Thermally Assisted Generation of Protein–Poly(ethylene sodium phosphate) Conjugates with High Mineral Affinity

Protein therapeutics has recently attracted interest in various medical treatments. However, the structure and function preservation in proteins under physiological conditions is still an important issue and reliable immobilization techniques are required. In this study, the thermally assisted complexation of proteins with amphiphilic polyphosphoesters is proposed as a new methodology for their durability improvement. Amphiphilic cholesterol-terminated poly­(ethylene sodium phosphate) (CH-PEP·Na) was synthesized via the organocatalytic ring-opening polymerization of 2-methoxy-2-oxo-1,3,2-dioxaphospholane initiated by cholesterol as the hydrophobic molecule and followed by demethylation and neutralization. For the protein nanocarrier preparation, a complex of the amphiphilic CH-PEP·Na with bovine serum albumin (BSA) was formed through the hydrophobic interactions between the lipophilic moieties of the protein and the cholesteryl groups of the CH-PEP·Na chains, which were induced by thermal treatment at 90 °C. The resulting complex size ranged between 27 and 51 nm, as confirmed by dynamic light scattering. The complexes dispersed in an aqueous medium exhibited a high stability in size for up to 1 month of storage. CH-PEP·Na efficiently inhibited the thermal aggregation and sedimentation of BSA, unlike poly­(ethylene sodium phosphate) (PEP·Na) and cholesterol-terminated poly­(ethylene glycol) (CH-PEG). In addition, CH-PEP·Na was able to protect the complexed BSA against proteolytic digestion and the BSA–CH-PEP·Na complexes well adsorbed onto hydroxyapatite even in the presence of BSA (5.5 g/dL). Hence, thermally induced protein–CH-PEP·Na complexes can be a potential tool for the development of bone and dental applications.