Drug–protein interactions assessed by fluorescence measurements in the real complexes and in model dyads

In the present work, a systematic fluorescence study on supramolecular systems using two serum albumins (HSA or BSA) as hosts and the nonsteroidal antiinflammatory drugs carprofen (CPF) or naproxen (NPX) as guests has been undertaken. In parallel, model dyads containing Tyr or Trp covalently linked to CPF or NPX have also been investigated. In HSA/(S)-CPF and BSA/(S)-CPF (λexc = 266 nm), at 1:1 M ratio, an important degree (more than 40%) of singlet–singlet energy transfer (SSET) was observed to take place. The distance (r) calculated for energy transfer from the SAs to (S)-CPF through a FRET mechanism was found to be ca. 21 Å. In the case of HSA/(S)-NPX and BSA/(S)-NPX, energy transfer occurred to a lower extent (ca. 7%), and r was determined as ca. 24 Å. In order to investigate the possible excited state interactions between bound ligands and the relevant amino acids present in the protein binding sites, four pairs of model dyads were designed and synthesised, namely (S, S)-TyrCPF, (S, R)-TyrCPF, (S, S)-TrpCPF, (S, R)-TrpCPF, (S, S)-TyrNPX, (S, R)-TyrNPX, (S, S)-TrpNPX and (S, R)-TrpNPX. A complete SSET was observed from Tyr or Trp to CPF, since no contribution from the amino acids was present in the emission of the dyads. Likewise, a very efficient Tyr or Trp to NPX energy transfer was observed. Remarkably, in (S, S)-TrpNPX and (S, R)-TrpNPX a configuration-dependent reduction in the emission intensity was observed, revealing a strong and stereoselective intramolecular quenching. This effect can be attributed to exciplex formation and is dynamic in nature, as the fluorescence lifetimes were much shorter in (S, R)- and (S, S)-TrpNPX (1.5 and 3.1 ns, respectively) than in (S)-NPX (11 ns).