Formation of carbamates and cross-linking of microbial poly(ε-l-lysine) studied by ¹³C and ¹⁵N solid-state NMR

Abstract

Formation of carbamates by amino groups of poly(ε-l-lysine) (ε-PL) and cross-linking of ε-PL were studied by using ¹³C and ¹⁵N solid-state NMR. It is a characteristic found in ε-PL cast from basic aqueous solution exposed to the air or gaseous CO₂. It is not observed in ε-PL cast from acidic aqueous solution and ε-PL cast from degassed aqueous solution under CO₂ free environment. The carboxyl carbon and amide nitrogen appear at 164 ppm in ¹³C spectrum and 92 ppm in ¹⁵N spectrum, respectively, which arise when some amino groups of ε-PL react with gaseous CO₂ to make carbamates. In addition to these peaks a peak at 171 ppm appears. We assigned it to amide C=O carbons which can not make intermolecular hydrogen bondings since there exist bulky carbamates groups close to these C=O groups. Self-assembly of ionic pairs of ammonium groups and carbamate anions leads to cross-linking of ε-PL.