Colorful methods to detect ion channels and pores: intravesicular chromogenic probes that respond to pH, pM and covalent capture

The activity of synthetic pores, ion channels, transporters and carriers is usually determined with fluorescent probes in vesicles or by conductance measurements in planar lipid bilayers. Elaborating on more colorful alternatives, we here introduce 5(6)-carboxynaphthofluorescein (CNF) as an intravesicular pH probe for the colorimetric detection of activity, selectivity and cooperativity of ion channels such as gramicidin A. We further report that intravesicular pyrocatechol violet (PV), together with extravesicular Cu2+, extravesicular 4-carboxyphenylboronic acid (CBA) or intravesicular 4-(benzyl-N-glutamate)boronic acid (BGBA) can detect the activity of synthetic pores or cell-penetrating peptide (CPP) sensors. Their response to analytes such as dodecylphosphate, hyaluronan or IP6 are reported as high-contrast color changes from yellow to blue, from yellow to red, or from red to green.