Stereodifferentiation in the intramolecular singlet excited state quenching of hydroxybiphenyl–tryptophan dyads

Abstract

The photochemical processes occurring in diastereomeric dyads (S,S)-1 and (S,R)-1, prepared by conjugation of (S)-2-(2-hydroxy-1,1'-biphenyl-4-yl)propanoic acid ((S)-BPOH) with (S)-and (R)-Trp, have been investigated. In acetonitrile, the fluorescence spectra of (S,S)-1 and (S,R)-1 were coincident in 10 shape and position with that of (S)-BPOH, although they revealed a markedly stereoselective quenching.Since singlet energy transfer from BPOH to Trp is forbidden (5 kcal mol -1 uphill), the quenching was attributed to thermodynamically favoured (according to Rehm-Weller) electron transfer or exciplex formation. Upon addition of 20% water, the fluorescence quantum yield of (S)-BPOH decreased, while only minor changes were observed for the dyads. This can be explained by an enhancement of the excited 15 state acidity of (S)-BPOH, associated with bridging of the carboxy and hydroxy groups by water, in agreement with the presence of water molecules in the X-ray structure of (S)-BPOH. When the carboxy group was not available for coordination with water, as in the methyl ester (S)-BPOHMe or in the dyads, this effect was prevented; accordingly, the fluorescence quantum yields did not depend on the presence or absence of water. The fluorescence lifetimes in dry acetonitrile were 1.67, 0.95 and 0.46 ns for (S)-20 BPOH, (S,S)-1 and (S,R)-1, respectively, indicating that the observed quenching is indeed dynamic. In line with the steady-state and time-resolved observations, molecular modelling pointed to a more favourable geometric arrangement of the two interacting chromophores in (S,R)-1. Interestingly, this dyad exhibited a folded conformation in the solid state.

Switching from ultrafast electron transfer to proton transfer in excited drug–protein complexes upon biotransformation

“…12 By contrast, since the metabolite is a phenol, although electron transfer might also occur, as it has been observed for GFT-MB/Tyr (or Trp) mixtures in organic solvent, excited state proton transfer (ESPT) to form phenolate-like excited species might be a competitive process within the protein; these species are known to absorb and emit at wavelengths longer than phenols. [44][45][46] In this regard, not only emission from GFT-MB@HSA was shied to longer wavelengths, but also its absorption spectrum revealed a red-shied shoulder at ca. 375 nm compared to GFT-MB in the bulk aqueous solution (Fig.…”

Mechanistic Studies on the Photoallergy Mediated by Fenofibric Acid: Photoreactivity with Serum Albumins

“…In this context, it has been demonstrated in previous reports that dyads composed of a drug covalently linked to an amino acid may provide key information on the processes that occur in the binding sites between the two chromophores (such as energy transfer, electron transfer, exciplex formation, etc. ). − …”

Looking for chiral recognition in photoinduced bimolecular electron transfer using ultrafast spectroscopy

“…[10][11][12][13] For example, Miranda and coworkers investigated diastereomeric dyads consisting of an hydroxybiphenyl drug linked to a tryptophan donor and found variations in the fluorescence lifetime as large as a factor of four depending on the diastereomer. 11 This effect as well as that observed with other diastereomeric dyads were attributed to the linker that limits the conformational space available to the reactants. This leads to marked differences in the electronic coupling and thus in the ET quenching dynamics.…”