Crystal Structures of Expanded Poly(L-leucine) Isomers Containing Bis(pyridine)silver(I) Moieties: Precise Formation of Secondary Structure Depending on the Side Chain

Precise construction of a three-dimensional molecular structure is key for functional macromolecules, such as enzymes or proteins. Previously, a new concept, expanded poly(alpha-amino acid)s containing rigid spacers, was proposed for strategic construction of chiral helices. Herein, expanded poly(L-leucine) isomers containing bis(pyridine)silver(I) moieties were synthesized, and their crystal structures were determined by X-ray analysis. Each expanded polypeptide forms a unique secondary structure, a left-handed 6(1) helix or zigzag chain (2(1) helix), precisely depending on the chemical structure of the side chain, that is, slight branching. Distinct conformations were indicated by two main areas in the Ramachandran plot. These results suggest that the appropriate selection of the amino acid sequence and rigid spacers will lead to a new expanded protein with a tailor-made three-dimensional structure and desired functions.