Syntheses and conformational studies of poly‐β,N‐alkyl L‐asparagines

Several β, N ‐alkyl L ‐asparagines were prepared from the phthalyl and benzyloxycarbonyl derivatives. High‐molecular‐weight poly‐β, N ‐benzyl L ‐asparagine and poly‐β, N ‐(1)‐phenethyl L ‐asparagine were prepared from the corresponding N ‐carboxyanhy‐drides. From the results obtained by a study of the infrared absorption spectra and the optical rotatory dispersion, poly‐β‐ N ‐benzyl L ‐asparagine was found to be a random coil structure in dichloroacetic acid and the optical rotatory dispersion curves gradually changed into the left‐handed α‐helix structure when chloroform was added to the solution. The coil‐to‐helix transition was observed in the vicinity of 20% dichloroacetic acid in a dichloroacetic acid‐chloroform mixture. Poly‐β, N ‐( d ), ( l ), and ( d + l , 1:1) ( d + l , 1:1): mixed polymer containing the same weighed poly‐β, N ‐( d ) and ( l )‐(1)‐phenethyl L ‐asparagines. ‐(1)‐phenethyl L ‐asparagines showed a nearly constant specific rotation in the dichloroacetic acidchloroform solvent system. Poly‐β, N ‐( dl )‐(1)‐phenethyl L ‐asparagine caused a gradual folding of the helix at dichloroacetic acid content of less than 20%.