Sodium Poly(α,L-glutamate)/Ethylene Oxide-Propylene Oxide (Block and Random) Copolymer Macromolecular Complexes. Method of Preparation and Structure of Complex

Abstract

Macromolecular complexes of sodium poly(α,L-glutamate) (PGNA) (molecular weight (MW) 1, 49 and 71 k) and ethylene oxide-propylene oxide tri-block copolymer (MW 8 400) have been prepared by a novel method involving dehydration of reverse micelles (DRM method). This series of complexes was compared with the complexes of PGNA (MW 1, 49 and 71 k)/ethylene oxide-propylene oxide random and tri-block copolymers prepared by the common method involving evaporation of aqueous mixtures (EAM method). By the DRM process fifteen times more copolymer was incorporated in the pure macromolecular complex than by the EAM process. CD spectra of the EAM series of complexes showed formation of α-helical PGNA conformation as evidenced by the observation of +ve peak at 194 nm and two –ve peaks at 201 and 221 nm. Formation of the α-helical conformation is further supported by FT-IR spectroscopy. On the other hand, CD spectra of the DRM macromolecular complexes showed neither α-helical nor random conformation, and the spectra may be attributed to a distorted helical PGNA conformation. DSC studies revealed that the copolymers in EAM macromolecular complexes were intimately blended with PGNA, while in the DRM series only 65% of the copolymer were blended at the molecular level, with the rest present as a pure copolymer domain. 23Na NMR spectra of both series of complexes showed presence of free sodium ions indicative of dissociated Na+—O dipolar interactions in aqueous solution. Hydrophobic interaction between PGNA and copolymer remained intact even in very dilute solutions of both series of complexes as observed by strong 2D-NOESY 1H NMR correlation between β and γ CH2 groups of PGNA and CH2 groups of copolymers. However, in the DRM series, only the CH2 groups of PEO blocks of the PEO-PPO-PEO copolymer showed the 2D-NOESY 1H NMR correlation indicating that only the PEO blocks are involved in the complex formation. The PPO block that had no interaction with PGNA may have formed pure PPO domains. NMR data combined with the DSC, CD and FT-IR data suggest that the structure of both series of macromolecular complex is a composite composed of copolymer molecules intimately interacting with PGNA chains. Interactions between β and γ groups of PGNA side groups with CH2 groups of the copolymer are involved in forming the complex. 2D-NOESY 1H NMR correlation further indicate that both the DRM and EAM series of macromolecular complexes are stable in water for at least seven weeks.