Effect of ionic strength on the interfacial properties of cytochrome c

The surface tension behaviour of oxidised cytochrome c (cyt c) solution was characterised at various pH and ionic strength at the air/water interface. The pendant drop method employing digital image analysis of the drop shape was applied to the measurement of the surface tension. The adsorption prop erties of cyt c were utilised to study the protein conformation change effe cted by acidification and ionic strength. At high ionic strength, the satur ated steady-state surface tension shows a cooperative change centred around 3.6 induced by a decrease in pH. Using spectroscopic experiments, the appa rent pK of the acid-induced transition of horse cyt c from the native to th e molten globular state is equal to 3.5. This fact indicates that the satur ated steady-state surface tension is a parameter which might be used to mon itor conformation changes of cyt c. (C) 1999 Elsevier Science B.V. All righ ts reserved.