L-band ESR spectra of copper(II) complexes with CuN4 configurations

L-band ESR spectrometry has been developed for detecting free radicals or paramagnetic metal ions in intact animals or biological samples which contain much water. Several studies by L-band ESR on free radicals have been reported, but few on paramagnetic elements such as copper(II) or iron(III). We measured L-band ESR spectra of Cu(II) for the first time to investigate the coordination structure around the metal ion in aqueous solution. In X- and L-band ESR, both spectral patterns and parameters have been found to reflect the coordination structure of Cu(II)-polyamine complexes depending on the structures such as square-planar, axially coordinate square-planar, tetrahedral and distorted square-planar (Cu(II) ion is displaced from CuN4 plane). In addition, in L-band ESR, the coordination structures of Cu(II) complexes relate with spectral pattern, ESR parameter (LA-value), stability constants and halfwave potentials in terms of the electron density on Cu(II). Thus, L-band ESR spectrometry was indicated to be useful to know the coordination structure of Cu(II) proteins or enzymes in aqueous solution.