<p>The FRET signal of TN L15 (Troponin C bound to the FRET pair Cerulean and Citrine) was measured by FLIM in buffered solutions of different free Calcium concentrations in the range 0 µM to 39 µM (Ca Calibration Buffer Kit, Invitrogen, Germany). The fluorescence decays were biexponentially approximated. In all cases, the fluorescence lifetime of the FRET-quenched Cerulean amounts to 808 ps and that of the unquenched Cerulean to 2491 ps. These values well agree according to the Strickler-Berg dependence on refractive index to the values measured in brain slices and in the brain stem of live mice. The ratio a1/(a1+a2) of FRET-quenched to unquenched Cerulean represents the FRET signal and is depicted on the ordinate. The inset shows the values for K<sub>d</sub> and the Hill slope.</p
