Microgels as carriers of antimicrobial peptides – effects of peptide PEGylation

Abstract Delivery systems are likely to be central for the translation of antimicrobial peptides (AMPs) towards therapeutics. Addressing AMP interactions with microgel carriers, we here investigate how poly(ethylene glycol) conjugation (‘PEGylation’) of AMPs affect their loading and release to/from microgels, combining structural studies using nuclear magnetic resonance (NMR) with ellipsometry, circular dichroism spectroscopy (CD), and light scattering. Such studies demonstrate that poly(ethyl acrylate-co-methacrylic acid) microgels bind considerable amounts of the positively charged AMP KYE28 (KYEITTIHNLFRKLTHRLFRRNFGYTLR) and its PEGylated variants KYE28-PEG48, PEG48-KYE28, and PEG24-KYE28-PEG24. Z-potential measurements indicate that KYE28 resides primarily inside the microgel core, and that localization of the PEGylated peptides is shifted towards the microgel corona. Furthermore, while all peptides are disordered in solution, CD measurements report on helix induction on microgel binding, particularly so for the PEGylated peptides. Addressing such conformational changes in more detail, NMR structural studies showed that peptide-microgel interactions are facilitated by a hydrophobic domain formed by the peptide after microgel binding, and with modulating electrostatic/salt bridge interaction between the positively charged peptide residues and negative microgel charges. As the microgels remain negatively charged also at high peptide load, membrane disruption and antimicrobial effects necessitates peptide release, demonstrated to be promoted by PEGylation and high ionic strength. Importantly, microgel loading, as well as peptide localization, conformation, and release, did not depend significantly on PEG conjugation site, but instead seems to be dictated by the PEG content of the peptide conjugates.