Poly(<scp>L</scp>‐lysyl‐<scp>L</scp>‐alanyl‐α‐<scp>L</scp>‐glutamic acid). II. Conformational studies

Abstract

Abstract The solution characterization of poly(Lys‐Ala‐Glu) is described. This polytripeptide is zwitterionic at neutral pH and is shown to take on a conformation which is dictated by the state of ionization, molecular weight, temperature, and solvent. The polypeptide is almost entirely α‐helical at low pH and temperature for polymers of greater than 25,000 molecular weight. Melting profiles for these conditions show t m ∼ 20°C. Analysis of circular dichroism curves shows the α‐helical content to vary in a linear manner with molecular weight in the range 3000–30,000. At neutral pH the charged polypeptide is essentially random, but substantial α‐helix could be induced by addition of methanol or trifluoroethanol. At temperatures where the sequential polypeptide is a random coil, addition of trifluoroethanol produces a polymer which is mostly α‐helical but also contains an appreciable ammount of β‐structure. The infrared spectrum of a low‐molecular‐weight fraction assumed to be cyclo (Lys‐Ala‐Glu) 2 was tentatively assigned a β‐pleated sheet structure. A comparison of this polytripeptide in various ionization states with other polytripeptides containing L ‐alanine and L ‐glutamate or L ‐lysine shows the α‐helix directing properties for the (uncharged) residues to lie in the order Ala &gt; Glu &gt; Lys.