Unexpected aqueous UCST behavior of a cationic comb polymer with pentaarginine side chains

Abstract

Thermoresponsive polymers, undergoing a reversible chemical or physical change using temperature as stimulus, attract increasing interest in particular as adaptable biomaterials. Except for zwitterionic polymers, fully charged polymers require the presence of specific ions to exhibit an upper critical solution temperature (UCST) in water. Herein, we report the discovery of an UCST in pure water for fully cationic comb polymers based on oligoarginine pendent grafts. These polymers were prepared using an original strategy based on solid-phase peptide synthesis of pentaarginine methacrylate-based macromonomer and its polymerization through reversible addition-fragmentation chain transfer. Despite their cationic nature, guanidinium groups from the arginine have the ability to self-associate at low temperature through hydrophobic interactions into stacked pair configuration defying the expected Coulomb interactions. These results pave the way to biomedical applications such as antimicrobial materials and drug delivery systems through the tuning of the polymer structure.

Ionic-Group Dependence of Polyelectrolyte Coacervate Phase Behavior

“…We infer that salt-dependent additional interaction exists for the guanidinium-based coacervates, and this results in the reversible dehydration at lower salt concentration. As shown in Figure S10, 1% w/v G(50) polyelectrolyte solution becomes turbid at 0.5 M of NaCl concentration, which is consistent with the previous observations where the guanidinium moieties can form the “like-charge” ion pair upon the addition of salt. − Lteif et al reported that polyelectrolytes containing the thiouronium group show phase separation upon the addition of salt in aqueous solvent, which is presumably attributed to the like-charge ion pairs . Zydziak et al also observed the phase separation of a cationic comb polymer with penta-arginine side chains, and they attributed this to the formation of ion pairs between guanidinium groups .…”

“…As shown in Figure S10, 1% w/v G(50) polyelectrolyte solution becomes turbid at 0.5 M of NaCl concentration, which is consistent with the previous observations where the guanidinium moieties can form the “like-charge” ion pair upon the addition of salt. − Lteif et al reported that polyelectrolytes containing the thiouronium group show phase separation upon the addition of salt in aqueous solvent, which is presumably attributed to the like-charge ion pairs . Zydziak et al also observed the phase separation of a cationic comb polymer with penta-arginine side chains, and they attributed this to the formation of ion pairs between guanidinium groups . This indicates that the “like-charge” ion pair of the guanidinium moieties decreases the geometric size of the guanidinium-functionalized polyelectrolytes in aqueous media with increasing salt concentration.…”

“…74 Zydziak et al also observed the phase separation of a cationic comb polymer with pentaarginine side chains, and they attributed this to the formation of ion pairs between guanidinium groups. 85 This indicates that the "like-charge" ion pair of the guanidinium moieties decreases the geometric size of the guanidinium-functionalized polyelectrolytes in aqueous media with increasing salt concentration. We suspect that the dehydration at lower salt concentration (<2.5 M of NaCl) observed from the guanidinium-based coacervates is attributed to the saltdependent behavior of guanidinium-based polyelectrolytes in aqueous solution.…”

Polyelectrolyte Complex Coacervation across a Broad Range of Charge Densities

“…This was observed most dramatically for high f values, revealing the hydrophobic nature of the guanidinium-functionalized AGE monomer upon the screening of charges with exogenous salt. 72 , 73 Polyelectrolyte complexes prepared from poly(Guan)/poly(Sulf) could not be dissociated with NaCl as poly(Guan) itself became visibly insoluble at [NaCl] ∼ 0.4 M ( Figure S13 ). We did however find that poly(Guan) remained soluble in monobasic sodium phosphate (NaH 2 PO 4 ) solutions and that the corresponding complexes could be dissociated at [NaH 2 PO 4 ] ∼ 4 M ( Figure S14 ).…”

Poly (ethylenimine)/(phenylthio) acetic acid ion pair self-assembly incorporating indocyanine green and its NIR–responsive release property

“…The ion pair selfassembly (IPSAM) can scatter the visible light (e.g. 500 nm) and render the suspension turbid, giving rise to a low transmittance [27,28]. If the suspension is heated, the phenyl group of PTA could become more hydrated, the ion pair would lose its amphiphilic property, thus the IPSAM would be disintegrated.…”