Time-Resolved Photoelectron Imaging of Model Biological Chromophores
Lead Research Organisation:
Heriot-Watt University
Department Name: Sch of Engineering and Physical Science
Abstract
This is a PhD research project in Physics. Time-Resolved Photoelectron Imaging (TRPEI) will be employed to study the relaxation dynamics of excited states in model biological chromophores. Such systems are of great interest since they play a significant role in the process of photoprotection through efficient dissipation of excess energy following absorption of ultraviolet (UV) radiation. Typical examples of relevant species include the DNA bases, the melanin pigments and phenylpropanoids that act as natural sunscreens in plants. Molecules will be prepared in the gas-phase and excited using UV pump pulses, provided by a femtosecond laser system. Subsequently, the excited stated are then probed using photoionization by a second UV laser pulse. The delay time between the pump and probe pulses is then varied, allowing for the real time observations of the dynamic evolution of the system. The project will also involve developing novel probe sources exploiting the vacuum ultraviolet (VUV) region of the electromagnetic spectrum. This will provide a more expanded view along the various critical reaction co-ordinates that mediate the UV relaxation process, thereby providing a greater level of mechanistic insight. Work will also be undertaken to begin exploring the effects of inter- and intra-state photoionization detection sensitivity within the TRPEI approach. The aim here is to learn more about important photoproduct branching yields and also to correlate various physical molecular properties that are easy to evaluate computationally (eg polarizability volume) with photoionization cross sections (which are notoriously difficult to calculate accurately). In addition to the experimental TRPEI measurement, ab initio computational studies using commercial software packages such as Gaussian will provide information to support and predict experimental observations.
Studentship Projects
| Project Reference | Relationship | Related To | Start | End | Student Name |
|---|---|---|---|---|---|
| EP/N509474/1 | 01/10/2016 | 30/09/2021 | |||
| 1822047 | Studentship | EP/N509474/1 | 01/07/2016 | 31/12/2019 | Lisa Saalbach |
| Description | To date, several publications related to this award have been published. This includes publications in peer-reviewed journals, one recently published review article which refers to work attributed to this award, and finally a PhD thesis. This award aimed to investigate the relaxation dynamics of excited states in biologically and environmentally relevant molecular motifs using time-resolved photoelectron imaging (TRPEI) and related methods. The reports published (to date) in peer-reviewed journals address several groups of molecules. The first published in 2019 (Larsen et al., J. Phys. Chem., 150, 2019) is an experimental and theoretical study of three small amide systems (formamide, dimethylformamide and dimethylacetamide). This work was carried out in collaboration with researchers at both the National Research Council Canada and the University of Copenhagen. Amides are molecules that form parts of the bonds linking peptides and proteins and appear as motifs in the DNA bases. It is therefore of great interest to develop an understanding of how these molecules are able to dispose of excess energy (induced through the absorption of ultraviolet (UV) light) efficiently and rapidly, preventing mutation and damage to the molecular structure which could result in an inhibition of their function in the body (a process known as 'photoprotection'). The consequences of such undesirable structural damage may be contributors to the causes of illnesses such as cancer. Furthermore, the study aimed to draw parallels to previous investigations on trends observed in small amine systems (a structurally very similar group of molecules). These have been found to display trends in their relaxation dynamics and assessing these for further molecular groups could allow researchers to use these patterns to predict the energy-releasing processes in larger biologically relevant systems. Using both the experimental and theoretical results obtained for this study, it was concluded that all three systems undergo rapid fragmentation, exhibiting extremely short lifetimes (< 100 fs). Furthermore, it was found that these processes are facilitated by different electronic states in the primary system formamide compared to the larger systems: dimethylformamide and dimethylacetamide. The second set of molecules investigated were three monohydroxyindoles: 4-, 5- and 6-hydroxyindole (Crane et al., Chem. Phys. Lett., 738, 2020), studied using time-resolved ion-yield measurements. Again, these species are biologically relevant as they are closely related to 5,6-dihydroxyindole which is a building block of the eumelanin pigment. This pigment is found in the skin of many animal species and acts to protect them from the potentially harmful effects of UV radiation. A specific technique known as laser-based sample desorption was required to bring these relatively large structures into the gas phase so that their dynamics could be studied without the presence of a solvent. Furthermore, theoretical quantum chemistry calculations were carried out to support this work, providing the first of such kind for 6-hydroxyindole. The results reveal a clear trend for the three molecules, showing that the excited state lifetimes increase by an order of magnitude between 4-, 5- and 6-hydroxyindole. The relaxation dynamics in the molecule acetylacetone were investigated in a third publication (Kotsina et al., Phys. Chem. Chem. Phys., 22, 2020). Acetylacetone is a constituent of various UV light-absorbing sites found in molecules such as avobenzone, which is used in commercial sunscreens. Understanding why this molecule is proficient at absorbing UV light and dispersing it on such rapid timescales could support the design of novel sunscreen components. The relaxation dynamics in acetylacetone have been studied previously, however, this current study provides an extended view of the processes taking place, quantitatively linking these earlier investigations. This was possible due to the employment of a high-energy (160 nm) pulse for the ionisation of the sample. The increased energy pulse enables us to see further 'along' the reaction coordinate, potentially allowing for the observation of the complete relaxation process. The results obtained for acetylacetone show four dynamical signatures, one of which was not previously observed. The signatures show the various steps involved in the overall relaxation process in acetylacetone and the study furthermore illustrates the advantages and highlights the need for high-energy ionising pulses in these time-resolved photoelectron imaging experiments. Furthermore, the above work has been included in a recent review article (M.J. Paterson & D. Townsend, Int. Rev. Phys. Chem.,39:4, 517-567). Finally, the work outlined above formed the basis of a PhD thesis (L. Saalbach, "Time-Resolved Studies of Excited State Molecular Dynamics", 2020). The thesis additionally reported results from an investigation into the influence of steric effects on the excitation dynamics in nitrobenzene derivatives, also attributed to this reward. A full in-depth discussion of these results has recently been published (Saalbach et al., J. Phys. Chem. A, 125, 33, 7174-7184, 2021). The ultrafast relaxation dynamics in nitrobenzene and three of its methyl-substituted derivatives (2,4-, 2,6- and 3,5-dimethylnitrobenzene) were studied using TRPEI as well as computational methods. The methylation of nitrobenzene served to modify the alignment of the NO2 group plane with respect to the benzene ring plane which has been linked to affecting branching ratios of the two major photodissociation pathways (NO and NO2 elimination). Deliberate NO-release is of interest for applications in clinical medicine where NO is known to support numerous biological functions. In this TRPEI investigation, no variation in the ultrafast dynamical behaviour of the methylated nitrobenzene systems was observed within the experimental window of 200 ps. The findings imply that localised motions (mainly on the NO2 group) are responsible for the initial energy redistribution in these systems (in agreement with previous theoretical studies) and suggest that NO and NO2 are released on longer timescales, where dynamics occurring on such timescales affect the photoproduct branching ratio. |
| Exploitation Route | The molecules investigated in the above studies are sub-units of larger, often biologically relevant systems. Biological molecules such as peptides, proteins and DNA are large, complex structures and understanding the relaxation processes taking place in these systems (after excitation using UV light) is an extremely complex task. It is therefore beneficial to initially study the relaxation processes of smaller systems (such as those outlined above) in the gas-phase to allow us to understand the fundamental processes taking place. From there the structures can be increased in size and eventually studied in their natural liquid-phase environments. This notion is known as the 'bottom-up approach'. Therefore, the work presented here can be used as a basis on which to build-up towards understanding the complex dynamics in large biological systems. Furthermore, there are different, complementary techniques required to understand the whole of the reaction coordinate from photo-reactant to photo-product. This is illustrated in particular by the study on hydroxyindoles (Crane et al., Chem. Phys. Lett., 738, 2020), which despite revealing the signatures and lifetimes of the relaxation processes, could not resolve all elements of the extremes of the reaction coordinate (the most short- and long-lived lifetime components). This provides a motivation for future studies using wavelength-dependent techniques to specifically probe the photo-reactant and photo-products of molecular systems. The study of nitrobenzene and its methyl-substituted derivatives (Saalbach et al., J. Phys. Chem. A, 125, 33, 7174-7184, 2021) has also highlighted directions for further experimental investigations on the photodynamics of nitroaromatic molecular systems. |
| Sectors | Chemicals,Environment,Pharmaceuticals and Medical Biotechnology |
| Description | Travel Grants for PhD Students and Early Career Scientists - Faraday Division |
| Amount | £370 (GBP) |
| Organisation | Royal Society of Chemistry |
| Sector | Charity/Non Profit |
| Country | United Kingdom |
| Start | 08/2018 |
| End | 09/2018 |
| Description | Vacuum Ultraviolet Time-Resolved Study of Small Amides |
| Organisation | University of Copenhagen |
| Department | Department of Chemistry |
| Country | Denmark |
| Sector | Academic/University |
| PI Contribution | I provided theoretical support to the experimental data by performing and analysing ab initio quantum chemistry calculations to produce excited state potential surfaces for the molecules under experimental study. |
| Collaborator Contribution | The experiment was conducted at the facilities of the collaborator at the University of Ottowa (group of A. Stolow). The resulting experimental data was analysed by the collaborators at the University of Copenhagen (group of T. Solling). |
| Impact | Publication in the Journal of Chemical Physics titled: "Vacuum ultraviolet excited state dynamics of small amides", citation: J. Chem. Phys. 150, 054301 (2019) |
| Start Year | 2016 |
| Description | Vacuum Ultraviolet Time-Resolved Study of Small Amides |
| Organisation | University of Ottawa |
| Country | Canada |
| Sector | Academic/University |
| PI Contribution | I provided theoretical support to the experimental data by performing and analysing ab initio quantum chemistry calculations to produce excited state potential surfaces for the molecules under experimental study. |
| Collaborator Contribution | The experiment was conducted at the facilities of the collaborator at the University of Ottowa (group of A. Stolow). The resulting experimental data was analysed by the collaborators at the University of Copenhagen (group of T. Solling). |
| Impact | Publication in the Journal of Chemical Physics titled: "Vacuum ultraviolet excited state dynamics of small amides", citation: J. Chem. Phys. 150, 054301 (2019) |
| Start Year | 2016 |
| Description | "Hot-Topic" Talk at Stereodynamics 2018 |
| Form Of Engagement Activity | A talk or presentation |
| Part Of Official Scheme? | No |
| Geographic Reach | International |
| Primary Audience | Postgraduate students |
| Results and Impact | I presented a "hot topic" talk on my current PhD research at Stereodynamics 2018, Arosa. The purpose of these talks was to provide opportunities for non-invited speakers and mainly younger researchers (Postgraduate students) to present their work to the community which was successful for me as I was approached with several questions not only after my presentation but also during the rest of the day. |
| Year(s) Of Engagement Activity | 2018 |