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. Author manuscript; available in PMC: 2014 Jun 10.
Published in final edited form as: Methods Mol Biol. 2014;1071:29–47. doi: 10.1007/978-1-62703-622-1_3

Fig. 2.

Fig. 2

A sensor’s sensitivity to changes in Ca2+ or Zn2+ concentration is related to its dissociation constant (Kd’). (A) When the ion concentration ([ion]) is the same as the sensor’s Kd’, 50% of a population of sensors (typically 1-20 μM in cells) is bound to the ion. The fraction of sensor bound changes the most when the [ion] varies within 10-fold of the sensor’s Kd’. (B) The FRET ratio is a linear function of the fraction bound, and so the midpoint of Rmin and Rmax corresponds to an [ion] equal to the sensor’s Kd’. The SNR and the Kd’ limit the range of the [ion] that can be quantified by the sensor.