Cobalt(II) complexation with small biomolecules as studied by 57Co emission Mössbauer spectroscopy

Abstract

In the emission (⁵⁷Co) variant of Mössbauer spectroscopy (EMS), the ⁵⁷Co radionuclide (with a half-life of 9months) is used that undergoes a nuclear decay ⁵⁷Co→⁵⁷Fe via electron capture followed by the emission of a γ-quantum, the energy of which is modified by the chemical state and the close coordination environment of the parent ⁵⁷Co atom. While EMS has been used largely in materials science and nuclear chemistry, its high sensitivity can also be of great advantage in revealing fine structural features and for speciation analysis of biological complexes, whenever the ⁵⁷Co²⁺ cation can be used directly as the coordinating metal or as a substitute for native cobalt or other metal ions. As such EMS applications are yet rare, in order to reliably interpret emission spectra of sophisticated ⁵⁷Co²⁺-doped biosystems, model EMS studies of simple cobalt biocomplexes are necessary. In this work, EMS spectroscopic data are analysed and discussed for ⁵⁷Co²⁺ complexes with a range of small biomolecules of different structures, including 4-n-hexylresorcinol, homoserine lactone and a few amino acids (spectra measured in rapidly frozen dilute aqueous solutions or in the dried state at T=80K). The EMS data obtained are discussed with regard to the available literature data related to the coordination modes of the biocomplexes under study.

Keywords: (57)Co radionuclide; Cobalt(II) biocomplexes; Emission Mössbauer spectroscopy.