Effect of pH and Au Nanoparticles on Cytochrome c Investigated by Electrochemistry and UV-Vis Absorption Spectroscopy

In this paper, gold nanoparticles (NPs) with an average size of (4.7 +/- 0.6) nm, capped with mercaptopropionic acid (MPA) ligand, are prepared. The effect of pH and Au NPs on cytochrome c (Cyt c) is investigated by electrochemistry and UV-Vis absorption spectroscopy. UV-Vis absorption spectra indicate that the structures of Cyt c and Cyt c-Au NPs complex do not change appreciably between pH=7.5 and pH=3.0, but their Soret band positions change markedly at pH=2.0, indicating that low pH value induces a conformational change in Cyt c. Cyclic voltammetry (CV) result shows that Au NPs capped with MPA enhance electron transfer between Cyt c and the electrode. The data also reveals that the biocompatibility of Au NPs is improved when citric acid ligand is replaced with MPA. The change in pH value causes a change of peak currents in CV and a shift of peak potential. When pH value deviates from 7.0, levels of electroactive Cyt c decrease. Significant pH change induces irreversible denaturing of Cyt c. The pH at which Cyt c-Au NPs complex denatures completely is one unit higher than that of Cyt c. Combining the results from UV-Vis spectroscopy and CV, we find that addition of Au NPs makes adsorbing state Cyt c more vulnerable to pH.National Natural Science Foundation of China [20833005, 20873116, 60936003, 21021002