Amphiphilic peptides are great focus owing to potential applications including drug delivery systems and antimicrobials. E5 peptide, one of amphiphilic peptides, forms amphiphilic α-helix at low pH, and exhibits membrane permeabilizing activity.^1,2) However, the activity of E5 peptide is low attributed to low solubility in aqueous media. In the previous paper, we showed that the cationic comb-type copolymer, poly(allylamine)-graft-dextrane (PAA-g-Dex) formed soluble inter-polyelectrolyte complex (sIPEC) with E5 peptide at both low and natural pH, and E5/PAA-g-Dex exhibited strong membrane disruptive activity.³⁾ In this study, we designed a novel comb-type cationic copolymer composed of a PAA main chain and thermoresponsive poly(N-isopropylacrylamide) graft chains (PAA-g-PNIPAAm). We hypothesized that the thermoresponsive hydrophilic/hydrophobic transition of the grafted polymer would regulate the membrane disruption activity of E5 peptide. The target copolymer was prepared by activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP). The ratio of AA units to Initiator to NIPAAm units was estimated to be 100:9:421 by ¹H NMR. Thus, the number-averaged molecular weight (M _n) of PNIPAAm and the PNIPAAm contents were 1.0 × 10⁴ and 94.3 wt %, respectively. Circular dichroism (CD) spectra measurement revealed that PAA-g-PNIPAAm formed sIPEC with E5 peptide, resulting in α-helix formation of E5 peptide. Furthermore, the complex formation of E5 and PAA-g-PNIPAAm was investigated by measuring the fluorescence spectra of triptophan residure in E5 peptide. From these investigations, we found that the interaction between E5 peptide and PAA-g-PNIPAAm at 35 °C (above LCST) was stronger than that at 25 °C (below LCST). These results suggest that the hydrophobicity of the environment around the cationic polymer chain influences complex formation. E5 peptide in the presence of PAA-g-PNIPAAm induced the leakage from liposomes, whereas E5 peptide alone did not. Importantly, E5/PAA-g-PNIPAAm induced more significant leakage at 35 °C than at 25 °C. Thus, membrane disruptive activity of E5/PAA-g-PNIPAAm sIPEC was regulated in response to temperature coupled with the phase transition of the thermo-responsive grafted chain.⁴⁾ References: 1) M. Murata, et al., Ohnishi, J. Biol. Chem. 1991, 266, 14353. 2) M. Murata, et al., Biophys. J. 1993, 64, 724. 3) N. Shimada et al., J. Controlled Release 2015, 218, 45. 4) T. Masuda et al., Biomacromolecules 2018, 19, 1333. Figure 1