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Towards understanding the photochemistry of tyrosine

By Azhar Iqbal


The H-atom detachment driven through the 1πσ* states of biological\ud chromophores containing an X-H bond (where X = N or O) upon UV absorption is\ud ubiquitous in nature. Understanding the role of this dissociative state in the\ud chromophores and their respective amino acids following UV excitation would\ud enable a step change towards establishing a better understanding of the mechanisms\ud of photostability of larger peptides in the gas-phase. The work presented in this thesis\ud focuses on the H-atom elimination of phenol and indole, the chromophores of the\ud amino acids tyrosine and tryptophan, respectively. The H-atom elimination has also\ud been carried out in tyrosine and its sub-units p-ethylphenol and tyramine upon\ud excitation at 200 nm. In all these systems the O-H bond fission on the phenol ring\ud results in a range of H-atoms kinetic energy release.\ud Using a combination of femtosecond pump-probe spectroscopy, time-offlight\ud mass spectroscopy (TOF-MS) and velocity map ion imaging (VMI) reveals\ud that H-atom elimination in all these systems occurs on an ultrafast timescale (~200\ud fs) for both fast and slow H-atoms. This casts considerable doubt over the previously\ud assigned statistical origin of the slow H-atoms and suggests direct pathways to their\ud formation. The H-atom kinetic energy spectrum in tyrosine also implies that H-atom\ud elimination is occurring through the same coordinate i.e. O-H bond as exhibited by\ud it’s phenol chromophore, thus confirming the active participation of the 1πσ* states\ud from the chromophore of the amino acid to the amino acid itself. These findings are\ud of great importance enabling one to compare these results with existing calculations\ud on the chromophores which often model the system in an isolated environment.\ud These results also provide ground work for more complex calculation to be carried\ud out, in particular on the amino acids and the di/tri peptides

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