Protonation induced pH increase at the tri-block copolymer micelle interface for transient membrane permeability at neutral pH
Achieving a controlled membrane permeability using pH-responsive block-copolymers is crucial for selective intercellular uptake activities. We have shown that pH at the tri-block-copolymer micelle interface compared to its bulk pH regulates the membrane permeability properties. pH dependent acid/base equilibrium of two different interface interacting pH-probes are exploited to measure interface pH for a pH-responsive tri-block copolymer (TBP) micelle under a wide bulk pH range (4.5-9.0). According to 1H-NMR studies, both pH-probes provide interface pH at a similar interface depth. We revealed that the protonation of amine moiety at the micelle interface, and subsequently positive charge formation causes the interface relatively less acidic than the bulk, and and bulk-to-interface pH-deviation (delpH) increased from ~0.9 to 1.9 with decreasing bulk pH from 8.0 to 4.5. From delpH vs. interface and bulk pH plots, the apparent and intrinsic protonation or positive charge formation pKa for the micelle were estimated to be ~7.3 and 6.0, respectively. When TBP micelle interacted with an anionic large unilamilar vesicle (LUV) of binary lipids (neutral and anionic) system at bulk pH 7.0, fluorescence leakage studies revealded that pH increase at the micelle interface from that of the LUV interface (pH ~5.5) makes the micelle interface partially protonated/cationic to exhibit a transient membrane permeability. Although the increasing interface protonation causes the interface relatively less acidic than the bulk at any bulk pH below 6.5, the pH increase at the micelle interface may not be sufficiently large enough to maintain the threshold amine-protonated condition for the transient leakage, and a continuous leakage was observed due to slow lipids bilayer disruption.