Ultrafast photoinduced processes in metal - containing molecular complexes and in proteins

The aim of this thesis is to investigate the ultrafast inter¿ and intra¿molecular processes, occurring in both metal¿containing molecular complexes and in proteins, by means of ultrafast Transient Absorption (TA) spectroscopy. The first part of the thesis focuses on understanding the origin of the solvent dependent Intersystem Crossing (ISC), from the lowest excited singlet state (1A2u) to the lowest excited triplet state (3A2u), in [Pt2 (P2O5H2)4]4¿ (Pt(pop)) and its perfluoroborated derivative [Pt2 (P2O5(BF2)2)4]4¿ (Pt(pop)¿BF2). Our UV pump¿Visible probe TA experiments (exciting both in the 1A2u state and in higher¿lying states) highlight the presence of an intermediate state, in agreement with the previous hypothesis of Milder and Brunschwig. The energy of the latter state is strongly modulated by both the solvent and the ligands present in the pop cage, causing the large difference in ISC timescales (upon excitation in the 1A2u state) between Pt(pop) in acetonitrile (MeCN) (< 1 ps), Pt(pop) in water (~13 ps) and Pt(pop)¿BF2 in MeCN (1.6 ns). Excitation at higher¿lying excited states populates both the 1A2u and the 3A2u state in different ratios, depending on the investigated system. Finally, we report on the presence of wave¿packets in the ground and excited states, as well as the coherence transfer from the initially excited state to the 3A2u state. The second part focuses on the photo¿excited tryptophan (*Trp) quenching by metal complexes in protein systems, such as ferrous Myoglobins (Mbs) and Prion Proteins (PrPs). Mb is a small globular protein composed by 153 amino acids and containing two Trp residues (positions 7 and 14). Trp7 is ~20 Å far away from the heme and its fluorescence is only quenched via Förster energy transfer (FRET), while Trp14 is closer (~15 Å) and it is quenched via both FRET and electron transfer to the heme. Our results demonstrate that *Trp14¿to¿heme electron transfer process occurs, surprisingly, also in ferrous Mbs (e.g. deoxy¿Mb and MbCO) and highlight the generation of a long¿lived product, namely a FeII¿porphyrin¿¿. We also report the interesting case of MbNO, in which the *Trp14 transfers the electron on the NO instead of the heme. Prion proteins are also globular proteins composed of 209 amino acids (in humans) and involved in metal binding. Even though a large amount of experiments and theoretical work have been performed on these proteins their main physiological role is not completely determined, but it is clear that the formation of a scrapie isoform (PrPSC) of the cellular PrPs (PrPC) is the infectious agent and functions as a template for the PrPC ¿ PrPSC conversion. Our UV pump¿Visible probe TA experiments aim to unravel the ultrafast processes occurring in the octarepeat region of PrPs and fill the gap between ultrashort and biological timescales. We find that both the minimum Cu binding sequence HGGGW (OPS) and the total octarepeat region (PHGGGWGQ)4 (OP4) display a non¿exponential quenching of the Trp residues, which was assigned to *Trp¿to¿nearby amino acids (H, G or Q) in different peptide¿s conformations. Upon Cu2+ complexation by OPS, a feature related to the formation of a Cu+ complex was detected. The same was not detected upon Cu2+ complexation by OP4, showing longer quenching timescales than the free peptide. Our results highlight the relationship between structural conformation of the peptide and quenching timescales of the *Trp.

Chergui, Majed
Lausanne, EPFL
Other identifiers:
urn: urn:nbn:ch:bel-epfl-thesis6739-3

 Record created 2015-09-02, last modified 2018-03-17

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