Synthesis and conformation of poly( L ‐lysyl‐ L ‐alanyl), a sequence‐ordered water‐soluble copolymer

The sequence‐ordered copolymer poly‐(Lys‐Ala‐Ala) was synthesized by polycondensation of the N ‐hydroxysuccinimide ester of ε,Z‐Lys‐Ala‐Ala and deprotection of the polymerization product. A fraction of molecular weight 13,000 obtained by ion‐exchange chromatography was investigated. The polymer is freely soluble in water at all p H values, and is completely digested by trypsin and elastase. From CD and ORD data it was concluded that in water at 1°C the ionized form (at p H 6.5) of the polymer is helical. On heating, helix‐coil transition curves were obtained with a midpoint, T m , depending on salt concentration. In salt‐free water T m = 12.3°C and in 0.2 M NaCl T m = 28.5°C. Adding MeOH, causes an increase in the helical content of the polymer (half helicity at 20% MeOH, without salt, at 29°C). Guanidine·HCl was shown to decrease the helicity. At 1°C half helicity. The nonionized polymer helix is more stable ( T m ∼90°C). At the high pH, at 60°C, when concentration of the polymer is higher than 1.9 × 10 ‐2 M , a precipitate is formed which redissolves on cooling with the original helicity. This does not occur in the presence of 50% MeOH. By comparison with polylysine it was concluded that replacing two‐thirds of the lysine residues in polylysine by alanine leads to a polymer forming a more stable α‐helix, when fully ionized. This is essentially due to the diminished coulombic repulsion. Uncharged lysine residues are comparable to alanine residues in their helix‐forming tendency since the sequential polymer as well as one‐third ionized polylysine are helical to approximately the same extent at room temperature.