We report the first UV laser photodissociation spectra (4.0-5.8 eV) of gas-phase deprotonated adenosine 5'-triphosphate, diphosphate and monophosphate anions. The photodepletion spectra of these anions display strong absorption bands across the region of 4.6-5.2 eV, consistent with excitation of a primarily adenine-centered π-π* transition. The spectra appear insensitive to the charge of the species (i.e., the spectrum of [ATP-2H] closely resembles that of [ATP-H]), while the spectral profile is affected to a greater extent by the variation of the molecular structure, i.e. the [AMP-H] and [ADP-H] photodepletion spectra display similar profiles while the [ATP-H] spectrum is distinctive. The photodepletion cross-section also decreases for the ATP anions compared to both the AMP and ADP anions, reflecting a high intrinsic photostability of ATP versus both AMP and ADP. A range of photofragments are produced across the 4.0-5.8 eV spectral range for all of the ATP analogues studied. These fragments are primarily associated with fragmentation on the ground-state electronic surface, indicative of a statistical decay process where ultrafast decay is followed by ergodic dissociation. However, while the photofragments observed following photoexcitation of the monoanionic species, [AMP-H] to [ADP-H] to [ATP-H] are entirely consistent with statistical decay, an additional group of photofragments are observed for the dianionic species, [ADP-2H] and [ATP-2H], that we associate with electron detachment, and subsequent fragmentation of the resulting electron-detached photofragment. TDDFT calculations are presented to support the interpretation of the experimental data, and confirm that the electronic structure of the adenine moiety is relatively unperturbed by varying the overall charge.

Laser photodissociation spectroscopy of the I·thymine (I·T) and I·cytosine (I·C) nucleobase clusters has been conducted for the first time across the regions above the electron detachment thresholds to explore the excited states and photodissociation channels. Although photodepletion is strong, only weak ionic photofragment signals are observed, indicating that the clusters decay predominantly by electron detachment. The photodepletion spectra of the I·T and I·C clusters display a prominent dipole-bound excited state (I) in the vicinity of the vertical detachment energy (∼4.0 eV). Like the previously studied I·uracil (I·U) cluster [W. L. Li et al., J. Chem. Phys. 145, 044319 (2016)], the I·T cluster also displays a second excited state (II) centred at 4.8 eV, which we similarly assign to a π-π* nucleobase-localized transition. However, no distinct higher-energy absorption bands are evident in the spectra of the I·C. Time-dependent density functional theory (TDDFT) calculations are presented, showing that while each of the I·T and I·U clusters displays a single dominant π-π* nucleobase-localized transition, the corresponding π-π* nucleobase transitions for I·C are split across three separate weaker electronic excitations. I and deprotonated nucleobase anion photofragments are observed upon photoexcitation of both I·U and I·T, with the action spectra showing bands (at 4.0 and 4.8 eV) for both the I and deprotonated nucleobase anion production. The photofragmentation behaviour of the I·C cluster is distinctive as its I photofragment displays a relatively flat profile above the expected vertical detachment energy. We discuss the observed photofragmentation profiles of the I·pyrimidine clusters, in the context of the previous time-resolved measurements, and conclude that the observed photoexcitations are primarily consistent with intracluster electron transfer dominating in the near-threshold region, while nucleobase-centred excitations dominate close to 4.8 eV. TDDFT calculations suggest that charge-transfer transitions [Iodide n (5p) → Uracil σ*] may contribute to the cluster absorption profile across the scanned spectral region, and the possible role of these states is also discussed.

Flavin chromophores play key roles in a wide range of photoactive proteins, but key questions exist in relation to their fundamental spectroscopic and photochemical properties. In this work, we report the first gas-phase spectroscopy study of protonated alloxazine (AL∙H+), a model flavin chromophore. Laser photodissociation is employed across a wide range (2.34–5.64 eV) to obtain the electronic spectrum and characterize the photofragmentation pathways. By comparison to TDDFT quantum chemical calculations, the spectrum is assigned to two AL∙H+ protomers; an N5 (dominant) and O4 (minor) form. The protomers have distinctly different spectral profiles in the region above 4.8 eV due to the presence of a strong electronic transition for the O4 protomer corresponding to an electron-density shift from the benzene to uracil moiety. AL∙H+ photoexcitation leads to fragmentation via loss of HCN and HNCO (along with small molecules such as CO2 and H2O), but the photofragmentation patterns differ dramatically from those observed upon collision excitation of the ground electronic state. This reveals that fragmentation is occurring during the excited state lifetime. Finally, our results show that the N5 protomer is associated primarily with HNCO loss while the O4 protomer is associated with HCN loss, indicating that the ring-opening dynamics are dependent on the location of protonation in the ground-state molecule.

Laser photofragmentation spectroscopy of the I-•adenine (I-•A) and H2PO3-•adenine (H2PO3-•A) clusters has been utilized for the first time across the electron detachment thresholds to explore how the anion identity affects intra-cluster electron transfer. Ionic photofragmentation is weak for both clusters, despite strong photodepletion, revealing that both clusters decay predominantly by electron detachment. The spectra of I-•A display a prominent dipole-bound excited state in the region of the vertical detachment energy, which relaxes to produce deprotonated adenine. In contrast, photoexcitation of H2PO3-•A in the near-threshold region does not access a dipole-bound state, but instead displays photofragmentation properties associated with ultrafast decay of an adenine-localized -* transition. Notably, the experimental electron detachment onset of H2PO3-•A is around 4.7 eV. This value is substantially reduced compared to the VDE expected for detachment of a simple anion-dipole complex. The lower VDE of H2PO3-•A can be traced to initial ionization of the adenine, which is followed by significant rearrangement of a hydrogen atom on the neutral surface. We conclude that these dynamics quench access to a dipole-bound excited state for H2PO3-•A. Despite the fact that the excess negative charge is located on H2PO3in the ground-state cluster, the anion in this cluster does not act as a free electron source to initiate free electron attachment or dissociation in the nucleobase. However, the H2PO3-•A cluster represents an important new example of an anionic cluster where ionization occurs from the initial neutral moiety of the cluster, and where photodetachment initiates intra-molecular hydrogen atom transfer.

We present the first study to measure the dissociative photochemistry of 2-thiouracil (2-TU), an important nucleobase analogue with applications in molecular biology and pharmacology. Laser photodissociation spectroscopy is applied to the deprotonated and protonated forms of 2-TU, which are produced in the gas-phase using electrospray ionization mass spectrometry. Our results show that the deprotonated form of 2-thiouracil ([2-TU-H]−) decays predominantly by electron ejection and hence concomitant production of the [2-TU-H]· free-radical species, following photoexcitation across the UVA-UVC region. Thiocyanate (SCN−) and a m/z 93 fragment ion are also observed as photodecay products of [2-TU-H]− but at very low intensities. Photoexcitation of protonated 2-thiouracil ([2-TU·H]+) across the same UVA-UVC spectral region produces the m/z 96 cationic fragment as the major photofragment. This ion corresponds to ejection of an HS· radical from the precursor ion and is determined to be a product of direct excited state decay. Fragment ions associated with decay of the hot ground state (i.e., the ions we would expect to observe if 2-thiouracil was behaving like UV-dissipating uracil) are observed as much more minor products. This behaviour is consistent with enhanced intersystem crossing to triplet excited states compared to internal conversion back to the ground state. These are the first experiments to probe the effect of protonation/deprotonation on thionucleobase photochemistry, and hence explore the effect of pH at a molecular level on their photophysical properties.