Fluorescein terbium chelates

To achieve the rapid diffusion limit requires a very long-lived donor. Chelates of terbium, which typically have lifetimes of 1.5-2.2 msec, are frequently used. Fluorescein and rhodamine make excellent acceptors for terbium. Chelates of europium also have long lifetimes, 0.5-2.3 msec, although these appear not to have been used. (Cronce and Horrocks have used europium in a calcium-binding site as a donor in a rapid diffusion... 

Fig. 10. Structure or terbium chelate, namely, terbium diethylenetriaminepentaacetic acid coupled to carbostyril 124. The carbostyril absorbs light (maximally at 327 nm) and transfers energy to the terbium. The net result is an increase in the effective absorbance of the terbium by several thousandfold. The DTPA chelate shields the terbium from the quenching effects of water and allows for easy attachment to macromolecules. Here the macromolecule is an 8-mer DNA oligomer modified with a primary amine on the 5 end. The acceptor, fluorescein, is attached to the 5 end of a complementary DNA oligomer. 

Jeyakumar and Katzenellenbogen developed a dual-acceptor assay to monitor a nuclear receptor (thyroid hormone - retinoid X receptor heterodimer) regulation by measuring both coactivator and corepressor binding by using fluorescein and Cy-5 as acceptors for a terbium chelate-labeled response element-bound receptor complex [47]. 

We have developed an energy transfer system which overcomes these difficulties. We use a luminescent lanthanide chelate as donor and an organic dye such as fluorescein, rhodamine, or CY-5 as acceptor. A number of workers have noted that the luminescent lanthanide elements terbium and europium are attractive donors because they have multiple transition dipole moments such that they act as randomized donors even in the absence of any rotational motion. This limits (1/3 < < 4/3)... 

In Figures 11 and 12 we show an experiment where a terbrium chelate (terbium diethylenetriaminepentaacetic acid coupled to carbostyril 124) transfers energy to fluorescein. The donor and acceptor are separated... 

Lanthanide chelates with long decay times can also be used to make efficient tandem probes where both emission wavelength and decay times can be stipulated by selecting the spacer arm and distances between dmior moiety and acceptor. Selvin reported a series of tunable probes composed of fluorescent terbium- and europium-chelates coupled to fluorescein or Cy-5, respectively [32]. 

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