Biophysical Journal 64: 1122-1132 (1993)
© 1993 the Biophysical Society

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Mechanism of response of potential-sensitive dyes studied by time-resolved fluorescence

Chemical Physics Group, Tata Institute of Fundamental Research, Homi Bhabha Road, Bombay 400 005, India

ABSTRACT

The mechanism of response of two potential-sensitive dyes, diOC₂(5) (3,3'-diethyloxadicarbocyanine iodide) and oxonol V (bis-[3-phenyl-5-oxoisoxazol-4-yl]pentamethine oxonol), were studied by using steady-state and time-resolved fluorescence techniques. The lipid concentration dependence of the (membrane potential)-induced change in total fluorescence intensity was quite different for these two dyes. Time-resolved fluorescence measurements showed that the fluorescence decay of these dyes in membranes could be resolved into at least three exponentials. -induced changes in the levels of these three populations were also measured under a variety of conditions. In the case of diOC₂(5) an inside negative increased the levels of the bound forms. This shows that diOC₂(5) responds to mainly by an "on-off" mechanism whereby perturbs the membrane-water partition coefficient of the dye. The -induced changes approached zero when the dye was totally membrane bound. In contrast, the -induced response of oxonol V increased with increased membrane binding. An inside negative decreased the level of the bound form with a longer lifetime. This shows that the mechanism of response of oxonol V is a -induced shift in the equilibrium between bound forms of the dye.