Licochalcone A bound to bovine serum albumin: a spectroscopic, photophysical and structural study

The interaction of Licochalcone A (LA) with bovine serum albumin (BSA) was studied by circular dichroism (CD), UV-Vis absorption, fluorescence and laser flash photolysis. The most stable 1 : 1 and 2 : 1 LA : BSA complexes were spectroscopically characterized. Two protein sites of similar affinity are involved in the LA association with both stoichiometries. The fluorescence quantum yield was found to be higher by ca. one order of magnitude for bound LA than for the free molecule. The emission lifetime strongly depended on the protein site. Binding induced an increase in the radiative rate constant for one location and a decrease of the non-radiative rate in the other location. Two LA triplets were evidenced in the protein environment, the first one shorter-lived and the second one longer-lived than in buffer. The BSA fluorescence was quenched in the complexes. FRET was found to be inefficient. Molecular mechanics (MM), molecular dynamics (MD) and quantum mechanical calculations of rotational strength combined with CD data afforded the likely structures of the complexes. One, involving the interaction of LA with Trp212 in domain II, was able to account for most of the observed photophysical effects. MM and MD calculations also showed LA associates in domain III, the lowest energy complex being one with the drug close to Tyr409.