Lanthanide complexes, first-shell coordination

VII. TOTAL ENERGETIC REQUIREMENTS TO INDUCE THE GLASS TRANSITION VIA CONSIDERATION OF THE FIRST-SHELL COORDINATION SPHERE IN TRANSITION METALAND LANTHANIDE COMPLEXES... 

Another technique that uses the fluorescence properties of trivalent lanthanides is that of the detection of fluorescence emission decay induced by pulsed dye laser excitation. Horrocks and Sudnick (17) have applied this technique to the study of water molecules bound to metal ions in small complexes and proteins. In one study they found that the exponential decay of Tb3+ fluorescence is altered when H20 is replaced by D20 and that this change can be used to determine the number of coordinated water molecules on the metal ion. With thermolysin, bound Tb3+ had 1-2 water molecules in the first coordination shell. This number is consistent with the x-ray structure. 

Binding sites for Ca " and ATP have been explored by the use of metal probes and nucleotide analogues. The Mn ion substitutes for Mg but also binds at the Ca " sites. Such complications have led to the use of lanthanides as probes for the Ca sites. Thus Gd " and Tb " compete with Ca for the high affinity site. Luminescence studies with laser-excited Tb " at the Ca sites show that two water molecules are present in the first coordination shell. Earlier work with Gd shows that the Ca sites are a maximum of 16.1 A apart, and that both sites involve a low level of hydration, consistent with a hydrophobic site. Gd has also been used as an ESR probe, and, under certain conditions, evidence has been produced for two forms of an E-Gd complex, in accord with current mechanistic views. 

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