| The spectroscopic and redox properties of lanthanide ions, Ln 3+, are used determine if the anions carbonate and phosphate are able to displace water molecules from the first coordination sphere of Eu 3+ analogs of Gd3+ magnetic resonance imaging, MRI, contrast agents and to study the electron transfer, ET, reaction between a photoexcited tryptophan residue, Trp, of the Ca2+ binding protein parvalbumin and the Eu3+ or Yb3+ ions substituted for Ca2+ in the two metal ion binding sites of parvalbumin.; In the study of the MRI contrast agents, a lengthening of the Eu 3+ excited state lifetime in the presence of millimolar concentrations of carbonate or phosphate indicates that water molecules are displaced by an anion. Only those metal complexes that contain negatively charged ligands and more than one water molecule in the first coordination sphere of Eu 3+ have their water molecules displaced by saturating concentrations of carbonate or phosphate. Our results show that under physiological conditions, phosphate and carbonate will, on average, displace less than half of a water molecule from the first coordination sphere of a typical contrast agent and suggest that the effect on proton spin relaxation is likely to be minimal.; The study of electron transfer required the characterization of mutants of the protein parvalbumin from carp. The removal of the first, second and last acidic residue of each of the classic EF-loop Ca2+ binding sites of parvalbumin is found to be sufficient to rid the loop of the ability to bind Ln3+ ions.; The mechanism of ET between the Trp and the two Ln3+ ions bound to parvalbumin, both 11 Å from the Trp, is found adhere to the β theory of exponential decay of the ET rate constant with distance, k ET = 1013 e−βr, rather than the Pathway theory which states that the structural motif of the protein, rather than the distance between the electron donor and acceptor, dictates the distance dependence of ET. The measured value for the reorganization energy, λ, from Marcus ET theory for our system is high, at 2 eV, compared to typical λ value of 1 eV measured in ET proteins. |
