Poly(ethylene oxide-co-beta-benzyl L-aspartate) block copolymers: Influence of the poly(ethylene oxide) block on the conformation of the poly(beta-benzyl L-aspartate) segment in organic solvents

The conformation of the poly(beta-benzyl L-aspartate) segment in the poly(ethylene oxide-co-beta-benzyl L-aspartate) (PEO/PBLA) block copolymers (M(w) of PEO = 5000 and 20 units of beta-benzyl L-aspartate) was investigated by H-1 NMR, by specific optical rotation measurements, and by 2D H-1,H-1 NOESY NMR in chloroform, dimethyl sulfoxide (DMSO), and mixtures of chloroform/DMSO. The comparison between the H-1 NMR spectra of the block copolymer in CDCl3 and the one in DMSO-d(6) showed that the PBLA blocks adopt a different conformation depending upon the solvents. The specific rotation of the block copolymer at 546 nm demonstrated that the PBLA segments adopt a left-handed alpha-helix conformation in chloroform. PBLA homopolymer with the same molecular weight as the PBLA segment in the block copolymer showed no evidence of alpha-helix formation in the same conditions, indicating that the PEO segment in the block copolymer is essential to allow PBLA to take the alpha-helix structure. On the other hand, the specific rotation of the PEO/PBLA block copolymer in DMSO at the same wavelength showed that the PBLA segments have a random-coil conformation in this solvent. Moreover. the measurement of the specific rotation of the block copolymer in mixtures of x% CHCl3/(100 - x)% DMSO (0 less than or equal to x less than or equal to 100) demonstrated that the left-handed alpha-helix conformation adopted by the PBLA segments in chloroform is stable. Conversely, the PBLA homopolymer cannot be solubilized in mixtures of CHCl3/DMSO, although this homopolymer is soluble in pure CHCl3 and in pure DMSO. The 2D H-1,H-1 NOESY NMR spectrum in CDCl3 gave evidence of interactions between the methylene protons (CO2CH2C6H5) of the PBLA segments and the methylene protons of the PEO blocks on one hand and between the benzyl protons of the PBLA segments and the methylene protons of the PEO blocks on the other. Such interactions are not observed in DMSO-d(6). Thus, the stability of the left-handed alpha-helix conformation of the PBLA blocks (having low molecular weights) in chloroform can be explained by the interactions existing between the PBLA and the PEO blocks, Moreover, these interactions also allow the solubilization of the block copolymer in mixtures of CHCl3/DMSO.