Periodic Reporting for period 1 - Photochem-RS-RP (Unraveling the Photochemistry of Radiosensitizers and Radioprotectors in Free Biomolecular Complexes)
Reporting period: 2022-07-01 to 2024-06-30
Dramatically increasing cancer cases around the world call for extra research efforts to improve cancer therapies. Radiation therapy or radiotherapy is one of the most common treatment methods. A way to enhance radiotherapy is inserting ‘radiosensitizers (RSs)’ and ‘radioprotectors (RPs)’ into the patient’s body. RSs in tumor cells make them more sensitive to radiation damage, allowing one to use reduced radiation doses, thus minimizing side effects. In contrast, RPs inhibit the damage of healthy cells from radiation. RSs and RPs are actively studied mostly in clinical trials. However, the fundamental mechanisms causing damage or death of cancer cells are not fully understood. Therefore, this project aimed to elucidate the elementary steps of radiation damage, their enhancement by RSs, and their inhibition by RPs. The technique combines beams of free gas phase molecules, mixed molecular clusters, and doped helium nanodroplets uniquely with synchrotron spectroscopy, electron spectroscopy, and ion mass spectrometry. The main goals are to unravel the photochemistry of selected RS compounds in the state of controlled microhydration and contact with DNA components (thymine, cytosine, tetrahydrofuran). Emission of slow electrons, water fragmentation, and anions formation are observables for radiation damage enhanced by RSs. A better understanding of the radiochemistry of RPs and RSs obtained with this project may help develop new schemes for efficient cancer treatment and identify new types of molecules or nanoparticles with improved RS or RP properties.
There are several scientific activities performed during this project. Some of the activities are shortly described below:
1. Hydrogen migration in inner-shell ionized halogenated cyclic hydrocarbons
In this study, the fragmentation of brominated cyclic hydrocarbons (bromocyclo-propane, bromocyclo-butane, and bromocyclo-pentane) upon ionization of inner-shell electrons (Br(3d) and C(1s)) was investigated using coincidence ion momentum imaging. Notably, a significant production of CH3+ fragments was observed, indicating intramolecular hydrogen migration, particularly evident with larger molecules. This contrasts with prior findings in linear hydrocarbon molecules. By analyzing the momentum correlations of fragment ions in three-body fragmentation pathways, it was deduced that CHx+ fragments with increasing hydrogen count are more likely formed through sequential fragmentation routes. The study's findings on molecular size-dependent kinetic energy releases and fragment kinetic energies were rationalized using classical Coulomb explosion simulations.
2. Dopant ionization and efficiency of ion and electron ejection from helium nanodroplets
The study focuses on effective ion and electron detection in photoionization spectroscopy and mass spectrometry of doped helium (He) nanodroplets. Utilizing a commercial quadrupole mass spectrometer and a photoelectron–photoion coincidence spectrometer, the research systematically measures ion and electron yields in pure and doped He nanodroplets across varying sizes and ionization methods (direct and secondary after resonant photoexcitation). The optimal droplet size for maximum ejected ion yield is determined for two dopants: oxygen molecules (O2) and lithium atoms (Li). Optimal droplet sizes are identified, with peak detection efficiency of dopant ions relative to primary photoabsorption events reaching up to 20% for O2 and 2% for Li in charge-transfer ionization, and 1% for O2 and 4% for Li in Penning ionization. These findings offer crucial insights for refining conditions in mass spectrometric and photoionization spectroscopy investigations of molecules and complexes isolated within He nanodroplets.
3. Electron energy loss and angular asymmetry induced by elastic scattering in helium droplets
This study focuses on the interaction of ionizing radiation with helium nanodroplets, serving as an ideal model system for condensed matter. The research investigates how purely elastic electron scattering impacts the angular and energy distributions of photoelectrons emitted from He nanodroplets of varying sizes (10-109 atoms per droplet). In larger droplets, photoelectrons exhibit notable anisotropy in the direction of incident light due to shadowing within the droplets. However, the detected photoelectron spectra remain minimally affected. This discovery suggests potential for photoelectron spectroscopy of embedded dopants within droplets, as long as the dopants are smaller than the light's penetration depth and the emitted electrons' trapping range.
4. Secondary ionization of pyrimidine nucleobases and their microhydrated derivatives in helium nanodroplets
The study delves into radiation-induced damage in biological systems caused by ionizing radiation, predominantly through secondary processes like charge and energy transfer, resulting in DNA bond breakage. The investigation centers on the fragmentation of cytosine (Cyt) and thymine (Thy) molecules, clusters, and microhydrated versions following direct and indirect ionization from extreme-ultraviolet (XUV) irradiation. Comparative analysis of photofragmentation mass spectra and photoelectron spectra of free Cyt and Thy molecules with those of Cyt/Thy clusters and microhydrated forms formed in superfluid helium (He) nanodroplets highlights the impact of Penning ionization after resonant excitation, revealing reduced fragmentation compared to direct photoionization and charge-transfer ionization. The addition of water molecules to Cyt/Thy molecules suppresses fragmentation effectively, a trend similarly observed in homogeneous Cyt/Thy clusters formed within He nanodroplets. Penning ionization electron spectra (PIES) for Cyt/Thy exhibit broad features, while microhydrated derivatives indicate sequential ionization, leading to intact microsolvated Cyt/Thy cations.
5. Fragmentation of Water Clusters Formed in Helium Nanodroplets by Charge Transfer and Penning Ionization
Helium nanodroplets (HNDs) are commonly employed to create customized clusters and molecular complexes within a cold, transparent, and minimally interacting matrix. Evaluating species within HNDs using mass spectrometry is hindered by ion fragmentation and trapping. This study investigates fragment ion mass spectra of HND-aggregated water and oxygen clusters, focusing on charge transfer ionization (CTI) and Penning ionization (PEI). PEI exhibits approximately 10 times lower efficiency than CTI in ionizing embedded clusters, yet it generates larger water cluster sizes and higher relative yields of unprotonated cluster ions, constituting a 'soft ionization' method. However, the propensity of ions to remain bound to HNDs results in diminished detection efficiency for larger HNDs with over 10,000 helium atoms. These findings play a pivotal role in optimizing conditions for mass spectrometry and photoionization spectroscopy of molecular complexes and clusters aggregated within HNDs.
1. Hydrogen migration in inner-shell ionized halogenated cyclic hydrocarbons
In this study, the fragmentation of brominated cyclic hydrocarbons (bromocyclo-propane, bromocyclo-butane, and bromocyclo-pentane) upon ionization of inner-shell electrons (Br(3d) and C(1s)) was investigated using coincidence ion momentum imaging. Notably, a significant production of CH3+ fragments was observed, indicating intramolecular hydrogen migration, particularly evident with larger molecules. This contrasts with prior findings in linear hydrocarbon molecules. By analyzing the momentum correlations of fragment ions in three-body fragmentation pathways, it was deduced that CHx+ fragments with increasing hydrogen count are more likely formed through sequential fragmentation routes. The study's findings on molecular size-dependent kinetic energy releases and fragment kinetic energies were rationalized using classical Coulomb explosion simulations.
2. Dopant ionization and efficiency of ion and electron ejection from helium nanodroplets
The study focuses on effective ion and electron detection in photoionization spectroscopy and mass spectrometry of doped helium (He) nanodroplets. Utilizing a commercial quadrupole mass spectrometer and a photoelectron–photoion coincidence spectrometer, the research systematically measures ion and electron yields in pure and doped He nanodroplets across varying sizes and ionization methods (direct and secondary after resonant photoexcitation). The optimal droplet size for maximum ejected ion yield is determined for two dopants: oxygen molecules (O2) and lithium atoms (Li). Optimal droplet sizes are identified, with peak detection efficiency of dopant ions relative to primary photoabsorption events reaching up to 20% for O2 and 2% for Li in charge-transfer ionization, and 1% for O2 and 4% for Li in Penning ionization. These findings offer crucial insights for refining conditions in mass spectrometric and photoionization spectroscopy investigations of molecules and complexes isolated within He nanodroplets.
3. Electron energy loss and angular asymmetry induced by elastic scattering in helium droplets
This study focuses on the interaction of ionizing radiation with helium nanodroplets, serving as an ideal model system for condensed matter. The research investigates how purely elastic electron scattering impacts the angular and energy distributions of photoelectrons emitted from He nanodroplets of varying sizes (10-109 atoms per droplet). In larger droplets, photoelectrons exhibit notable anisotropy in the direction of incident light due to shadowing within the droplets. However, the detected photoelectron spectra remain minimally affected. This discovery suggests potential for photoelectron spectroscopy of embedded dopants within droplets, as long as the dopants are smaller than the light's penetration depth and the emitted electrons' trapping range.
4. Secondary ionization of pyrimidine nucleobases and their microhydrated derivatives in helium nanodroplets
The study delves into radiation-induced damage in biological systems caused by ionizing radiation, predominantly through secondary processes like charge and energy transfer, resulting in DNA bond breakage. The investigation centers on the fragmentation of cytosine (Cyt) and thymine (Thy) molecules, clusters, and microhydrated versions following direct and indirect ionization from extreme-ultraviolet (XUV) irradiation. Comparative analysis of photofragmentation mass spectra and photoelectron spectra of free Cyt and Thy molecules with those of Cyt/Thy clusters and microhydrated forms formed in superfluid helium (He) nanodroplets highlights the impact of Penning ionization after resonant excitation, revealing reduced fragmentation compared to direct photoionization and charge-transfer ionization. The addition of water molecules to Cyt/Thy molecules suppresses fragmentation effectively, a trend similarly observed in homogeneous Cyt/Thy clusters formed within He nanodroplets. Penning ionization electron spectra (PIES) for Cyt/Thy exhibit broad features, while microhydrated derivatives indicate sequential ionization, leading to intact microsolvated Cyt/Thy cations.
5. Fragmentation of Water Clusters Formed in Helium Nanodroplets by Charge Transfer and Penning Ionization
Helium nanodroplets (HNDs) are commonly employed to create customized clusters and molecular complexes within a cold, transparent, and minimally interacting matrix. Evaluating species within HNDs using mass spectrometry is hindered by ion fragmentation and trapping. This study investigates fragment ion mass spectra of HND-aggregated water and oxygen clusters, focusing on charge transfer ionization (CTI) and Penning ionization (PEI). PEI exhibits approximately 10 times lower efficiency than CTI in ionizing embedded clusters, yet it generates larger water cluster sizes and higher relative yields of unprotonated cluster ions, constituting a 'soft ionization' method. However, the propensity of ions to remain bound to HNDs results in diminished detection efficiency for larger HNDs with over 10,000 helium atoms. These findings play a pivotal role in optimizing conditions for mass spectrometry and photoionization spectroscopy of molecular complexes and clusters aggregated within HNDs.
The primary objective of this project is to shed light on the underlying photophysical and photochemical mechanisms that determine the dual behavior of specific molecules, either as reactive species (RSs) or as reactive promoters (RPs). Despite extensive exploration within macroscopic samples and clinical trials, comprehensive investigations into the intricate dynamics of RSs and RPs, particularly their intricate interplay with radiation and their immediate surroundings, remain limited. During this project, we used a pioneering avenue for unraveling these intricate molecular processes lies in employing XUV and X-ray spectroscopy techniques on molecular complexes within water clusters or helium nanodroplets, coupled with electron-ion coincidence detection. This innovative approach holds the key to unveiling these fundamental processes at the molecular scale.
This project is able to explore some trends in these complex molecular processes that will help different life sciences research communities to develop the RSs and RPs for different applications in biological sciences as briefly explained.
This project is able to explore some trends in these complex molecular processes that will help different life sciences research communities to develop the RSs and RPs for different applications in biological sciences as briefly explained.