Quantification of Fundamental Weak Base and Ion Permeabilities in the Acidic pH Region Utilizing Conjugated Oregon Green in Liposome‐Based Assays

Abstract

Ammonia and ammonium play an essential role in the metabolism of almost all living organisms. Despite their importance for human health as well as plant growth, knowledge about passive NH3/NH4+ membrane permeabilities is scarce. Large unilamellar vesicles (LUVs), a popular model membrane system, have the potential to be a game changer. However, despite a variety of environmental sensitive dyes awaiting encapsulation into membrane vesicles, a pH sensor for the acidic pH region is currently missing. This gap is filled by introducing conjugated Oregon Green (OG) which can be encapsulated into lipid‐ or polymer‐based vesicles. This allows the quantification of proton or weak base permeabilities across the vesicular membrane and functionally characterizes pH sensitive membrane proteins at an acidic pH as low as pH 4.0. Furthermore, the expanded pH range enables simultaneous estimation of ion permeabilities without the use of membrane potential uncouplers due to an increased ion sensitivity. The utility of the sensor is demonstrated by quantifying passive NH3, NH4+, and Cl− permeabilities through vastly different lipid and polymer membranes. Moreover, its performance is benchmarked against carboxyfluorescein, an established pH‐sensor in the neutral pH‐range.