Incorporation of d-alanine into poly(ethylene glycol) and l-poly(alanine-co-phenylalanine) block copolymers affects their nanoassemblies and enzymatic degradation

Poly(ethylene glycol)–poly(alanine-co-phenylalanine) (PEG–PAF) block copolymers with similar molecular weights were synthesized to investigate the effect of the partial incorporation of D-alanine into PEG-L-PAF. The ratio of L-alanine to D-alanine of the polymer varied over 100/0, 80/20, 60/40, 50/50, and 0/100. Circular dichroism and FTIR spectra indicated that the PEG–PAFs with the mixed composition of L-alanine and D-alanine exhibited dominantly random coil structures, whereas the PEG–PAF with the enantiomeric alanine (100/0 or 0/100) exhibited right-handed or left-handed α-helical structures as well as β-sheet structures, respectively. Dynamic light scattering of the polymer aqueous solution indicated that the size of nanoassemblies significantly decreased as a result of the partial incorporation of D-alanine into PEG-L-PAF. The most probable diameters were 20–40 nm and 80–105 nm for the PEG–PAFs with a mixed composition (L-alanine/D-alanine ratios of 80/20, 60/40, and 50/50) and an enantiomeric composition (L-alanine/D-alanine ratios of 0/100 and 100/0), respectively. As the temperature increased, the relative β-sheet content decreased for PEG–PAF with the enantiomeric alanine, while there was no significant change in random coil structures of PEG–PAFs with a mixed composition. 13C-NMR spectra suggest the dehydration and decrease in molecular motion of PEG during the sol-to-gel transition of PEG–PAF. Through the partial incorporation of D-alanine into PEG-L-PAF, thermogelling behaviour was also observed at much higher temperatures and concentrations due to the difference in the secondary structure and nanoassembly of the polymers. The enzymatic degradability of PEG–PAF also decreased as a result of the partial incorporation of D-alanine into PEG-L-PAF. The paper suggests that partial incorporation of D-alanine into an L-polypeptide block copolymer can be a useful method in designing a biodegradable material as well as controlling nanoassemblies of the polymer.