Insulin complexes with PEGylated basic oligopeptides

Biodegradable oligolysine and oligoarginine-type homopeptides functionalized with PEG of two different molecular weights interact with insulin, at physiological pH, affording complexes studied by dynamic light scattering, ζ-potential, circular dichroism, FTIR spectroscopy, and isothermal titration calorimetry (ITC). High levels of insulin complexation efficiencies (>99.5%) were determined for all derivatives. FTIR spectra suggest that the positively charged homo-oligopeptide derivatives interact with B chain C-terminus of insulin leading to the formation of nanoparticles than can be traced even at low oligopeptide/insulin molar ratios. The ITC profiles are complex, displaying significant endothermic and exothermic contributions. Oligoarginine-type derivatives exhibit the strongest interactions, while PEGylation of either oligopeptide with the high molecular weight chains significantly affects the ITC profiles and leads to larger enthalpy changes. This may be attributed to PEG-induced aggregation of insulin due to the depletion attraction effect leading to the formation of stable nanocomplexes. Stabilization of complexed insulin against enzymatic degradation by trypsin and α-chymotrypsin is observed especially for the high molecular weight PEGylated arginine-based derivative. Insulin release rates in simulated intestinal fluid are controlled by the length of PEG chains and the presence of arginine end-groups. Released insulin retains its secondary structure as established by circular dichroism spectroscopy.