Head‐to‐Tail Interactions in Tyrosine/Benzophenone Dyads in the Ground and the Excited State: NMR and Laser Flash Photolysis Studies

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.

Intramolecular H-atom transfer reactions in rigid peptides — Correlated solvent and structural effects

“…The aromatic side chains in linear peptides preferably populate the rotamer E N in polar solvents such as DMSO, with p(E N ) 0.7. [61][62][63][64] In contrast, E N appears to be much less favorable in the cyclic peptide 1, where both aromatic residues preferably populate the back-folded F conformation instead. This behavior is quite common for the DKPs that carry aromatic amino acids and has been associated with dipoleinduced dipole interactions between the DKP amides and the aromatic ring.…”

“…This behavior interferes with biradical accumulation and is contrary to the general observation that biradicals decay on a much longer time scale than do their parent triplets. 12,16,19,61 In contrast to the largely solvent-independent behavior of 2, the evolution of the transient spectra of dyad 1 is strongly dependent on the nature of the solvent. In particular, the spectrum obtained after 200 ns in the ACN-H 2 O (2:1 v/v) mixture differs dramatically from the one in pure ACN and is fully consistent with efficient formation of ketyl and tyrosyl radicals.…”

“…The quantum yields for the formation of the respective biradicals were reported to be unity. 61 Thus, any significant spectral evolution with a spectral shift from 525 to 540 nm and accompanied by a growth at 405 nm would be indicative of the formation of ketyl and tyrosyl radicals, respectively. We found that the two dyads showed distinctly different spectral evolution during and after the decay of the triplet state.…”

Efficient Photochemical Oxidation of Anisole in Protic Solvents: Electron Transfer driven by Specific Solvent–Solute Interactions

“…(Triplet) Exciplexes that are based on charge-transfer interactions between the reactants have often been invoked as prominent intermediates on the electron-transfer or hydrogenatom transfer coordinates of excited-state quenching reactions. [8,29,[51][52][53][54][55][56][57][58] Whereas quenching of triplet excited BP by anisole certainly is due to charge transfer interactions in nonprotic solvents, there was no evidence for the formation of strongly bound exciplexes. In particular, the Stern-Volmer plots that have been employed to evaluate the bimolecular quenching rate constants showed no significant deviation from linearity even at high quencher concentrations, an observation that holds for all solvents (mixtures) used herein.…”

Unambiguous evaluation of the relative photolysis rates of nitro indolinyl protecting groups critical for brain network studies

“…The initial protons (6, 5, 2, Me, i and j) disappear over time while new species (6 0 , 5 0 , 2 0 , Me 0 , i 0 , and j 0 ) appear. These NMR peaks and integrals provide qualitative and quantitative information [10,[18][19][20][21][22][23][24][25].…”