ABSTRACT
A description of the molecular mechanisms by which probes respond to a change in membrane potential will provide a convenient framework for the discussion of response characteristics such as sensitivity, calibratibility, and speed. A hydrophobic ionic dye will have some equilibrium distribution between the external aqueous medium, the cell plasma membrane, the cytosol, and the membranes and aqueous compartments of intracellular organelles. The membrane potential plays a direct role in governing the distribution across and within the plasma membrane — the more negative the potential the greater the accumulation of positively charged dye. A dipolar probe structure may adopt a variety of orientations with respect to the two-dimensional surface of the membrane to which it is bound. Electrochromism is the direct coupling of the membrane electric field with the electronic redistribution in a chromophore that accompanies excitation or emission. The cyanines are slow dyes which adopt the redistribution mechanism in their response to membrane potential changes.
