Direct probing of the folding/unfolding event of bovine hemoglobin at montmorillonite clay modified electrode by adsorptive-transfer voltammetry

An efficient way was proposed for probing the folding/unfolding event of bovine hemoglobin (Hb) through adsorptive-transfer voltammetry. Hb molecules in native and pre-unfolded in different urea conditions for 23 h were adsorbed onto the montmorillonite clay modified glassy carbon electrode (Hb/clay/GCE and uHb/clay/GCE, respectively). Cyclic voltammograms of Hb/clay/GCE and uHb/clay/GCE showed that the unfolding of Hb caused great change in the direct electron transfer between the heme irons within Hb and electrode surface, which was facilitated on clay film. From the amount of the electroactive Hb (W_Hbe) and the adsorbed Hb (Γ) on clay per unit mass, the minimal electroactive portion (MEP) of the adsorbed Hb was calculated to assess the unfolding state of Hb. With the increase of urea concentration, MEP showed a sigmoid curve. Thermodynamic parameters related to the unfolding event of Hb were also obtained based on the linear free energy model (LEM), including the free energy of folding in water (ΔG_U^water), the slope of the Santoro-Bolen equation (m), and the urea concentration required in for achieving half of the total change (S_m) in the unfolding curves. This work gave the first try for investigating protein unfolding at nano-materials modified electrode using adsorptive-transfer voltammetry, which improved the sensitivity of analysis and avoided the disadvantages involved in the existing electrochemical methods for protein unfolding. The proposed method will benefit the electrochemical studies of protein.