Solution properties of synthetic polypeptides. Synthesis and conformational properties of poly(N∈-acetoacetyl-L-lysine), poly(Nδ-acetoacetyl-L-ornithine), and poly(Nγ-acetoacetyl-L-diaminobutyric acid) in aqueous solution

Poly(N-acetoacetyl-L-lysine) (PALL), poly(Nδ-acetoacetyl-L-ornithine) (PALO), and Poly(Nγ-acetoacetyl-L-diaminobutyric acid) (PADB) have been synthesized by reaction of poly(L-lysine), poly(L-ornithine), and poly(L-diaminobutyric acid) with diketene in aqueous solution at pH ∼9. All polymers are water soluble at pH >3.5, the order of solubility being PALL < PALO < PADB. With time PALL tends to become insoluble in water probably because of the formation of β structures. The conformational properties of three polymers have been investigated by uv absorption and circular dichroism techniques. In acid or neutral aqueous solution the polymers assume the right-handed α-helical conformation, while the random coil form is predominant in alkaline solution. The conformation of the peptide backbone does affect appreciably the optical activity of the side-chain chromophores in all the three polymers. In the case of PADB the side-chain groups have also substantial perturbing effects on the CD pattern of the backbone.