Abstract
BackgroundThe aqueous solution behavior of thermosensitive PEG-PA block copolymers as well as secondary structure of PA is expected to significantly change through modification of the hydrophobic PA by long chain alkyl (C18) groups with different configurations.MethodOleoyl and stearoyl (C18) groups were conjugated to poly(ethylene glycol)-poly(L-alanine) (PEG-PA; EG45A16) diblock copolymers to compare their conjugation effect on nano-assemblies and corresponding aqueous solution behavior of the polymers.ResultsDue to the nature of a hydrophilic PEG block and a hydrophobic PA or C18-modified PA, PEG-PA, oleoyl group-conjugated PEG-PA (PEG-PAO), and stearoyl group-conjugated PEG-PA (PEG-PAS) block copolymers form micelles in water. Compared with PEG-PA, the micelle size of PEG-PAO and PEG-PAS increased. Circular dichroism and FTIR spectra of aqueous polymer solutions showed that β sheet content increased, whereas α helix content decreased by C18 modification of PEG-PA. PEG-PAS showed better performance in ice crystallization inhibition than PEG-PAO. The sol-to-gel transition temperatures of aqueous PEG-PAO solutions were 25–37 °C higher than those of aqueous PEG-PA solutions, whereas aqueous PEG-PAS solutions remained as gels in the temperature range of 0–80 °C. 1H-NMR spectra indicated that the oleoyl groups increased core mobility, whereas stearoyl groups decreased the core mobility of the micelles in water. The difference in micromobility between PAO and PAS interfered or promoted gelation of the aqueous polymer solutions, respectively.ConclusionsThis study suggests that a hydrophobic C18-modification of polypeptide induces α helix-to-β sheet transition of the polypeptide; however, aqueous solution behaviors including ice recrystallization inhibition and gelation are significantly affected by the nature of the hydrophobic molecule.Graphical abstract
Highlights
The aqueous solution behavior of thermosensitive poly(ethylene glycol)-poly(L-alanine) (PEG-PA) block copolymers as well as secondary structure of PA is expected to significantly change through modification of the hydrophobic PA by long chain alkyl (C18) groups with different configurations
Apparent sizes of the micelles were provided by dynamic light scattering of aqueous PEG-PA, PEG-PAO, and PEG-PAS solutions (0.01 wt.%), where the peak average micelle sizes were 16, 28, and 28 nm, respectively (Fig. 2a)
About two times excess amount of oleoyl chloride and stearoyl chloride reacted with the amino-end group of PEG-PA to prepare PEG-PAO and PEG-PAS, respectively, which led to the polymers without contamination of unreacted PEG-PA. 1H-NMR and GPC confirm the correct structure of PEG-PA, PEG-PAO and PEG-PAS
Summary
The aqueous solution behavior of thermosensitive PEG-PA block copolymers as well as secondary structure of PA is expected to significantly change through modification of the hydrophobic PA by long chain alkyl (C18) groups with different configurations. Oleoyl and stearoyl groups are among the most common alky chains in the cellular membrane. Even though both oleoyl and stearoyl groups are C18, the structural configurations and biological functions are quite different due to the presence of a cis double bond at the middle of the oleoyl group. The oleoyl group has a 9,10-cis kink along the C18 alkyl chain. It is abundant in human adipose tissue as the most common unsaturated fatty acid ester [1, 2]. The fully saturated fatty acid ester of the stearoyl group provides rigidity as well as stability of the cell membrane [5]
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