Remarkable Microenvironmental Difference between Monolayer and Bilayer Membrane Interfaces. Dissociation Behavior of a Lysine Residue Placed on the Membrane Surface

The dissociation constant (pK) of an ε-amino group of the l-lysine residue in a cationic peptide lipid was evaluated for both the bilayer vesicle formed in aqueous media and monolayer membrane assembled at the air-water interface. The pK value of 9.4 in the aqueous vesicle evaluated using 1H-NMR spectroscopy was relatively close to that observed for the ε-amino group of lysine in water (10.5). In contrast, π-A isotherm measurements for the monolayer membrane on pure water revealed a remarkably shifted pK (5.1).