Terbium chelate structure

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. 

The absorption spectrum of the acceptor is shown in the bottom panel of Figure 15.19. A favorable aspect of retinal is that it can be photobkached. thus decreasing the acceptor concentration in the disk membranes. Rrior to pho-tobleaching, the absorption spectrum of retirtal overlaps with that of the Tb donor. The emission spectrum of terbium is typical of the lanthanides. The emission is from /orbitals of the atom, and the emission spectra are typically highly structured line spectra. Also, it is cormnon to use lanthanide chelates, rather than lanthanides alone. This is because the lanthanides are extremely weak absorbers, with extinction coefricients near 0.1 cm. Fortu-... 

Bifunctional chelating agents, which are structurally similar to H edta, have been used to attach stable chelates of terbium and europium to human serum albumin, permitting the lanthanide fluorescence of the products to be studied at micromolar concentrations using standard instrumentation. The use of these metal complexes in energy-transfer experiments should greatly reduce the uncertainty in estimating the separation between the donor and the acceptor. 

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