Head-to-tail interactions in tyrosine/benzophenone dyads in the ground and the excited state: NMR and laser flash photolysis studies |
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Authors: | Hörner Gerald Hug Gordon L Pogocki Dariusz Filipiak Piotr Bauer Walter Grohmann Andreas Lämmermann Anica Pedzinski Tomasz Marciniak Bronislaw |
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Affiliation: | Faculty of Chemistry, Adam Mickiewicz University Poznan, Grunwaldzka 6, Poznan, Poland. hoerner@amu.edu.pl |
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Abstract: | The formation of head-to-tail contacts in de novo synthesized benzophenone/tyrosine dyads, bp logical sum Tyr, was probed in the ground and excited triplet state by NMR techniques and laser flash photolysis, respectively. The high affinity of triplet-excited ketones towards phenols was used to trace the geometric demands for high reactivity in the excited state. A retardation effect on the rates with increasing hydrogen-bond-acceptor ability of the solvent is correlated with ground-state masking of the phenol. In a given solvent the efficiencies of the intramolecular hydrogen-atom-transfer reaction depend strongly on the properties of the linker: rate constants for the intramolecular quenching of the triplet state cover the range of 10(5) to 10(8) s(-1). The observed order of reactivity correlates to a) the probability of close contacts (from molecular-dynamics simulations) and b) the extent of the electronic overlap between the pi systems of the donor and acceptor moieties (from NMR). A broad survey of the NMR spectra in nine different solvents showed that head-to-tail interactions between the aromatic moieties of the bp logical sum Tyr dyads already exist in the ground state. Favourable aromatic-aromatic interactions in the ground state appear to correspond to high excited-state reactivity. |
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Keywords: | ketones peptides photochemistry pi interactions tyrosine |
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