Effect of detergents and hexafluoroisopropanol on the conformation of a non-helical and a helical plant protease inhibitor.

We have earlier reported the purification of a non-helical proteinase inhibitor from Cajanus cajan and a helical proteinase/amylase inhibitor from Phaseolus aureus. The effect of detergents, viz. sodium dodecyl sulfate (SDS), sodium deoxycholate (DOC) and 3-[(3-cholamidopropy) dimethylammonio]-1-propane sulfonate (CHAPS) and hexafluoroisopropanol on the conformation of these proteinaceous inhibitors was investigated using circular dichroism spectroscopy. The present report focuses on changes in the polypeptide backbone conformation with respect to induction of helical structure. SDS causes minimal changes in the tertiary as well as secondary structure of C. cajan proteinase inhibitor. In the presence of anionic bile salt, deoxycholate, minor changes in the far-UV CD spectrum were accompanied by loss in inhibitory activity while CHAPS did not affect the inhibitor function. As judged from the changes in circular dichroic curves ([Theta](MRW) at 208 and 222 nm), the primarily disorganized polypeptide chain of C. cajan proteinase inhibitor was converted by 3,3,3,3',3',3'-hexafluoro-2-propanol (HFIP) into helical conformation. The P. aureus inhibitor showed increased helicity in the presence of SDS ([Theta](MRW) at 208 nm) as well as sodium deoxycholate and CHAPS ([Theta](MRW) at 222 nm). Fluorescence measurements show slight alterations in the emission intensities. HFIP caused a cooperative increase in alpha-helical secondary structure in the P. aureus inhibitor.