Embodying a stable alpha-helical protein structure through efficient chemical ligation via thioether formation

A new approach was developed to embody the alpha-helical protein structure having an arbitrary combination and arrangement of helices by the successive ligation of a haloacetyl peptide segment with a cysteinyl peptide. A four-helix-bundle protein was efficiently constructed by the repetitive ligation of alpha-helical peptide segments. The use of HPLC-purified unprotected peptide segments facilitated the purification of the intermediates to afford the highly homogeneous desired protein. The use of the bromoacetyl moiety and the chloroacetyl moiety for the ligation was judged to make no difference in practice. A trial of introducing an additional intramolecular disulfide cross-link was also examined. The resulting protein showed high stability in the chaotropic and thermal denaturation and in enzymatic degradation.