Synthesis and conformation of poly( L -azetidine-2-carboxylic acid- L -proline) and poly([ L -proline] 3 - L -azetidine-2-carboxylic acid)

Abstract

The synthesis, characterization, and conformational assessment of po L y( L -Aze- L -Pro) and poly[( L -Pro) 3 - L -Aze] are reported. The polymers were prepared by using the pentachlorophenol active ester as the polymerizable tetrapeptide derivatives. The copolymer, poly( L -Aze- L -Pro), assumes a Form II helix in polar solvents, and is converted into a form I-like helix at a critical solvent composition of ethanol to trifluoroethanol. The CD spectrum of this Form I-like conformation of poly( L -Aze- L -Pro) is similar to that of poly( trans -5-isopropyl- L -proline), indicating that the rigid four-membered ring at the alternating position can lock in the structure by a mechanism similar to that of a bulky substituent at the trans-5-position of proline. The helix conformation of this copolymer was unfolded in a 0.2 M CaCl 2 aqueous solution. In contrast to poly( L -Aze- L -Pro), the copolymer of poly[( L -Pro) 3 - L -Aze] contains both cis and trans peptide bond geometry when dissolved in a 90:10 ETOH-H 2 O mixture. The conversion of the mixed conformation of poly[( L -Pro) 3 - L -Aze)] into a polyproline Form II-like structure occurred in highly polar solvent environments such as water.