Reversible heat coagulation of some water‐soluble amino acid copolymers

Poly‐ε‐aminocaproyl‐DL‐alanine, [HN(CH2)5COHNCH(CH3)CO]n, was prepared by the polymerization of N‐carbothiophenyl‐ε‐aminocaproyl‐DL‐alanine. The copolymer synthesized precipitates from an aqueous saturated solution on heating (40 to 60°). The precipitate formed goes back into solution on cooling. Repeated precipitation and dissolution by change of temperature show that the heat coagulation is reversible. The coagulation phenomenon discussed is pH‐dependent, but practically independent of ionic strength or the presence of denaturing agents such as urea. The ability of the following polymers to coagulate from an aqueous solution on heating was investigated: H[HN(CH2)mCOHNCH(CH3)CO]nOH (I), m = 1,2,3,4,5; and H[HN(CH2)mCO]nOH (II), m = 1.2,3,4,5. None of the polymers investigated, except poly‐ε‐aminocaproyl‐DL‐alanine (I, m = 5) showed reversible heat coagulation. The polymers of group I dissolve in water more readily than those of group II. The aqueous solutions of I, m = 1; I, m = 3; and II, m = 1, give a distinct positive biuret reaction. In a series of copolymers of α‐amino acids with ε‐aminocaproic acid it was observed that reversible heat coagulation in water occurs in copolymers containing L‐proline or α‐aminoisobutyric acid, but not in copolymers containing glycine, L‐leucine, or L‐phenylalanine. Reversible heat coagulation was also observed in the case of poly‐L‐proline.