S C

IE N

T IF

IC H

IG H

LI G

H T

S S

T R

U C

T U

R E

O F

M A

T E

R IA

L S

1 3 9 I H I G H L I G H T S 2 0 2 2

PRINCIPAL PUBLICATION AND AUTHORS

Ion-Triggered Hydrogels Self-Assembled from Statistical Copolypeptides, B. Wu (a,b), S.B. Hanay (a), S.D. Kimmins (c), S.-A. Cryan (a), D. Herida-Merino (b), A. Heise (a), ACS Macro Lett. 11(3), 101-108 (2022); https:/doi.org/10.1021/acsmacrolett.1c00774 (a) RCSI University of Medicine and Health Sciences, Dublin (Ireland) (b) Dutch-Belgian Beamline (DUBBLE), ESRF (c) Pontificia Universidad Católica de Valparaíso, Valparaíso (Chile)

two worm-like polymer chain form factor models with a structure factor of a mass fractal object. This type of form factor model has been frequently applied to fibrous hydrogel networks, while the fractal structure factor has also been used for branched fibrous hydrogel structures self-assembled from short polypeptides. Figure 131b shows the evolution of the fitted fractal dimension (Df) over the gelation period. The whole gelation can be described as a process of gradual branching of the network structure.

Based on these results, the following gelation model is proposed for this type of random copolymerised polypeptides. As shown in Figure 131c, due to the presence of charged ions, the conformational structure (e.g., α-helix) of the polymer formed between the oppositely charge Lys and Tyr units gets disrupted. Furthermore, these charged salts can significantly increase the repulsion between adjacent positively

charged Lys units, which in turn increase the amount of unfolding of the polypeptides. When the unfolded polypeptides reach a certain concentration, they start to form an intermolecular β-sheet through H-bonding. As a result of the gradual increase in the number of the intermolecular connections between individual polypeptides, a worm-like bundle structure is produced over time. Finally, when a critical concentration of polypeptides and ions were used, after consuming most of the free polymer units, a hyper-branched network was formed by the end of the hydrogelation. Furthermore, the diffusion and mechanical properties of these hydrogels can be tuned through tailoring the polypeptide chain length and ion strength.

Fig. 131: a) Scattering profiles of the solution composed by poly(Lys80Tyr20) and PBS buffer over time. b) The fitted fractal dimensions. c) Illustration for the proposed gelation process.