Gadolinium as an EPR and NMR Probe of Ca2+ Sites in Biological Systems

Abstract

In spite of an increasing awareness of the importance of calcium in biological systems (1), Ca2+ has received little attention compared to most of the biologically relevant transition metals. Ca2+ is not easily studied by physical methods: the absence of unpaired electrons makes it unsuitable for electron paramagnetic resonance (EPR) studies; its NMR signal is extremely weak, and its electronic transitions cannot be studied by conventional optical spectroscopic methods. Biochemists as well as inorganic chemists have used the trivalent lanthanides as analogues for Ca2+. The ability of the lanthanides to replace Ca2+ in, for example, Ca2+ binding proteins is well established (2). Several investigators, including Horrocks (3) and Martin and Richardson (4) have utilized lanthanide ions, including terbium (Tb3+) and europium (Eu3+), as luminescence probes of Ca2+ binding sites in biological systems. Gd3+, with seven unpaired electrons, has been used extensively as a paramagnetic probe in NMR studies (5,6) and in single crystal EPR (7,8). Until recently (9,10), however, its usefulness as a EPR probe in biological systems has been overlooked or discounted (11). We have conducted an extensive study of the EPR properties of Gd3+ and have found that EPR spectra of solutions and glasses can be obtained which are highly sensitive to the symmetry, ligands and coordination geometry of Gd3+.