The novel "g' = 1.74" EPR spectrum of pink and purple uteroferrin.
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Low temperature (T less than or equal to 20 K) EPR measurements have revealed the presence of a heretofore undetected signal in uteroferrin, a purple protein bearing a single iron atom per molecule. All three of its principal g values (1.923, 1.738, and 1.583) lie well below the free electron value of 2.0023. Magnetic susceptibility data from 2-77 K confirm that the novel EPR spectrum arises from a paramagnetic center with a single unpaired electron spin. Quantitative correlation of the EPR, susceptibility, and optical data point to chromophoric iron as the source of the rhombic EPR spectrum. Furthermore, close agreement between the concentration of iron and the integrated intensity of the rhombic EPR signal show that the iron in the paramagnetic center is mononuclear. Reduction of the protein to its pink form leaves the rhombic signal essentially unaltered. The previously reported g' = 4.3 EPR signal accounts for only a small fraction of the total iron in the protein and undoubtedly arises from adventitious iron. Collectively, these results strongly suggest that uteroferrin represents a new class of low spin iron proteins.