Critical chain length for the formation of α- and β-polypeptide conformations in the solid state. Syntheses and conformations of oligopeptides with alternate L-alanyl and L-leucyl residues

Abstract

A series of oligopeptides with alternate L-alanyl and L-leucyl residues Nps-(L-Leu-L-Ala)_n-OEt and Nps-L-Ala(L-Leu-L-Ala)_m-OEt (n= 1–6 and 8, and m= 1–5) have been prepared by stepwisesynthesis with o-nitrophenylsulphenyl N-carboxy-α-amino-acid anhydrides for lower oligopeptides and fragment condensation with N-hydroxysuccinimide and dicyclohexylcarbodi-imide for higher oligopeptides; assignment of conformation to the products in the solid state has been made on the basis of i.r. and for i.r. spectroscopic results and X-ray diffraction measurements. α-Helical conformations have been found in deca- and higher peptides. The β-structure, on the other hand, appears to begin at the heptapeptide level.