Dynamical and structural changes of an anesthetic analogue in chemical and biological nanocavities

We report on photophysical studies of the interaction between an anesthetic analogue, methyl 2-amino-4,5-dimethoxybenzoate (ADMB), with the human serum albumin (HSA) protein and the normal micelle of n-octyl-beta-D-glucopyranoside (OG) in water solutions. We used steady-state and picosecond time-resolved emission spectroscopy to follow the dynamical and structural changes due to their hydrophobicity and confinement on the photophysical behavior of ADMB. The formed 1:1 complex with the protein is robust with an equilibrium constant of 9.6 x 10(4) M(-1) at 293 K. The fluorescence lifetimes of the 1:1 entity become longer (up to approximately 10 ns), and the emission transients show complex behavior due to the heterogeneity of the media. Rotational time (45 ns) from picosecond anisotropy measurements clearly indicates strong confinement in the robust ADMB:HSA complex. For the ADMB:OG one, the anisotropy decays give time constants of 50 and 980 ps, assigned to free and restricted rotors within the micelle, respectively. The process of energy transfer from the excited tryptophan 214 (Trp214) of HSA to the trapped ADMB occurs with an efficiency of 50%, and the calculated distance between both chromophores is 19 A. We believe that these results are important for a better understanding of processes occurring in encapsulated drugs and thus should be relevant to nanopharmacodynamics.