and it can be formed in interstellar space by oligomerization of HCN. 11,12 Fig .…”