Patent ID: 8198043

Claim:
An isolated polypeptide comprising a two helix segment derived from the Z domain of protein A that is capable of binding the Fc region of IgG and wherein the isolated polypeptide comprises a disulfide bond between two members of a thiol-containing pair of residues, wherein both members of the disulfide pair are positioned outside of the binding interface of the polypeptide and wherein the first amino acid of the binding interface is positioned eleven amino acids in an N-terminal direction relative to a conserved proline, wherein the conserved proline is an internal residue within the isolated polypeptide and wherein the isolated polypeptide comprises more than eleven amino acids in the N-terminal direction relative to the conserved proline, wherein the isolated polypeptide comprises a variant of SEQ ID NO. 2 modified by a substitution that comprises substituting two amino acids for the thiol-containing pair of residues and wherein Xaa is any naturally occurring amino acid except cysteine.