Foerster radius

Changes in FRET may be caused by a change in the distance of the two fluorophores or by alteration of the two dipole moments. Distance changes are particularly influential on the FRET efficiency around the Foerster radius Rq, and structural information on the sensor protein might permit intelligent guesses... 

For an efficiency of energy transfer of 0.5, one obtains the Foerster radius, i.e., the distance for that energy transfer between donor and acceptor part in the interaction. This distance is given by... 

The critical perturbation predicted by the reaction-diffusion equations with the shortest recurrence time occurs when [Br ] fluctuates from its steady state value to [Br ]g within a sphere of radius 0.5 fim in an excitable system. In the experiments of Foerster et al. [8] and of Showalter et al. [5], the mechanism by which chemical waves are induced consists of depletion of bromide ions as well. In both the experiments of Foerster et al. and of Showalter et al., the bromide concentration is presumably reduced to at least [Br ] due to the silver ions released in solution. The region in which the bromide is depleted to its critical concentration is estimated to be about 5 /xm surrounding a capillary 16 fim in radius in the experiments of Foerster et al. and a minimum of 2 /xm surrounding an electrode 150 /xm in radius in the experiments of Showalter et al. Both of these volumes necessary to perturb for wave initiation are larger than the volume of a sphere of radius 0.5 /xm which has the minimum recurrence time for the calculations of the deterministic equations. Hence, the recurrence time for a fluctuation to induce a concentration change in these larger volumes would be larger than the minimum calculated recurrence time of 10 seconds. 

Effects of a fluctuation occurring in a sphere with radius smaller than 0.05 /xm are not examined numerically here, as the total number of particles involved is too small to warrant a deterministic description [21]. However, since the experiments of Foerster et al. and of Showalter et al. indicate that a region on the order of several microns needs to be perturbed in order for a wave to propagate, the lack of calculations regarding fluctuations in spheres with radius less than 0.05 fim is justified. 

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