MAPPING THE INTRINSIC PHOTOCHEMISTRY OF PhotoCORMS VIA GAS-PHASE LASER SPECTROSCOPY

We perform, for the first time, gas-phase laser photodissociation spectroscopy on a series of metal carbonyls that can lose CO upon irradiation. These molecules (PhotoCORMs) can be used for delivering and releasing CO molecules for medicinal purposes, such as in cancer therapy and as antimicrobials. Photodepletion (PD) and photofragmentation (PF) spectra of [CpRu(Ph$_{3}$)$_{2}$CO]$^{+}$ and [CpRu(dppe)CO]$^{+}$ were acquired between 230 and 400 nm, and the range 230-500 nm was explored for [Mn(CO)$_{4}$Br$_{2}$]$^{-}$. All the PhotoCORMs lose CO after irradiation, accessing different fragmentation channels when different excited states are populated. Indeed, while scanning the wavelength range in our laser-interfaced electrospray mass spectrometer, we observe the production spectra of the photofragments and can track the variation in the intensity of their production. [Mn(CO)$_{4}$Br$_{2}$]$^{-}$ loses 3 CO molecules in the key visible region and 4 COs in the UV. [CpRu(Ph$_{3}$)$_{2}$CO]$^{+}$ fragments into [CpRuPh$_{3}$]$^{+}$ via the loss of CO and \chem{Ph_3}. This observation can be used to improve the design of new CO-releasing molecules, as we demonstrate in the [CpRu(dppe)CO]$^{+}$ system where we successfully observe only the CO loss across the whole explored wavelength range. Finally, solution-phase irradiation results are presented for 365 nm photoexcitation, showing comparable photofragmentation results to the ones obtained in the gas-phase

Electron detachment dynamics of the iodide-guanine cluster: does ionization occur from the iodide or from guanine? : does ionization occur from the iodide or from guanine?

Laser photodissociation spectroscopy of the I‐·guanine complex has been conducted for the first time across the regions above the electron detachment threshold to explore the excited states and whether vertical ionisation occurs from the iodide or the nucleobase. The photofragment spectra reveal a prominent dipole-bound excited state (I) close to the calculated vertical electron detachment energy (∼4.0 eV) and a second excited (II) centred around 4.8 eV, which we assign to π-π* nucleobase-localised transitions. The ionic photofragments are identified as I‐ and I‐·[G-H], with the later fragment being produced significantly more strongly than the former. Both photofragments are observed across the two excited states, with production of the iodide being attributed to internal conversion to the ground state followed by evaporation. We trace the formation of the I‐·[G-H] photofragment to initial vertical ionisation of guanine, followed by ejection of a proton. This two-step process is important as it follows known steps in radiation-induced damage to DNA, namely initial formation of a guanine radical cation which then forms a free radical [G-H] moiety through deprotonation. Production of the I‐·[G-H] photofragment is pronounced through II indicating that its formation is enhanced by coupling of the π-π* transitions to the electron detachment continuum

Protomer-Dependent Electronic Spectroscopy and Photochemistry of the Model Flavin Chromophore Alloxazine

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

Photoexcitation of iodide ion-pyrimidine clusters above the electron detachment threshold : Intracluster electron transfer versus nucleobase-centred excitations

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 (5p6) → Uracil σ∗] may contribute to the cluster absorption profile across the scanned spectral region, and the possible role of these states is also discussed

Photoexcitation of Adenosine 5'-Triphosphate Anions in Vacuo : Probing the Influence of Charge State on the UV Photophysics of Adenine

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](2-) 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](2-) and [ATP-2H](2-), 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

Observation of Enhanced Dissociative Photochemistry in the Non-Native Nucleobase 2-Thiouracil

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

Near-threshold electron transfer in anion-nucleobase clusters : Does the identity of the anion matter?

Laser dissociation spectroscopy of I − ·adenine (I − ·A) and H 2 PO − 3 ·adenine (H 2 PO − 3 ·A) has been utilised for the first time to explore how the anion identity impacts on the excited states. Despite strong photodepletion, ionic photofragmentation is weak for both clusters, revealing that they decay predominantly by electron detachment. The spectra of I − ·A display a prominent dipole-bound excited state in the region of the detachment energy which relaxes to produce deprotonated adenine. In contrast, near-threshold photoexcitation of H 2 PO − 3 ·A does not access a dipole-bound state, but instead displays photofragmentation properties associated with ultrafast decay of an adenine-localised π→π* transition. Notably, the experimental electron detachment onset of H 2 PO − 3 ·A is around 4.7 eV, which is substantially lower than the expected detachment energy of an ion-dipole complex. The low value for H 2 PO − 3 ·A can be traced to initial ionisation of the adenine followed by significant geometric rearrangement on the neutral surface. We conclude that these dynamics quench access to a dipole-bound excited state for H 2 PO − 3 ·A and subsequent electron transfer. H 2 PO − 3 ·A represents an important new example of an ionic cluster where ionisation occurs from the neutral cluster component and where photodetachment initiates intra-molecular hydrogen atom transfer

Direct Measurement of the Visible to UV Photodissociation Processes for the PhotoCORM TryptoCORM

PhotoCORMs are light‐triggered compounds that release CO for medical applications. Here, we apply laser spectroscopy in the gas phase to TryptoCORM, a known photoCORM that has been shown to destroy Escherichia coli upon visible‐light activation. Our experiments allow us to map TryptoCORM’s photochemistry across a wide wavelength range by using novel laser‐interfaced mass spectrometry (LIMS). LIMS provides the intrinsic absorption spectrum of the photoCORM along with the production spectra of all of its ionic photoproducts for the first time. Importantly, the photoproduct spectra directly reveal the optimum wavelengths for maximizing CO ejection, and the extent to which CO ejection is compromised at redder wavelengths. A series of comparative studies were performed on TryptoCORM‐CH3CN which exists in dynamic equilibrium with TryptoCORM in solution. Our measurements allow us to conclude that the presence of the labile CH3CN facilitates CO release over a wider wavelength range. This work demonstrates the potential of LIMS as a new methodology for assessing active agent release ( e.g. CO, NO, H2S) from light‐activated prodrugs

A “one pot” tool for characterizing solution-phase and gas-phase photochemical reactions by electrospray mass spectrometry

The characterization of new photochemical pathways is important to progress the understanding of emerging areas of light-triggered inorganic and organic chemistry. In this context, the development of platforms to perform routine characterization of photochemical reactions remains an important goal for photochemists. Here, we demonstrate a new instrument that can be used to characterize both solution-phase and gas-phase photochemical reactions through electrospray ionization mass spectrometry (ESI-MS). The gas-phase photochemistry is studied by novel laser‐interfaced mass spectrometry (LIMS), where the molecular species of interest is introduced to the gas-phase by ESI, mass-selected and then subjected to laser photodissociation in the ion-trap. On-line solution-phase photochemistry is initiated by LEDs prior to ESI-MS in the same instrument with ESI-MS again being used to monitor photoproducts. Two ruthenium metal carbonyls, [Ru(η5-C5H5)(PPh3)2CO ][PF6] and [Ru(η5-C5H5)(dppe)CO][PF6] (dppe = 1,2-bis(diphenylphosphino)ethane) are studied using this methodology. We show that the gas-phase photofragmentation pathways observed for the ruthenium complexes via LIMS (i.e. loss of CO + PPh3 ligands from Ru(η5-C5H5)(PPh3)2CO ]+ and loss of just CO from [Ru(η5-C5H5)(dppe)CO]+, mirror the solution-phase photochemistry. The advantages of performing the gas-phase and solution-phase photochemical characterizations in a single instrument are discussed

Application of laser-interfaced mass spectrometry to biological and pharmaceutical molecules

This thesis demonstrates the use of laser photodissociation spectroscopy to explore the photochemistry of molecular and cluster ions isolated in a commercially adapted mass spectrometer. Chapter 3 describes gas-phase photodissociation experiments on adenosine nucleotides with phosphate chains of varying lengths with single and double negative charges. Direct comparison between the photochemistry of this series of compounds shows that the adenine π → π* transition is relatively unaffected by the charge state. In addition, the experiments show that photostability is enhanced as the number of phosphate groups increases. Near-threshold excitation of I-·A and H2PO3-·A is investigated in Chapter 4 to explore whether it leads to electron transfer from the anion to the adenine. It is shown that the adenine dipole-bound excited state is prepared from excitation of I-·A. In contrast, electron detachment in H2PO3-·A occurs from the neutral nucleobase and not from the anion. We identify H2PO3-·A as an example of an anion-molecule cluster where photodetachment initiates intra-molecular hydrogen atom transfer. Chapters 5 and 6 describe the application of photodissociation spectroscopy to organometallic complexes for CO-releasing applications