Garbuio, Luca (2011) Fullerenes, peptides and radicals. [Laurea specialistica biennale]
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this Thesis concerns the synthesis, characterization and investigation of two series of compounds which are characterized by the presence of a fullerene moiety and / or of a nitroxide function covalently linked at the extremity of a rigid peptide. The structured peptide sustains a strong oriented dipole moment located along the main axes of the molecules and its direction is opposite in the two series, named 2+ and 2-. The dipole orientation was expected to cause opposite effects, for example, on the redox properties of the moieties located at its extremity. The syntheses required a considerable amount of experimental work and the method chosen to functionalize the fullerene was the Maggini-Prato reaction. We could synthesize the following compounds: the fullerene-peptides 2+ and 2-, the fullerene-peptides-reference 2+ and 2- and the peptides-reference 2+ and 2-. Besides other two fullerene-peptides were synthesized using the precious endofullerene that bears a hydrogen molecule entrapped in its cage. Characterization in solution (Mass Spectrometry, FT-IR absorption spectroscopy, 1H NMR spectroscopy) proves the identity of compounds under investigation. It also provides evidence of the presence of structured peptides in both series 2- and 2+. 1H NMR spectra of systems bearing the nitroxide function were acquired after quenching of the free radical with diphenylhydrazine. Cyclic voltammetries clearly show that the first reduction peak of fullerene is anticipated in the 2+ series with respect to the 2- series. This experimental outcome is in full agreement with the effect caused by the peptide macrodipole. An even more accentuated deviation is found on the oxidation side (TOAC oxidation) where the potential shifts are once again on the direction expected for an effect of the helix dipole moment, namely, easier reduction and easier oxidation for the 2+ series while more difficult reduction and more difficult oxidation for the 2- series. It is thus possible to state that in these compounds the helix dipole moment plays an important role on the tuning of the potentials of the active redox groups (the nitroxide function and the fullerene). Thank to the unique rigidity of the peptide spacer, an accurate estimation of the distance is obtainable and this parameter is crucial for several investigations where the interactions between the fullerene (or the hydrogen molecule encapsulated inside its cage) and the nitroxide are distance-dependent. Some of the compounds were thus studied by TR EPR (Time-Resolved Electron Paramagnetic Resonance) to investigate the coupling between the doublet ground state of the nitroxide and the triplet state of the fullerene generated after photo-excitation and by 1H NMR spectroscopy to obtain the nuclear spin relaxations and the ortho / para conversion rates of the encapsulated hydrogen.
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