Topological effects in ultrafast photoinduced processes between flurbiprofen and tryptophan in linked dyads and within human serum albumin

Abstract

The interaction dynamics between flurbiprofen (FBP) and tryptophan (Trp) has been studied in covalently linked dyads and within human serum albumin (HSA) by means of fluorescence and ultrafast transient absorption spectroscopy. The dyads have proven to be excellent models to investigate photoinduced processes such as energy and/or electron transfer that may occur in proteins and other biological media. Since the relative spatial arrangement of the interacting units may affect the yield and kinetics of the photoinduced processes, two spacers consisting of amino and carboxylic groups separated by a cyclic or a long linear hydrocarbon chain (1 and 2, respectively) have been used to link the (S)- or (R)-FBP with the (S)-Trp moieties. The main feature observed in the dyads was a strong intramolecular quenching of the fluorescence, which was more important for the (S,S)- than for the (R,S)- diastereomer in dyads 1, whereas the reverse was true for dyads 2. This was consistent with the results obtained by simple molecular modelling (PM3). The observed stereodifferentiation in (S,S)-1 and (R,S)-1 arises from the deactivation of ¹Trp*, while in (S,S)-2 and (R,S)-2 it is associated with ¹FBP*. The mechanistic nature of ¹FBP* quenching is ascribed to energy transfer, while for ¹Trp* it is attributed to electron transfer and/or exciplex formation. These results are consistent with those obtained by ultrafast transient absorption spectroscopy, where ¹FBP* was detected as a band with a maximum at ca. 425 nm and a shoulder at ∼375 nm, whereas Trp did not give rise to any noticeable transient. Interestingly, similar photoprocesses were observed in the dyads and in the supramolecular FBP@HSA complexes. Overall, these results may aid to gain a deeper understanding of the photoinduced processes occurring in protein-bound drugs, which may shed light on the mechanistic pathways involved in photobiological damage.