Synthesis of an iron(II)-braced proline-II tripod protein

This paper describes the engineering of braced tripod proteins for use as molecular frameworks. Specifically, a 30-residue tripod-shaped protein with three proline-II helical legs braced by an iron(II)tris(bipyridine) complex was modularly designed, chemically synthesized, and biophysically characterized. Three copies of a 10-residue leg peptide were covalently linked through sulfide bonds to an N-terminal apex (1,3,5-tris(methylene)benzene) and by amide bonds to the brace (FeII(Mbc)3: Mbc is 4-methyl-2,2-bipyridine-4-carbonyl). The leg peptide (H-Cys-Pro5-Pra(Mbc)-Pro3-NH2: Pra is cis-4-amino-l-proline) was assembled by the solid-phase method using Boc-Pra(Mbc)-OH, which was synthesized in 75% overall yield by coupling Mbc-OH to the 4-amino group of Boc-Pra-OCH3 and saponifying the methyl ester group.The iron(II)-braced tripod was assembled by S-alkylation of three copies of the leg peptide with 1,3,5-tris(bromomethyl)benzene followed by ligation of Fe2+ to the resulting unbraced tripod. The CD spectrum of the iron(II)-braced tripod showed a positive MLCT band at 570 nm and a negative –* band at 312 nm, so its FeII(Mbc)3 brace was predominantly in the configuration. In a mostly acetonitrile solution at 25 °C, the leg peptide and the unbraced tripod isomerized from the proline-II helical form into the proline-I helical form but the iron(II)-braced tripod remained in the proline-II helical form.