Stability of the helical conformation of random L‐alanine–L‐lysine copolymers in aqueous solution

The potentiometric titration of random copolymers of L ‐lysine and L ‐alanine containing 0–35% alanine was carried out. The standard free‐energy change for the transition of coil to helix was calculated from the titration curve, and was treated by taking account of first‐neighbor interactions. For uncharged lysine Δ G ° = −140 cal/mole, and for alanine Δ G ° = −50 cal/mole in 0.06 M NaBr at 25°C, indicating that the alanine helix is thermodynamically less stable than the lysine helix. The randomness in co‐polymerization was confirmed by trypsin treatment.