Synthesis and conformation of star-shaped poly(γ-benzyl-l-glutamate)s on a cyclotriphosphazene core

The preparation of star-shaped poly(gamma-benzyl-L-glutamate)s by the ring-opening polymerization of N-carboxy anhydride gamma-benzyl-L-glutamate (BLG-NCA) with hexakis(4-aminomethylphenoxy)- (4) and hexakis(4-aminophenoxy)cyclotriphosphazenes (6), and the conformation of resulting polymers has been studied. The six amino groups in 4 can initiate the polymerization of BLG-NCA to give star-shaped polyglutamates (7) with narrow molecular weight distributions ((M) over bar (w)/(M) over bar (n) = 1.10-1.33). For the polymerization of BLG-NCA with 6, however, a high ratio of [BLG-MCA]/[6] was required to obtain star-shaped polyglutamates (8), The conformation of 7 changed from a beta-sheet form to a right-handed alpha-helix form, depending on the degree of polymerization per chain (DP) over bar (n)/6). The helix content of hexa-armed poly (gamma-benzyl-L-glutamate-CO-L-glutamic acid)s (9), prepared by partial hydrolysis of 7, increased significantly compared with that of the corresponding linear analogue (10). As increasing of helix content of 9, the fluorescence spectra of 8-amilino-1-naphthalenesulfonic acid (ANS), a fluorescence probe, shifted to a short wavelength accompanied by the enhancement of intensity, suggesting that star-shaped polymers are liable to form hydrophobic domains. From these results and the structural feature of the cyclotriphosphazene core, the formation of a 3alpha-helix bundle structure of polyglutamates on both sides of the phosphazene ring has been suggested.