Ultraviolet photodissociation spectroscopy of cold, isolated adenine complexes with a potassium cation

Cryogenic ion spectroscopy of adenine complexes containing alkali metal cations

Cryogenic ion spectroscopy was used to characterize adenine complexes containing alkali metal cations (M⁺A, M = Cs, Rb, K, Na, and Li) produced by electrospray ionization. The ultraviolet (UV) photodissociation spectra of the complexes stored in a cryogenic ion trap exhibited well-resolved vibronic bands near their origin bands of the S₀-S₁ transition. The UV-UV hole-burning and infrared ion-dip spectra showed that all the M⁺A ions in the ion trap were single isomers of M⁺A7a, where the M⁺ ion was not bound to canonical 9H-adenine (A9) but bound to a rare tautomer, 7H-adenine (A7). Density functional theory calculations showed lower tautomerization barriers for M⁺A9 than for bare A9 in aqueous solution. We suggest that M⁺ ions not only play a catalytic role in the tautomerization of A9 to A7 but also increase the tautomerization yield by forming stable M⁺A7a isomers.

Undervalued N3 Coordination Revealed in the Cisplatin Complex with 2'-Deoxyadenosine-5'-monophosphate by a Combined IRMPD and Theoretical Study

The complex obtained by the reaction of cisplatin and 2'-deoxyadenosine-5'-monophosphate (5'-dAMP) in water has been isolated and detected by electrospray ionization mass spectrometry. The so-formed cis-[PtCl(NH₃)₂(5'-dAMP)]⁺ complex has been studied in detail by infrared multiple photon dissociation (IRMPD) spectroscopy in two spectral ranges, namely, 700-1900 and 2800-3800 cm^-1, backed by quantum-chemical calculations at the B3LYP/LACV3P/6-311G** level of theory. In agreement with the computational results, the vibrational spectroscopic characterization of cis-[PtCl(NH₃)₂(5'-dAMP)]⁺ shows that the sampled ionic population comprises two major isomers, differentiated in the X-H stretching region by their distinct fragmentation patterns. One of these species presents coordination of the platinum moiety at the N3 position of adenine, whereas in the second one, platinum is bound at the N1 position of adenine. IRMPD kinetics have allowed an estimation of their relative proportions. Surprisingly, the most abundant component of cis-[PtCl(NH₃)₂(5'-dAMP)]⁺ is the N3 isomer, although it is slightly less stable than the other potential isomers in the gas phase. In contrast, the lowest-energy species, namely, the one showing cisplatin binding to the N7 position of adenine, seems to be the one less represented in the sampled ionic population. These findings suggest that the reaction of cisplatin with 5'-dAMP is governed by the kinetics of the process occurring in solution rather than by the thermodynamic factors.