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

Abstract

We investigated the decay dynamics of the DNA base pairs adenine-adenine (A(2)), adenine-thymine (AT), and thymine-thymine (T(2)) produced in a supersonic jet by femtosecond (fs) time-resolved photoionization spectroscopy. The base pair was excited by a fs pump pulse at 267 nm and the population change of its excited state was monitored by non-resonant three-photon ionization using a fs probe pulse at 800 nm after a certain time delay. All of the transients recorded in the mass channel of the parent ion exhibited a tri-exponential decay, with time constants ranging from 100 fs to longer than 100 ps. Most of these time constants coincide well with the previous values deduced indirectly from the transients of protonated adenine (AH(+)) and thymine (TH(+)), which were assumed to be produced by fragmentation of the base-pair ions. Notably, for the transient of T(2), we observed a new decay component with a time constant of 2.3 ps, which was absent in the transient of TH(+). We suggest that the new decay component arises from the decay of stacked T(2) dimers that are mostly ionized to T(2)(+), whereas the decay signal recorded in the mass channel of TH(+) is merely from the relaxation of hydrogen-bonded T(2) dimers. From the amplitude of the new decay component, the population of the stacked T(2) dimers relative to the hydrogen-bonded dimers was estimated to be ∼2 % in the supersonic jet, which is about fifteen times higher than the theoretical value.

Internal conversion and intersystem crossing pathways in UV excited, isolated uracils and their implications in prebiotic chemistry

“…In comparison to the existing literature, there is good agreement with previously reported time constants. 59,60,[62][63][64][65]86 The nanosecond decay component of thymine was also observed by Schultz and co-workers, [62][63][64][65] Kim et al, 86 and Ligare et al 67 Only the latter study offers a spectroscopic assignment to the T 1 ( 3 pp*) based on double resonance measurements of the excited-state N-H stretch frequency and determines its lifetime as a few hundred nanoseconds. These observations for thymine further support the overall decay mechanism put forward above for uracil: S 2 ( 1 pp*) -S 1 ( 1 np*) -T 1 ( 3 pp*).…”

Radiation‐Induced Transfer of Charge, Atoms, and Energy within Isolated Biomolecular Systems

“…Interestingly, by comparing the data in the presence or absence of mixed clusters containing adenine and thymine, they also found that in AT, the proton can be transferred from A or from T, but with very different timescales: ionization of A forms [T+H] + with τ=1.3 ps, while ionization of T forms [A+H] + with τ≫1 ps (see Figure 5). Using a similar experimental approach, the Kim group confirmed that [T+H] + comes from H‐bound but not stacked T 2 after ionization [115] …”

“…Using a similar experimental approach, the Kim group confirmed that [T + H] + comes from H-bound but not stacked T 2 after ionization. [115] Larger nucleobase clusters have also been studied: thymine and methylthymine clusters in He droplets have been ionized by electron impact, and protonation was found to be a major process. [116] The authors also gave evidence for the destabilization of the resulting protonated clusters by methylation of thymine, which can be related to the importance of H bonds in these clusters.…”