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Abstract Early studies suggested that Fe^IIIcomplexes cannot compete with Gd^IIIcomplexes as T₁MRI contrast agents. Now it is shown that one member of a class of high‐spin macrocyclic Fe^IIIcomplexes produces more intense contrast in mice kidneys and liver at 30 minutes post‐injection than does a commercially used Gd^IIIagent and also produces similar T₁relaxivity in serum phantoms at 4.7 T and 37 °C. Comparison of four different Fe^IIImacrocyclic complexes elucidates the factors that contribute to relaxivity in vivo including solution speciation. Variable‐temperature¹⁷O NMR studies suggest that none of the complexes has a single, integral inner‐sphere water that exchanges rapidly on the NMR timescale. MRI studies in mice show large in vivo differences of three of the Fe^IIIcomplexes that correspond, in part, to their r₁relaxivity in phantoms. Changes in overall charge of the complex modulate contrast enhancement, especially of the kidneys. 

The development of MRI probes is of interest for labeling antibiotic-resistant fungal infections based on yeast. Our work showed that yeast cells can be labeled with high-spin Fe(III) complexes to produce enhanced T2 water proton relaxation. These Fe(III)-based macrocyclic complexes contained a 1,4,7-triazacyclononane framework, two pendant alcohol groups, and either a non-coordinating ancillary group and a bound water molecule or a third coordinating pendant. The Fe(III) complexes that had an open coordination site associated strongly with Saccharomyces cerevisiae upon incubation, as shown by screening using Z-spectra analysis. The incubation of one Fe(III) complex with either Saccharomyces cerevisiae or Candida albicans yeast led to an interaction with the β-glucan-based cell wall, as shown by the ready retrieval of the complex by the bidentate chelator called maltol. Other conditions, such as a heat shock treatment of the complexes, produced Fe(III) complex uptake that could not be reversed by the addition of maltol. Appending a fluorescence dye to Fe(TOB) led to uptake through secretory pathways, as shown by confocal fluorescence microscopy and by the incomplete retrieval of the Fe(III) complex by the maltol treatment. Yeast cells that were labeled with these Fe(III) complexes displayed enhanced water proton T2 relaxation, both for S. cerevisiae and for yeast and hyphal forms of C. albicans.