Excited State Interactions in Flurbiprofen-Tryptophan dyads

Abstract

[EN] Fluorescence and laser-flash photolysis measurements have been performed on two pairs of diastereomeric dyads that contain the nonsteroidal anti-inflammatory drug (S)- or (R)-flurbiprofen (FBP) and (S)-tryptophan (Trp), which is a relevant amino acid present in site I of human serum albumin. The fluorescence spectra were obtained when subjected to excitation at 266 nm, where similar to 60% of the light is absorbed by FBP and similar to 40% is absorbed by Trp; the most remarkable feature observed in all dyads was a dramatic fluorescence quenching, and the residual emission was assigned to the Trp chromophore. In addition, an exciplex emission was observed as a broad band between 380 and 500 nm, especially in the case of the (R,S) diastereomers. The fluorescence lifetimes (tau(F)) at lambda(em) = 340 nm were clearly shorter in the dyads than in Trp-derived model compounds; in contrast, the values of tau(F) at lambda(em) = 440 nm (exciplex) were much longer. On the other hand, the typical FBP triplet-triplet transient absorption spectrum was obtained when subjected to laser-flash photolysis, although the signals were less intense than when FBP was directly excited under the same conditions. The main photophysical events in FBP-Trp dyads can be summarized as follows: (1) most of the energy provided by the incident radiation at 266 nm reaches the excited singlet state of Trp ((1)Trp*), either via direct absorption by this chromophore or by singlet singlet energy transfer from excited FBP ((FBP)-F-1*); (2) a minor, yet stereoselective deactivation of (FBP)-F-1* leads to detectable exciplexes and/or radical ion pairs; (3) the main process observed is intramolecular (1)Trp* quenching; and (4) the first triplet excited-state of FBP can be populated by triplet-triplet energy transfer from excited Trp or by back-electron transfer within the charge-separated states.