UC Berkeley

Interactions between low energy electrons (LEEs) and biological chromophores are fundamental in many processes which take place in living organisms. UV irradiation of a cell, for example, can lead to LEE attachment to DNA and RNA, inducing single and double strand breakages. Transient nucleobase anions generated by electron attachment have been implicated as being key to damage mechanisms. The process of LEE attachment to nucleobases may be modeled by time resolved photoelectron spectroscopy (TRPES) of a cluster containing the nucleobase of interest and an electron donor species. The iodide acts as a controllable source of LEEs when excited by a UV pulse. A UV or IR probe pulse can detach an excess electron from a transient anion or photofragment, tracing the dynamics of electron transfer and accommodation. Small modifications to nucleobase structures can have a profound impact on electron dynamics. An excess electron can be accommodated as either a valence bound (VB) ion, in which a molecular valence orbital is populated, or a dipole bound (DB) ion, in which the electron is stabilized by interaction with the molecular dipole. Single atom replacements or addition of a methyl group can significantly change the binding energy associated with these anions. In this thesis, we employ TRPES to examine the electron dynamics for iodide-2-thiouracil (I–2TU), iodide-4-thiouracil (I–4TU), and iodide-thymine (I–T). Thiobases like 2-thiouracil and 4-thiouracil have an increased reactivity compared to the canonical nucleobases, leading to their potential application as sensitizers for phototherapies. Their properties can also clarify fundamental principles of nucleobases photophysics. The thiouracils have larger dipolar moments than uracil, but no evidence for thiouracil DB ions has been observed by single photon photoelectron spectroscopy. TRPES measurements of the corresponding iodide clusters enable us to determine whether these ions are fundamentally unstable or simply short-lived. The singly substituted thiouracils also have VB ions with significantly different properties than the VB ions of uracil or thymine. Thiouracil VB ions have much higher binding energies, facilitating rapid and complete DB to VB interconversion. 2-thiouracil has a pair of nearly degenerate ππ* transitions which other groups have associated with faster relaxation rates of the neutral species; 4-thiouracil has a particularly low energy ππ* transition that is accessible below the I–4TU VDE. Their study can therefore give us significant insight into the properties that are most important in electron accommodation of the canonical nucleobases by comparing changes in dynamics due to changes in these properties. Excitation of I–2TU near its vertical detachment energy (VDE) gives rise to DB and VB ion signals, with the DB signal appearing and decaying on ultrafast timescales (within the instrument response time) and bi-exponential decay of the VB signal. At 4.73 eV excitation, no DB signal is seen, but two signals are attributed to distinct but energetically similar VB anions arising from the nearly degenerate ππ* excitations. Utilizing a UV probe pulse, we find that the major decay product I– appears quite rapidly. Rice–Ramsperger–Kassel–Marcus (RRKM) theory predicts a cluster dissociation time in reasonable agreement with the measured I– rise time, indicating that iodide reforms rapidly by back electron transfer. TRPES of I–4TU was performed with excitation energies below the cluster VDE and near the cluster VDE. We find that 3.88 eV excitation, which is resonant with a 4TU ππ* transition but below the cluster VDE, facilitates the formation of valence bound anions without the mediation of a dipole bound state. This provides empirical evidence for a previously proposed mechanism wherein nucleobase localized excitation is key to VB ion formation. Near-VDE excitation of the cluster gives rise to DB and VB anion signal, with two distinct time regimes for VB anion formation. The first is ultra-fast formation associated with ππ* excitation and the second is a slower rise corresponding to DB to VB interconversion. Electron accommodation dynamics of I–T were previously measured with an IR (1.55 eV) probe energy. In light of results for the thiouracils, these dynamics with a UV proper energy merit analysis. The higher energy probe reveals an ion signal bound by 1–2 eV in addition to the canonical T VB ion. The VDE of this new ion signal is in reasonable agreement with the calculated VDE of a thymine tautomer anion. The UV probe pulse also enables us to measure the iodide dissociation time, which is found to be much slower than that of the other iodide nucleobase clusters we have measured but is similar to the decay time of the tautomer ion signal. This suggests that because the VB signal from the canonical tautomer of thymine is depleted rapidly and efficiently by electron autodetachment, reformation of iodide comes from decay of the thymine rare tautomer’s VB anion. The rise of the tautomer anion signal gives an approximate tautomerization time constant of 8 ps.