Steady-state fluorescence spectroscopy studies of BSA/polyfluorene: the influence of pH on equilibrium conformation

Abstract

Bovine serum albumin (BSA) is a globular protein, in neutral pH range, with 585 amino acid residues that mainly forms alpha helix secondary structure, connected by 17 isulfide bridges, which confer a relatively stability to the protein. BSA presents high structural homology to human serum albumin (HSA), by alignment of the two sequences, which crystallographic structure is known [1]. Both BSA and HSA undergo to different spatial arrangement in acidic pH solution, the N-F transition between pH 5.0 and 3.5, and the expanded forms (E transition) below this value. Furthermore, in alkaline pH conditions, these proteins can acquire another different conformation, known as N-B transition [2]. It has been proposed that, similar to the acid-induced one, the alkaline-induced transformation have a biological role, probably related to drugs, ions and ligands delivery. Therefore, the studies on pH variation comprise essential problems to explore, in both cases, to understand the protein conformational equilibrium and explore the polymers conjugation for biomaterials applications. Hence, this work is mainly focus on enlightening the conformational transitions promoted in BSA by pH changes, with and without the addition of Polyfluorene (PFO). For that, fluorescence emission spectra of BSA in buffer acetate/borate/phosphate 50mM from pH 2 to 12 were measured with PFO/Dioxane at ratio BSA/PFO 1:0.5. The results showed that energy transfer process depends on the pH due to the dependence in the emission efficiency of BSA reach a maximum approximated at pH ~5.5.