Lanthanides as Probes in Biological Systems

The lanthanides form a series of ions of closely related size and bonding characteristics and in many respects resemble Ca +, for which they often substitute isomorphously in biological systems. Since different Ln ions can be probed with particular spectroscopic techniques (e.g. Eu + and Tb +, fluorescence Gd +, ESR Nd +, electronic spectra), in favourable circumstances it should be possible to obtain information about the binding site of spectroscopically inactive Ca + in several ways. Systems smdied include the calciumbinding sites in calmodulin, trypsin, parvalbumin, and the Satellite tobacco necrosis virus. 

Showing the effect of zero-field splitting on the EPR transitions in Gd +. 

Lanthanide chelates have been used as labels of antigens and antibodies in fluoroimmuno-logical analysis, and in areas such as the determination of steroids in animals, of hormones and viral antigens in humans, and of herbicides in water. 

Question 5.3 Explain why f-f transitions in the electronic spectra of lanthanide complexes are weaker than d-d transitions in the corresponding spectra of transition metal complexes. 

Question 5.4 Why is theCe +(4f ion colourless whereas Ti + solutions (3d ) are purple  

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