Trifluoroethanol (2,2,2-trifluoroethanol, CF 3CH2OH, often abbreviated TFE ) is a substituted organic alcohol often used as a co-solvent. It is a liquid at room temperature and miscible with water. The property that makes it of interest in molecular biology is its ability to stabilize noncovalent structure in peptides. The a-helix is a common secondary structure motif in proteins, but it is only slightly stable in most short peptides (see Alpha-Helix Formation). TFE stabilizes helical structure in some peptides (1-3). Several proteins have been split into peptide fragments, and these peptide fragments have shown a tendency to form helices in TFE (4-6), even if they are unstructured in water. This tendency is particularly strong for peptides corresponding to helical regions in the intact protein, but it has also been present even in fragments that contained no helix in the intact protein. Relatively high concentrations of TFE are needed to get the full stabilization, and the amount of a-helical structure generally reaches a maximum around 40% to 60% (v/v) TFE. TFE has also been shown in certain instances to stabilize b-sheet secondary structure (3, 7).
Because TFE weakens hydrophobic interactions, it is a protein denaturant. However, the denatured state induced by TFE is usually structured. For example, the denatured state of the protein hen egg-white lysozyme in 70% TFE consists of regions of high helical content (8), corresponding to regions that were helical in the native protein, although this denatured state does lack tertiary structure interactions. Study of proteins denatured in TFE can give an idea of the local structure-forming tendencies of regions of the polypeptide chain and may suggest sites at which the folding of the protein is initiated.
The mechanism by which TFE stabilizes structure is unclear. One suggestion is that TFE interferes with the ability of water to solvate peptide groups that make up part of the peptide backbone (2, 9). Therefore the unstructured conformation of a peptide, whose backbone peptide groups are exposed to water, is less favored in TFE solution than the same conformation in pure water. Thus, structures featuring these groups hydrogen-bonded to other groups in the peptide (as in the a-helix) are favored.
