A method for purifying a target of interest comprising, a) contacting a sample comprising a target of interest with a composition comprising a polypeptide agent coupled to a solid support, wherein the polypeptide agent has an amino acid sequence comprising: i) ii) iii) iv) v) vi) vii) viii) ix) x) wherein X n is a natural or non-natural amino acid, and Z 1 and Z 2 comprise between 2 to 30 natural or non-natural amino acids, optionally wherein X n is not a cysteine or a proline, wherein the polypeptide agent has an amino acid sequence that differs from SEQ ID NO:1, wherein the polypeptide specifically binds the target of interest, wherein the polypeptide's specific binding to the target of interest is greater than binding of a polypeptide according to SEQ ID NO:1 to the target of interest; wherein the contacting is performed under conditions that permit binding of the composition to the target of interest, and optionally wherein the polypeptide agent is coupled to the solid support through non-covalent association or a covalent bond and the solid support comprises a bead, a glass slide, a chip, a gelatin, or an agarose; and b) removing a portion of the sample that is not bound to the composition; and c) optionally further comprising dissociating the composition from the target of interest and recovering the target of interest. a) contacting a sample comprising a target of interest with a composition comprising a polypeptide agent coupled to a solid support, wherein the polypeptide agent has an amino acid sequence comprising: i) ii) iii) iv) v) vi) vii) viii) ix) x) wherein X n is a natural or non-natural amino acid, and Z 1 and Z 2 comprise between 2 to 30 natural or non-natural amino acids, optionally wherein X n is not a cysteine or a proline, wherein the polypeptide agent has an amino acid sequence that differs from SEQ ID NO:1, wherein the polypeptide specifically binds the target of interest, wherein the polypeptide's specific binding to the target of interest is greater than binding of a polypeptide according to SEQ ID NO:1 to the target of interest; wherein the contacting is performed under conditions that permit binding of the composition to the target of interest, and optionally wherein the polypeptide agent is coupled to the solid support through non-covalent association or a covalent bond and the solid support comprises a bead, a glass slide, a chip, a gelatin, or an agarose; and i) ii) iii) iv) v) vi) vii) viii) ix) x) wherein X n is a natural or non-natural amino acid, and Z 1 and Z 2 comprise between 2 to 30 natural or non-natural amino acids, optionally wherein X n is not a cysteine or a proline, wherein the polypeptide agent has an amino acid sequence that differs from SEQ ID NO:1, wherein the polypeptide specifically binds the target of interest, wherein the polypeptide's specific binding to the target of interest is greater than binding of a polypeptide according to SEQ ID NO:1 to the target of interest; wherein the contacting is performed under conditions that permit binding of the composition to the target of interest, and optionally wherein the polypeptide agent is coupled to the solid support through non-covalent association or a covalent bond and the solid support comprises a bead, a glass slide, a chip, a gelatin, or an agarose; and wherein X n is a natural or non-natural amino acid, and Z 1 and Z 2 comprise between 2 to 30 natural or non-natural amino acids, optionally wherein X n is not a cysteine or a proline, wherein the polypeptide agent has an amino acid sequence that differs from SEQ ID NO:1, wherein the polypeptide specifically binds the target of interest, wherein the polypeptide's specific binding to the target of interest is greater than binding of a polypeptide according to SEQ ID NO:1 to the target of interest; wherein the contacting is performed under conditions that permit binding of the composition to the target of interest, and optionally wherein the polypeptide agent is coupled to the solid support through non-covalent association or a covalent bond and the solid support comprises a bead, a glass slide, a chip, a gelatin, or an agarose; and b) removing a portion of the sample that is not bound to the composition; and c) optionally further comprising dissociating the composition from the target of interest and recovering the target of interest.