Evaluation of critical packing parameter in the series of polytyrosine-PEG amphiphilic copolymers

Here, we report on the evaluation of critical packing parameters, P-C, of l-tyrosine and ethylene glycol (PEG) amphiphilic block copolymers. The copolymers were synthesized via l-tyrosine-N-carboxyanhydride ring-opening polymerization using different amino-terminated PEGs as macroinitiators. The size and shape of their associates formed in water were investigated by transmission electron microscopy and dynamic and static light scatterings. The copolymers containing 20-30 wt. % of Tyr formed rod-like micelles. The copolymers with 30-50 wt. % of Tyr formed spherical vesicles, while those with 50-80 wt. % Tyr formed irregularly shaped polymeric nanoparticles. P-C of the copolymers was estimated as the ratio of the van der Waals volume of the hydrophobic block (V) to its length (L) and the cross-sectional area of the polar block (S-H). The values of S-H for each copolymer were calculated according to the equation obtained from the correlation analysis of the published data. The hydrophobic block length L was calculated in three ways, i.e., assuming that polytyrosine adopts (1) stretched, (2) amyloid hairpin, or (3) Gaussian coil conformation. In the latter case, the best match of the calculated P-C values with the morphology of the copolymer assemblies was observed.