Terbium ion

Until very recently, studies of the use of luminescent lanthanide complexes as biological probes concentrated on the use of terbium and europium complexes. These have emission lines in the visible region of the spectrum, and have long-lived (millisecond timescale) metal-centered emission. The first examples to be studied in detail were complexes of the Lehn cryptand (complexes (20) and (26) in Figure 7),48,50,88 whose luminescence properties have also been applied to bioassay (vide infra). In this case, the europium and terbium ions both have two water molecules... 

Figure 9.53 DPA derivatives have been used as potent enhancers of lanthanide luminescence. Three DPA groups can coordinate with a terbium ion. The iodoacetate derivative of DPA has been used to label covalently molecules for lanthanide luminescence.

When used with europium or terbium ions, a carbostyril-based lanthanide chelate can be excited at 340 nm and provide sharp characteristic emission bands for transfer of energy to the appropriate acceptor fluor. Similar to the TMT chelator described previously, luminescence from terbium FRET signals well with Cy3 dyes and luminescence from europium can be used with APC or Cy5 dyes. Other fluorescent dyes that have similar excitation and emission ranges to these also can be used as acceptors in TR-FRET assays. For instance, terbium chelates can... 

Nonradiative relaxations in solutions are particularly interesting. Terpi-lovskii (38) made a calculation of the probabilities of nonradiative transitions for the trivalent terbium ion in aqueous solution. He considered that the transitions were caused by Brownian movement of vibrations of the solvate envelope. 

He assumed that the terbium ion was surrounded by six water molecules,... 

Since the backup ions other than aluminum are of a size similar to the terbium ion, it is reasonable to assume that the structure of the glass matrix over the whole series is the same. Therefore, the concentration dependence of the lifetime is unequivocally due to terbium ions being packed closer and closer together. Pearson and Peterson postulate that, as the ions are situated closer and closer together, the quenching mechanism of Dexter and Schulman (45) becomes operative. That is, the excitation jumps from ion to ion by a resonance process until it reaches a sink. 

The observations that the 5D4 state lifetime does not begin to drop below 2.5 mole per cent or more is reasonable compared to the 1 mole per cent value for pair formation, since it is proposed that the quenching of the 5D4 state depends on the formation of chains of terbium ions, which would necessitate a higher concentration of terbiums than would be required for the formation of isolated pairs. 

In another study, Kondrat eva (103) made a determination of the luminescent quantum yield of the 5D4 state of the terbium ion in aqueous solution. The method used was based upon fluorescent-lifetime measurements and had previously been used by Rinck (96) and Geisler and Hellwege (96) to determine the quantum yield of rare earths in crystals. Kondrat eva made his studies on chloride and sulfate solutions, using the electronic shutter technique of Steinhaus et al. (66). 

From the data of Hoogschagen and Gorter (104), the oscillator strength of the 5D4-+7F6 transition was obtained. By means of the Ladenburg formula, the spontaneous coefficient A46 was calculated. Using the relative-emission intensities, the rest of the A4J spontaneous-emission coefficients could be calculated. From these and a measured lifetime of 5.5 x 10 4 sec at 15°C, he calculated a quantum efficiency of 0.8 per cent. Kondrat eva concluded that the probability of radiationless transition for the trivalent terbium ion in aqueous solution is approximately two orders of magnitude greater than for the radiation transition. 

Figure 7 Energy level diagram of lowest energy excited states of europium and terbium ions. Only the emissive excited states within the 5Dj manifold are shown.

Fluorescent probes in reversed micellar aggregates proved, moreover, appropriate to study dynamic properties of micelles in nonpolar solvents42,44 The particularly suitable fluorescent label was the highly water sensitive terbium ion (Tb3+) (Fig. 26). 

Another example of a different type of correlation of structural to photophysical properties is shown in a study of a unique terbium compound [63]. This compound will be briefly discussed and is depicted in Figure 7.9 with its nonlinear emission properties with excitation at 800 nm. The photophysical properties are atypical and rather extraordinary due to the unusual molecular structure of the co-crystallization compound (4) of the organic chromophore and the terbium salt This compound shows both multiphoton absorption induced green f-f emission from the terbium ion as well as second-harmonic generation. However, unlike previously... 

The photoluminescence emission based on the terbium ion peaks at 491, 549, 582, and 618 nm correspond to the f-felectronic transitions 4 Fe, D4 ->- F5, D4 F4, and D4 ->- F3, respectively (Figure 11.16). Among them, D4 -> F5 is dominant, hence a bright green emission can be observed when terbium complexes are used as emission materials. 

The complex of A-(2-pyridinyl)-3-ketobutyramide with Tb + in methanol solution can emit the intrinsic fluorescence of Tb +. When EtsN and Zn + are added to the solution, the fluorescence (kex = 329 nm, ka = 546 nm) is significantly enhanced. This sensitive fluorescence enhancement system can be used for the determination of terbium ion ". ... 

Contrary to the previous case, parvalbumin binds only two Ca ions. Compared to the technical difficulties encountered at the K-absorption edge of calcium due to increased absorption of 3 A wavelengths, the quantitative replacement of Ca by terbium ions (Lj-edge at 1.648 A) offers an ideal way to obtain structural information about the ion binding sites of parvalbumin by resonance scattering experiments. In fact, Miake-Lye, Doniach and Hodgson were able to determine for the first time the distance between the center of mass of the parvalbumin and its two terbium ion binding sites in solution. 

Terbium clathrochelate showed green emission of very high intensity. The emission spectrum contains the D4 —> Fj transition bands of the encapsulated terbium ion. The same but less intense emission spectrum was observed at higher temperatures. The luminescence quantum yield is close to 1 at 4.4 K and is approximately 0.05 at room temperature [390]. The decrease in intensity of the terbium(III) ion luminescence starts at 100 K (higher than that of free macrobicyclic tris-bipyridine ligand and lower than that of the corresponding europium(III) compound, Fig. 69). It may be... 

Figure 2.21 A terbium ion complex used as a probe in biophysical studies...

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