We report on star polypeptide polymers that exhibit upper critical solution temperature (UCST) behavior in aqueous solution. First, star-polyornithine (star-PO) was synthesized via ring opening polymerization of N-δ-Cbz-L-ornithine N-carboxyanhydride (L-ornithine(Z)-NCA) using a generation-four polyamidoamine dendrimer (G4-PAMAM) as a precursor. To endow UCST behavior to star-PO, the primary amines in the side chains were converted into ureido groups, making star-poly(ornithine-co-citrulline) (star-POC). Figure 1b shows AFM height image of the star-POC. The star-POC polymers exhibited UCST transitions in aqueous solutions which were sharper (occurring within a narrow temperature range of 5 °C) than those typically observed with linear UCST polymers. Importantly, individual star polymers and their aggregates in 150 mM NaCl solutions at pH 7.0 demonstrated transitions at close-to-ambient temperatures as quantified by dynamic light scattering. Moreover, the UCST of the star polypeptides could be tuned via both the length of POC branches and the degree of ureido modification. For a fixed degree of ureido modification, the UCST of star-POC polymers increased with the length of the star-polymer branches. For example, in the case of 96% ureido-modified star-POC, UCST changed from 17 °C to 24 °C for star polymers with 55-unit to 100-unit branches (star-POC55 and star-POC100 respectively, shown in Figure 1a). At the same time, for star-POC polymers with fixed branch length, higher degrees of ureido modification resulted in an increase of UCST. Specifically, star-POC55 polymers with 93% and 96% modified units exhibited UCSTs of 14°C and 17°C, respectively. These results, taken together with potential biocompatibility and biodegradability, make star-POC polymers promising candidates for use as individually dispersed responsive containers or as building blocks for responsive assemblies.

UCST star polypeptides: Synthesis and solution behavior

256th National Meeting (2018)

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