Structural analysis of microbial poly ( e - L - lysine ) / poly ( acrylic acid ) complex by FT - IR , DSC , and solid - state 13 C and 15 N NMR

INTRODUCTION In recent years, there has been considerable interest in biopolymers due to the concern over the environmental impacts arising from the disposal of petroleum-based plastics. Poly(e-L-lysine) (e-PL) is one of the few poly(amino acid)s that are known to occur in nature.1,2 Microbial e-PL is a product of a variant of Streptomyces albulus. In e-PL molecules, the e-amino group of an L-lysine unit is linked to the a-carboxyl group of the other unit to form a peptide bond, leaving the a-amino group as a side chain. e-PL is water soluble, biodegradable, edible and non-toxic to humans and the environment, in addition to having broad-spectrum antibacterial activity. Thus, e-PL and its derivatives have been of great interest for a broad range of industrial and biomedical applications such as food preservatives, emulsifying agents, dietary agents, biodegradable fibers, water absorbent hydrogels, drug delivery carriers and anticancer agent enhancers.1 We have studied the molecular structure and the conformation of e-PL in aqueous solutions.3–5 The pH-dependent infrared (IR), circular dichroism and 1H solution nuclear magnetic resonance (NMR) spectra have indicated that e-PL assumes a b-sheet conformation in basic aqueous solutions and an electrostatically expanded conformation in acidic aqueous solutions.3–5 We have also characterized the structure and the conformation of e-PL and its derivatives in the solid state by 13C and 15N solid-state NMR.6,7 These results indicated that e-PL is a semi-crystalline polymer with a crystallinity of B63%, as estimated by the measurements of 13C spin-lattice relaxation time in the laboratory frame. A conformational model of e-PL was also proposed in which the main chain makes a parallel b-sheet similar to the g-form of nylon 6.6 As tools for side chain functionalization, chemically modified derivatives of e-PL, e-PL/methyl orange (MO) and e-PL/dabsyl chloride (DC), were prepared through reactions of ePL with MO and DC. In e-PL/MO, the side chain a-amino groups of e-PL are involved in ionic bonding with MO to form poly-ion complexes, (e-PL)-NH3 . . . SO3-(MO). On the other hand, e-PL reacts with DC in e-PL/DC to form covalent sulfonamide bonds, (ePL)-NH-SO2-(DC). These chemically modified e-PLs exhibit 15N NMR signals characteristic of the binding mode at the a-amino groups.7 Polymer blends are widely used as a means of tailoring and modifying the characteristics of polymeric materials for various industrial and biomedical applications.8 Recently, a few studies on e-PL-based polymer blends have been reported.9,10 In this work, a structural analysis of e-PL polymer complexes with poly(acrylic acid) is carried out using 13C and 15N solid-state NMR, Fourier transform infrared spectroscopy and differential scanning calorimetry (DSC) measurements.