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Disputation Matthias Wohlgemuth

11.02.2021 | 16:30
Thema der Dissertation:
Light-induced dynamics and spectroscopy of chiral biomolecules and excitonic nanostructures
Abstract: The interaction of biomolecules with UV light gives rise to a wide variety of fundamental processes in nature. For example, it is well known that biological molecules feature various relaxation pathways that efficiently dissipate the gained excess energy to their environment before any harmful, i.e., irreversible photoreactions can take place. The study of such processes may therefore extend the knowledge about why biological matter is built exactly the way as it is found on Earth today.
In the present thesis, novel quantum-classical methods for the simulation of light-induced excited state dynamics in complex (bio)molecular systems have been developed and applied to selected examples. In addition, the methodology has been employed to simulate time-resolved spectroscopic observables.
In particular, the still unsolved problem of enantiomeric enrichment of amino acids has been tackled by extending the field-induced surface-hopping (FISH) approach to include the interaction with circularly polarized light. This has been motivated by the observation of enantiomerically enriched left-handed amino acids on meteorites and comets, which are strongly exposed to polarized radiation in space. Such cosmic sources of enantiomerically enriched molecules may offer an explanation for the dominance of L-amino acids on Earth.
The developed methodology has been applied to the smallest chiral amino acid, alanine. It has been found that an enrichment of one of the stereogenic isomers due to fragmentation and recombination processes in excited electronic states can be induced by the irradiation with circularly polarized light.
The quantum-classical methodology has been further developed to study the photodynamics in multi-chromophoric molecular aggregates. In such systems excitation energy transport (EET) plays a substantial role and is important, e.g., for light-harvesting in the photosynthetic systems. EET processes and the formation of long-living delocalized excitons have been also made responsible for the photostability of DNA which prevents cancerogenic mutations. In the newly developed multi-chromophoric FISH (McFISH) method quantum-classical simulations in the frame of the extended Frenkel exciton model have been combined with QM/MM techniques in order to simulate the photodynamics of a double-stranded DNA decamer. In accordance with the experimental observations a multi-exponential excited state decay, involving the formation of long-lived delocalized as well as ultrafast decaying localized states resembling those of the bare nucleobases, has been found. While these aforementioned studies focused on the light-induced dynamics of the chromophores, the influence of the environment has to be considered as well. The interaction of water with biomolecules and the formation of complex hydrogen-bonded networks are important driving forces in biochemical reactions and structural rearrangements. In particular, the structure and function of proteins is intimately connected with the dynamics of the water network, and photoexcitation of a solute molecule may trigger a rearrangement of the solvent molecules. A valuable means to probe this type of dynamics is the picosecond time-resolved pump-probe infrared (ps-TRIR) spectroscopy. However, based on experimental data alone, the atomistic interpretation is difficult. Therefore, a generally applicable method for the simulation of pump-probe TRIR spectra based on molecular dynamics simulations has been devised. The developed methodology has been employed to investigate the migration dynamics of a single water molecule around the --CONH-- peptide linkage in two peptide analogues serving as models for biological proteins. The simulated spectra are in excellent agreement with the experimental observations and the simulations have allowed for an atomistic interpretation of the measured time-scales, migration pathways and their fingerprints in the experimental ps-TRIR spectra.

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11.02.2021 | 16:30