Forster radii

Kremers GJ, Goedhart J, van Munster EB, Gadella TWJ (2006) Cyan and yellow super fluorescent proteins with improved brightness, protein folding, and FRET Forster radius. Biochemistry 45 6570-6580... 

Here, rDA is the distance between donor and acceptor and R0 is the Forster radius for the donor-acceptor pair. The Forster radius is the key quality measure for a VFP-FRET pair. R0 is defined as the distance at which 50% FRET occurs and can be calculated from the following equation [105, 106] ... 

The fluorescence is measured in dilute solution and in glassy PMMA for polyesters in which 2,6-naphthalene dicarboxylate is the rigid unit, and (CF iy+t is the flexible spacer. The anisotropy in the rigid medium demonstrates the existence of intramolecular energy migration, which becomes more important as y decreases from 5 to 1. The Forster radius is about 12 A in the bichromophoric compounds and 14 A in the polyesters. 

Here the chromophores can be farther apart (10-100 A long-range interactions) since this mechanism does not require any overlapping of orbitals. The Forster radius designates the distance between donor and acceptor at which the efficiency of energy transfer amounts to exactly 50%. Half of the excited donor molecules are then deactivated by fluorescence resonance energy transfer, and the other 50% by fluorescence or phosphorescence. 

In these multichromophoric cyclodextrins the fluorophores are randomly oriented. Excitation of one of the naphthoate fluorophores is followed by efficient dipole-dipole excitation energy transfer between the seven fluorophores, with a Forster radius of 14 A. This process is not detectable by fluorescence intensity measurements, as neither the intensity nor the decay law are affected by energy transfer between identical fluorophores (also called homotransfer. The dynamics of energy hopping are on the other hand reflected in the fluorescence anisotropy. To avoid depolarization by rotational motion of the fluorophores, experiments were conducted in a low temperature and optically clear rigid glass (9 1 ethanol-methanol at 110 K). 

Analogous results were obtained for the uncomplexed CD-St heptachromophoric cyclodextrin discussed in Section 7.2.4.2. In this case however, and owing to the larger Forster radius for homotransfer (22 A), the leveling of the anisotropy occurs in ca. 100 ps, and the average reciprocal rate constant for transfer is only 2.4 ps [14]. Nevertheless, a similar nearest neighbor distance (7 A) is recovered. 

In subsection (B) we will discuss the results with the anthracene energy trap more extensively, from the point of view of intrachain energy migration. This system is more difficult to analyze with simple models because of the large Forster radius for naphthalene to anthracene single-step energy transfer (25A ) ... 

---