The secondary structures of poly (L-alanine) blocks in some diblock copolymers of poly(L-alanine)-b-poly(ethylene glycol) monomethyl ether in the solid state characterized by nuclear magnetic resonance and infrared spectrometry

摘要

The C-13 cross-polarization/magic-angle spinning (CP/MAS) spectra of the solid-state nuclear magnetic resonance (NMR) and the infrared spectra of three diblock copolymers, poly (L-alanine)-block-poly(ethylene glycol) monornethyl ether (PLA-b-MPEG), with various proportions of two blocks were studied in comparison with those of the homopolymer poly(L-alanine), PLA, and the blends of two blocks (PLA and MPEG). The secondary structures such as alpha-helix and beta-sheet of poly (L-alanine) (PLA) blocks in the block copolymers could be elucidated from the signals in the solid-state 13C CP/MAS NMR spectra and transmittance peaks in the Fourier-transformation infrared (FTIR) spectra. Dramatic differences in the secondary structures were observed for the diblock copolymers, homopolymer PLA and blend samples. It was found that with the increase of the fraction of PLA block in the block copolymers, the ratio of beta-sheet to a-helical conformation of PLA block went up although the alpha-helical conformation was much more than beta-sheet conformation in total. It contradicted the general prediction of the secondary structure of homopolypeptides or PLA/PEG blends, in which the beta-sheet conformation content decreased with the decrease of the polymerization degree of PLA. The investigation in FTIR spectrometry resulted in the same conclusion. (C) 2004 Elsevier B.V. All fights reserved.