Iodide quenching of fluorophores in solution. Spectrum of ALEXA-448 (left) and MTS-TAMRA (right) for different iodide concentrations. Each measurement was made from a 2 μl drop of solution containing the fluorophore (5 μM) and the iodide concentration indicated (see Materials and methods). The same optical components were used as for the recordings with labeled oocytes. LED current was set to 200 and 400 mA for ALEXA-488 and MTS-TAMRA, respectively. Inset shows Stern–Volmer plots for normalized spectral peaks. Straight lines are linear regression fits, forced to origin at F₀/F = 1.0, where F₀ is the control ([I⁻] = 0) peak fluorescence count and F is the value at the same wavelength for a given [I⁻]. The Stern–Volmer quenching constants were 0.012 ± 0.001 mM⁻¹ (ALEXA-488) and 0.009 ± 0.002 mM⁻¹ (MTS-TAMRA), indicating comparable quenching effects.