2,2,2-Trifluoroethanol induces helical conformation in an all beta-sheet protein

The effect of 2,2,2-trifluoroethanol (TFE) on the structure of an all beta-sheet protein, cardiotoxin analogue II (CTX II), from the Taiwan cobra (Naja naja atra) is studied. Using circular dichroism studies, it is found that higher concentrations of TFE induced a structural transition from beta-sheet to alpha-helix, both in the native state (nCTX II) and in denatured but not disulfide reduced CTX II (dCTX II) samples. The beta-sheet to alpha-helix conversion is shown to be cooperative. However, in denatured and reduced CTX II (rCTX II), a TFE transforms a portion(s) of the protein backbone a random coil to an alpha-helical conformation. Based on the solution structure of CTX II and the physical property of TFE, a possible mechanism for the observed backbone structural transitions induced by TFE is discussed. The results described in this paper question the significance of the structure of the "molten globule" intermediate(s) obtained in organic solvents such as TFE.