Mechanism of IR and UV laser desorption of nucleosides: A study by Fourier transform mass spectrometry

Laser desorption coupled with Fourier transform mass spectrometry (LD/FTMS) was employed to study the desorption characteristics of adenosine, guanosine, thymidine, and uridine at wavelengths of 266 and 1064 nm. At 266 nm near threshold, the two pyrimidines, thymidine and uridine, yield disodiated molecular ions, (M - H + 2Na)⁺, as the most abundant ions whereas guanosine given the sodiated base, (B + Na)⁺. No evidence of excited-state proton transfer was found to explain disodiated molecular ion formation of the pyrimidines. Instead, formation of the disodiated ions is driven by a shift in the lactim-lactam equilibrium toward the lactim form in the T₁ state, a feature not characteristic of the purine nucleosides. Cationization in the dilute gas phase is not a major means of forming these ions upon IR irradiation, unlike the desorption of sucrose. A direct desorption mechanism is proposed whereby molecular ions are formed by dissociation of clusters of nucleosides that are bonded by means of the base ring systems.