High Helical Propensity of the Peptide Fragments Derived from β-Lactoglobulin, a Predominantly β-sheet Protein

Abstract

It is generally assumed that peptide fragments derived from globular proteins are either unfolded, or adopt native-like secondary structures, in particular α-helix, which are similar to those occurring in the early stages of protein folding. Since the structured conformations of short peptides are unstable, 2,2,2-trifluoroethanol (TFE) is often used to stabilize them. To examine the folding process of β-proteins, we synthesized three fragments of β-lactoglobulin corresponding to two β-strands and one helix region, and examined their conformation by circular dichroism and nuclear magnetic resonance. These regions were chosen because, according to secondary structure prediction, all three should have high helix propensities. In aqueous solution, the three peptides had only a little ordered structure, but when TFE was added, they exhibited marked helical propensities, as also observed for the whole molecule of β-lactoglobulin. These results indicate that the intrinsic helical propensity of a peptide fragment elucidated by the addition of TFE is not necessarily related to the secondary structure in the native state. The results further suggest a case of non-hierarchical protein folding model, in which non-native structures may be involved in the early stage of folding.