Influence of PEGylation on the ability of carboxymethyl-dextran to form complexes with α-lactalbumin

• Poly(ethylene glycol) was attached to reducing end of carboxymethyl-dextran (CMD).
• Protein complex formation occurred at pH 5.9 for PEGylated CMD vs. pH 5.5 for CMD.
• Phase separation of complexes not influenced by amount of PEGylated CMD.
• Spheroidal complexes possessed hydrodynamic radius of 11–40 nm.

Electrostatic interactions between α-lactalbumin (α-lac) and carboxymethyldextran (CMD) in acidic solutions lead to phase-separated complexes. By adding a non-ionic poly(ethylene glycol) (PEG) chain onto the reducing end of CMD, forming carboxymethyl-dextran-block-poly(ethylene glycol) (CMD-b-PEG), the PEG block was hypothesized to reduce interactions with α-lac and promote formation of a micelle-like complex structure. Formation of complexes between α-lac and CMD-b-PEG or α-lac and CMD was determined following acidification by light scattering and electrophoretic mobility. Phase separation, size, and structure of α-lac/CMD-b-PEG complexes were characterized by turbidimetry, dynamic light scattering, and electron microscopy, respectively. Complexes of α-lac/CMD-b-PEG formed at pH values near pH 6, while α-lac/CMD complexes formed at pH 5.5. Both CMD and CMD-b-PEG decreased the charge of α-lac below pH 5.5 and led to phase separation below pH 5. Shift in charge and the critical pH of phase separation were both sensitive to the α-lac to CMD ratio, while the relative amount of CMD-b-PEG did not significantly influence either. Hydrodynamic radii of α-lac/CMD-b-PEG complexes was between 11 and 20 nm, which increased with increasing α-lac to CMD-b-PEG ratio and with decreasing pH. Spheroidal structures of ∼10 nm were also observed in micrographs that were attributed to α-lac/CMD-b-PEG complexes.