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

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.