High PEGylation efficiency of pentaethylenehexamine-end poly(ethylene glycol) (mPEG-N6) for active-ester surface

The chemical surface-modification of carboxylated polystyrene submicroparticles (sMPs) with α-methoxy-poly(ethylene glycol)-pentaethylenehexamine (mPEG-N6), which possesses multiple amino end-groups at one end, was explored with respect to modification efficiency. As a control, a PEG mono-aminated at one end (mPEG-N1) was employed in parallel experiments. Dynamic light scattering (DLS), electrophoretic mobility (μ(e)), Fourier transform infra-red (FT-IR) absorption, and 3-(p-carboxybenzoyl)quinoline-2-caboxaldehyde (CBQCA) assays were carried out. From reported pK(a) values of the amino groups, about 25% of the amino groups were protonated at pH9.5 for pentaethylenehexamine (N6), in other words, 1.5 amino groups were protonated and 4.5 amino groups were non-protonated on average for each mPEG-N6 molecule under the conditions. The multiple amino end-groups of mPEG-N6 played two different roles in modifying the sMPs: the protonated part offered electrostatic attraction between mPEG-N6 and the negatively charged sMPs; the non-protonated part covalently reacted with the active ester groups on the sMP surface after EDC-activation treatments. During the PEG-modification process, the former attractive force increased the local mPEG-N6 concentration surrounding the sMPs, which facilitated covalent conjugation to the sMPs. In contrast, protonated mPEG-N1 (80% under the same conditions) tends to cover the negatively charged sMPs, which retards the reaction of non-protonated mPEG-N1. These collaborative actions within each mPEG-N6 chain improved the PEG-modification efficiency, rending mPEG-N6 an ideal PEGylation agent relative to mPEG-N1.