Phase transfer mechanisms of fluorophore-labeled cell-penetrating peptide ε-poly-l-α-lysine at liquid|liquid interfaces

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The transfer reaction of a novel cell-penetrating peptide, ε-poly-l-α-lysine (εPL) as a bacterial polycationic isopeptide, was investigated at the water|1,2-dichloroethane (DCE) interface. Spectroelectrochemical analysis elucidated the interfacial reaction mechanism of covalently carboxyfluorescein (FAM)-labeled εPL (FAM-εPL) in detail. FAM-εPL showed the concentration-dependent interfacial mechanisms in the positive potential region, i.e., the adsorption from the aqueous side of the interface was occurred at the low concentration of FAM-εPL, while the ion transfer across the interface accompanied by the adsorption process at the organic side proceeded further as the concentration increased. The facilitated ion transfer of FAM-εPL was observed at the biomimetic interface with a phospholipid layer. The transfer process of FAM-εPL was accelerated by the specific interaction at the aqueous side of the biomimetic interface, that is, the polar headgroup region of the phospholipid layer. The interfacial and membrane permeation mechanisms of εPL were elucidated through the spectroelectrochemical measurements by the fluorescent FAM modification and the phase transfer of a membrane impermeable FAM was also achieved by forming the conjugate with εPL.