The Lanthanide Cations as Probes in Biological Systems

A study has been made over a range of temperatures at 20 MHz and 35 MHz of the spin relaxation rates of water protons in aqueous solutions of ethylene glycol bis‐(aminoethyl)‐tetraacetic acid containing Gd(III) and Eu(II) ions. The results suggest that the dominant relaxation process arises from rotation of the solvated complexes. Relaxation enhancement has been observed for Gd(III) in the presence of lysozyme and values of ɛ*, Kd and τc have been determined. An application of these results to obtain distances of substrate analogues from the active site of this and other enzymes is discussed.