Lanthanides calcium probes

An alternative probe procedure is to use the trivalent lanthanides 42). Some lanthanide cations can replace calcium in a functional sense in biological systems. On the other hand, it is generally true that lanthanides inhibit calcium systems in a competitive way and they can be used therefore to probe the calcium sites. The lanthanides, 15 probes of slowly graded ionic size, represent a wonderful experimental tool for the study of biological selectivity and a chance to uncover fine control mechanisms. Almost every physical method can be used in the study of one or the other of these cations. 

The use of thallium-205 as a probe for potassium has been suggested, - and Kayne and Reuben have used the broadening between the signals of the two paramagnetic ions thallium(i) and manganese(n) to estimate the distance between the monovalent and bivalent activators in rabbit muscle pyruvate kinase. Several cases of the inhibition of Ca + transport processes by lanthanides have been reported, and Williams and co-workers have proposed the use of the lanthanides as probes for calcium. 

Mikkelson, R.B., 1976, Lanthanides as Calcium Probes in Membranes, in Biological Membranes, Vol. 3, eds D. Chapman and D.F. Wallach (Academic Press, New York) pp. 153-190. 

Lanthanides are well-known probes for calcium, although they bind to sites more strongly than Ca2+ and do not enter cells. One useful feature is that a series of probes of slightly differing radii may be used. Tbni has a radius (1.00 A) very close to that of calcium (0.99 A), and has been used successfully in many instances. The Lnm ions have also been used to probe sites on nucleic acids normally filled with essential Mg2+ ions.107,108... 

The lanthanides and divalent calcium are similar in size and have a tendency for high coordination numbers and these similarities make lanthanide ions as structural probes in biochemical reactions in place of calcium. Further, lanthanide ions can be used as markers... 

There are a host of potential probes for and Ca, as these ions have about the same size as many divalent first row transition metals and divalent lanthanides (Table VI). Trivalent lanthanides have the correct radii although the wrong charge as substitutes for calcium. In isomor-phous replacement in minerals, the radius is more important than the charge this may also be true in biology as Na and (radius 1.0 A) and and Ba (radius 1.3 A) often compete for sites. This means that a lanthanide may be an excellent competitor for a divalent cation site in biology and there is already some evidence to this eflFect (Table IX) (29). 

Lanthanides possess some favorable properties as biochemical probes. They can substitute chemically for calcium in many calcium-dependimLpniiems. A main route of nonradiative decay is Ify cou dii to vibrations of water. For both Eu and Tb, the lifetiiite i n HjO and D]0 can be used to calculate the number of bound water molecules (n) ... 

Calcium, one of the most important and ubiquitous of the essential elements, has very few ehemical properties which can be used to probe its biochemistry in situ. The lanthanides, as Ln ions, make almost ideal biomimetic agents for calcium (11) as they are chemically hard and exhibit a range of ionic radii which encompass that of the Ca ion (Evans 1990). 

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