Target sequences for synthetic molecules

A method of purifying a bonding partner, comprising:

• a) providing a bonding partner comprising a target sequence wherein the target sequence comprises Cys-Cys-X1-X1-X1-X1-X3-X1-X1-Cys-X1-Cys-X2 (SEQ. ID NO:
• 3), wherein:
• X1 is an amino acid having a non-polar side chain,
• X2 is an amino acid having a basic side chain, and
• X3 is an amino acid having a non-ionic polar side chain, and
• wherein the target sequence reacts with a biarsenical molecule having the structure:
• and tautomers, anhydrides, and salts thereof;
• wherein:
• each X1 or X2 is independently Cl, Br, I, ORa, or SRa, or
• X1 and X2 together with the arsenic atom form a ring having the formula:
• Ra is H, C1-C4 alkyl, CH2CH2OH, CH2COOH, or CN;
• Z is 1,2-ethanediyl, 1,2-propanediyl, 2,3-butanediyl, 1,3-propanediyl,
• 1,2 benzenediyl, 4-methyl-1,2-benzenediyl, 1,2-cyclopentanediyl,
• 1,2-cyclohexanediyl, 3-hydroxy-1,2-propanediyl, 3-sulfo-1,2-propanediyl, or
• 1,2-bis(carboxy)-1,2-ethanediyl;
• Y1 and Y2 are each independently H or CH3, or
• Y1 and Y2 together form a ring such that the biarsenical molecule has the formula
• wherein:
• M is O, S, CH2, C(CH3)2, or NH;
• R1 and R2 are each independently ORa, OAc, NRaRb, or H;
• R3 and R4 are each independently H, F, Cl, Br, I, ORa, or Ra;
• or
• R1 together with R3, or R2 together with R4, or both, form a ring in which
• (i) one of R1 or R3 is C2-C3 alkyl and the other is NRa and
• (ii) one of R2 and R4 is C2-C3 alkyl and the other is NRa;
• Rb is H, C1-C4 alkyl, CH2CH2OH, CH2COOH, or CN;
• Q is CRaRb, CRaORb, C═O, or a spirolactone having the formula:
• wherein the Spiro linkage is formed at C1;
• b) contacting the bonding partner with the biarsenical molecule, wherein the biarsenical molecule is coupled to a solid phase, and
• c) eluting the bonding partner from the biarsenical molecule by contacting the biarsenical molecule with a dithiol.