Even-odd alternation in mass spectrum of thymine and uracil clusters: Evidence of intracluster photodimerization

Abstract

Multiphoton ionization of thymine and uracil clusters generated by a supersonic molecular beam gave rise to a remarkable alternation of mass spectral intensities between even-and odd-numbered clusters. Such alternation was observed in clusters of up to 30 molecules. Excitation to the two lowest electronically excited states seemed to be a strong prerequisite. In view of the well known photodimerization reaction of thymine and uracil in the bulk phase, it is proposed that such alternation in the mass spectral intensity resulted from formation of photodimer units within the cluster on intense UV irradiation. Several analogues of thymine with no known propensity for photodimerization in the bulk phase did not exhibit any sign of such alternation in the cluster mass spectrum. The intrinsic UV window for photodimerization, and hence photoinduced mammalian mutagenesis, was estimated to be approximately 210 -280 nm, significantly narrower than the previously reported bulk values of 150 -300 nm. S olar radiation is known to cause mutagenesis and carcinogenesis in mammalian cells (1-4). It is now widely accepted that DNA is the central target for such sunlight-induced lethality. In particular, pyrimidine base dimers are believed to play a key role. The adjacent bases of thymine in a DNA strand tend to undergo facile photodimerization on irradiation of UV light (1-5), and these photodimers cause a miscarriage of genetic information during replication of DNA. Despite many studies (6-11) and a recent heightened awareness of the ozone-hole problem, direct information on the intrinsic features of this process is lacking, mainly because it has never been investigated at an isolated molecular level. Here we generated molecular clusters of pyrimidine bases and found that strong UV irradiation of these cold clusters leads to a mass spectrum with a striking alternation of intensities between even-and odd-numbered clusters. Several derivatives of thymine and uracil with no known propensity for photodimerization did not yield such mass distribution when they were subject to the same irradiation condition. We therefore seemed to have found a strong correlation between the occurrence of the alternation pattern in cluster mass spectrum and the known tendency of the same molecule to undergo photodimerization in the bulk phase. By using the alternation as the criterion for photodimerization in an isolated molecular system, we estimated the intrinsic wavelength range of UV light causing photodimerization, a leading cause of photoinduced mammalian mutagenesis. It covers a range of 210-280 nm, which is significantly narrower than the previously reported bulk value of 150-300 nm.The experimental methods used in the present study have been described elsewhere (12, 13). Briefly, we used a molecular beam machine equipped with a photoionization time-of-flight (TOF) MS (Fig. 1). Clusters of thymine or uracil were generated by supersonic expansion of the sample with 2 atm (1 atm ϭ 101.3 kPa) argon carrier gas through a high-temperature pulsed nozz...

Dissociative multi-photon ionization of isolated uracil and uracil-adenine complexes

“…2a for conventional numbering of the ring atoms) [34,35]. MPI mass spectra of pure uracil clusters have previously been measured using 274 nm ns-timescale laser pulses but no information on fragment ions was reported [36]. The only experiment in the literature that probed fragment ion production from pure uracil clusters was performed for 100 keV O 5+ impact ionization [37] with an aggregation source that produced larger clusters than the present apparatus.…”

Intrinsic Lifetimes of the Excited State of DNA and RNA Bases

“…The four bases of DNA are the chromophores of DNA. When a DNA is irradiated by ultraviolet light, the bases are electronically excited and should, in principle, become prone to photochemical reactions that lead to mutagenesis and carcinogenesis. − Such lethal consequences are largely avoided, however, because the excited bases are believed to have a lifetime so short that they relax to their ground state before a photochemical reaction takes place. It has been suggested that the ultrashort lifetime of the photoexcited DNA bases is what led Nature to adopt the four nucleobases as the durable carriers of genetic information .…”

Femtosecond Decay Dynamics of Intact Adenine and Thymine Base Pairs in a Supersonic Jet

“…[30] UV-induced photodimerization of thymine in jet-cooled clusters has also been reported. [31] It is also worth noting that the formation of cyclobutane-type dimers is complete within 1 ps of UV irradiation at 272 nm in an all thymine, 18-mer DNA single strand. [32] On the basis of this assignment for t 2 T2 , the population of the stacked T 2 dimers in the molecular beam relative to that of the hydrogen-bonded ones was calculated from the relative ion signals of T 2 + and TH + and from the relative amplitude of decay components of t 2 T2 and t 2…”