Excited state intermolecular proton and energy transfer of 1-hydroxypyrene interacting with the human serum albumin protein

We report on pico to nanosecond studies of the excited-state intermolecular-proton transfer (ESPT) reaction and energy transfer of 1-hydroxypyrene (1-HP) with the human serum albumin (HSA) protein. The formed robust 1:1 complexes (Keq = (1.84 ± 0.1) × 106 M−1) show both photoacid and conjugated photobase emissions of the caged 1-HP in its protonated structure. No evidence of excited state equilibrium between the protonated and deprotonated forms and of ESPT in aqueous phosphate buffer (pH 7) has been found. However, the proton-transfer reaction in the 1-HP/HSA protein complexes proceeds with a time constant of ∼400 ps and is assigned to a direct acid–base reaction between 1-HP and carboxylate group of nearby amino acid residue of the HSA protein. Contrary to trisodium 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) that has three electronegative sulfonate groups and forms strong H-bonds with amino acid residues in binding site II, 1-HP interacts predominantly with the hydrophobic pocket of binding site I. The ps–ns anisotropy measurements further confirm the robustness of HPTS:HSA complexes. We studied also the possible energy transfer process from excited tryptophan residue (Trp214) to the encapsulated 1-HP. For 1-HP:HSA complex, the efficiency of the energy transfer process from excited Trp214 to 1-HP is significant (77%) with a rate constant of ∼9.8 × 108 s−1 and calculated interchromophore distance of ∼15 Å.