BETA-STRUCTURES OF POLYPEPTIDES WITH L-RESIDUES AND D-RESIDUES .1. SYNTHESIS AND CONFORMATIONAL STUDIES

A series of five alternating poly(leucyl-lysyl) samples with varying amounts of L- and D-residues randomly distributed along the chain, but evenly shared out amongst leucyl and lysyl residues were synthesized by condensation of a mixture of the four diastereoisomeric dipeptide p-nitro-phenylesters. Their behavior in aqueous solution at various ionic strengths was studied by infrared spectroscopy which allowed measurement of the total amount of β-structures, and by circular dichroism which gives the excess of L-residues over D-residues in the same structures. Comparison with the properties of the all L-poly(Lys-Leu-Lys-Leu) shows that incorporation of a few D-residues in a L-chain seems to reduce the width of the β-sheets obtained in presence of salt. Higher proportions of D-isomers prevent the coil →β transition from occurring when the ionic strength is increased except for segments containing at least 6 to 7 adjacent residues of the same configuration. © 1979 Springer-Verlag.