Structural investigation of microbial poly(ε-L-lysine) derivatives with azo dyes by solid-state ¹³C and ¹⁵ N NMR

Summary

Solid-state high-resolution ¹³C and ¹⁵N NMR spectra of microbial poly(ε-lysine) (ε-PL) derivatives with azo dyes have been measured. Chemically modified derivatives of ε-PL, ε-PL/MO and ε-PL/DC were prepared through reactions with methyl orange (MO) and dabsyl chloride (DC), respectively. Side chain α-amino groups of ε-PL in ε-PL/MO are involved in ionic bonds with methyl orange to form poly-ion complexes, (ε-PL)-NH₃⁺SO₃^--(MO). On the other hand, ε-PL is allowed to react with dabsyl chloride in ε-PL/DC to form covalent sulfonamide bonds, (ε-PL)-NH-SO₂-(DC). These chemically modified ε-PL’s exhibit ¹⁵N NMR signals characteristic of the binding mode at the α-amino groups. The spectral analysis reveals that the ε-PL/DC sample contains a small amount of ion complexes with MO. It has been shown that ¹⁵N solid-state NMR is a useful tool for structural determination of ε-PL and its derivatives.