Effect of Neutral Salt on the Stability of Horse Cytochrome c at Acidic pH

Abstract

Horse ferricytochrome c (ferricyt c) at mildly acidic pH (~3.8) behaves as a two state folding protein. This work examined the effect of salt (NaCl) on the stability of horse ferricyt c at pH~ 3.8 (250 C). Chemical unfolding studies in urea (based on fluorescence; excitation: 280 nm, and emission: 365 nm) show that NaCl significantly increases the chemical stability of the protein. On the other hand, thermal unfolding studies (based on absorbance at 399 nm) show that NaCl significantly decreases the thermal stability of ferricyt c. As [NaCl] is increased, the midpoint transition temperature, Tm, for unfolding of ferricyt c decrease mono-exponentially, plateau at ~0.5 M NaCl consistent with destabilization of ferricyt c by ionic screening of electrostatic interactions.