Bioinorganic transition metal compounds

Many but not all transition metal ions are bearing unpaired electrons. The ones with unpaired electrons are ESR-detectable by Magnettech's MS5000 due to the high sensitivity of the ESR spectrometer.

Detection of transition metal compounds with ESR spectrometer MS5000

In many cases still the detection provides information about the ligand field (high spin or low spin) or the oxidation state of the ion. 

For example an ESR-silent complex of Fe2+ possesses a d6-low spin electron configuration, were all electrons are paired. An Fe2+ complex showing an ESR spectrum has a high spin configuration. As Fe2+ low spin, Cu+ is also ESR silent, because of its d10-configuration, where all d-orbitals are fully occupied and all d-electrons are paired. In contrast Cu2+ bears one unpaired electron resulting in an one line spectrum. 

Beside the miniscope ESR spectrometer provides an ideal tool to compare the coordination geometry of active metal centres of enzymes and their low molecular weight model complexes by using the state of the art miniscope evaluation software. An example is shown below, displaying the ESR-spectra of the active Cu2+ centre of Cu(II)2Zn2 superoxide dismutase and Cu(II)PuPy a superoxide dismutase mimic.

Usage of the temperature controller for MS5000 allows to record spectra at liquid nitrogen temperature in frozen solutions of ethyleneglycol : water 1:1. The hyperfine splitting of these anisotropic signals provide information if the coordination geometry is tetragonal planar or tetrahedrally distorted.

Most of the enzymes and model complexes examined are copper and iron bearing compounds. But electron spin resonance (ESR) spectroscopy is useful for all paramagnetic transition metal centres with unpaired electrons in general.