PolyLproline

Poly-L-proline is a synthetic polymer composed of L-proline residues linked by peptide bonds, making it a homopolymer of the amino acid proline. It can adopt two principal helical forms depending on solvent and conditions: poly-L-proline I (PPI), in which all peptide bonds are in the cis conformation and which is typically favored in nonpolar environments, and poly-L-proline II (PPII), in which all peptide bonds are in the trans conformation and is common in aqueous or biological settings. PPII is a left-handed helix with roughly three residues per turn and an extended geometry that is often described as collagen-like; PPI is a more compact, right-handed helix that arises when cis content is high.

Synthesis and structure: Poly-L-proline is typically prepared by polymerizing protected L-proline derivatives, most commonly through polymerization of L-proline N-carboxyanhydride or related monomers, followed by deprotection. The resulting polymer’s molecular weight, tacticity, and environment influence the balance between PPI and PPII, as well as solubility and mechanical properties. The rigid pyrrolidine ring of proline contributes to a relatively high chain rigidity and persistence length compared with many other polypeptides.

Applications: The conformational versatility and rigidity of poly-L-proline make it useful as a structural scaffold and rigid spacer in biomaterials and peptide conjugates. It serves as a model system for studying proline-rich interactions, collagen-like motifs, and general aspects of polypeptide folding and molecular recognition. In materials science, it is explored as a component of synthetic biopolymers and hydrogel formulations.