Ratios of the acceptor (≈617 nm) and donor (≈534 nm) emission intensities as a function of pH for brain POPC/PI and brain POPC/PI-4P (85:15), respectively (total lipid concentration = 0.3 mM, 20°C). Labeled NBD-PC (0.6 mol %) and BODIPY-PI or BODIPY-PI-4P (0.7 mol %), respectively, were fused into the outer leaflet of the preformed unilamellar vesicles. () PI/PC; () PI-4P/PC.