Bulk Biopolyelectrolyte Complexes from Homopolypeptides: Solid "Salt Bridges"

Salt bridges, pairings between oppositely charged amino acids, are dispersed throughout proteins to assist folding and interactions. Biopolyelectrolyte complexes (BioPECs) were made between the homopolypeptides poly-l-arginine (PLR) and poly-l-lysine (PLK) with sodium triphosphate (STPP), as well as from polypeptide-only combinations. Viscoelastic measurements on these high salt bridge density materials showed many were solid, even glassy, in nature. Although the polypeptide-phosphate complexes had similar moduli at room temperature, the PLR-STPP complex displayed an unusual melting event above 70 °C not seen in PLK-STPP. This event was supported with differential scanning calorimetry. Infrared spectroscopy showed the PLK-STPP system contained β-sheets, while PLR-STPP did not. Stoichiometric, macroscopic BioPECs of PLR and PLK with poly-l-aspartic acid (PLD) and poly-l-glutamic acid (PLE) were made. PLR-PLD was found to undergo a melting event similar to that in PLR-STPP. ATR-FTIR studies showed that BioPECs made with PLD do not contain β-sheets, while those composed of PLE do. This work illustrates an expanded palette of unique properties from these biomaterials, such as strong viscoelastic differences between PECs containing PLE and PLD, even though they differ by only one carbon on the side chain.