ESR of metalloproteins and metalloenzymes

The intent of this section is to focus on various techniques employed to study ESR of metal ions. The literature cited is neither complete nor in many ways representative of the enormous volume of work utilizing ESR to study proteins and enzymes. Much of the early pioneering work done in applying ESR techniques to biochemical systems 

ESR of heme proteins has characterized both the high-spin (S = 5/2) and the low-spin (S=l/2) forms. The review by Palmer [226] on ESR of heme proteins is particularly useful, in which the model for low-spin iron complexes (for which the d y, and dy are split from the d 2 and orbitals) is clearly explained. The five 

Stable structures such as the naturally occurring ferric porphyrin complexes or porphyrins substituted with copper, cobalt, silver or vanadyl probe the active site of heme-containing enzyme [227]. Complexes of copper not associated with heme are also common. They are frequently formed at an amino terminus because the amino group provides a good primary amine donor atom. Two or three amino acid residues beginning at the N-terminus are often flexible until a more rigid portion of the polypeptide, such as the a helix, is encountered. A peptide nitrogen is available to 

The ESR spectrum for cupric ion bound to four equivalent imidazoles from carnosine ()8-alanyl-L-histidine dipeptide) in frozen solution shows the quality of information that can typically be obtained [233] (Fig. 15, top spectrum). The intense line on the right split by at least nine hyperfine lines is the gj feature resolved due to hyperfine splitting to copper and to nitrogen donor atoms. Three of the four copper hyperfine lines in the gy (2.25) region (left side) are observable with y4 [ = 175 G. The remaining ESR parameters can be estimated and confirmed by computer simulation. These values for carnosine aregy = 2.06, Af = 15 G and /4 = 15 G. For comparison, data for cupric ion bound to hemoglobin at the N-terminus are gy = 2.210, = 2.050, 

The case in which no reaction occurs has been somewhat neglected. In this case complementary complexes may be utilized as metallo spin probes. Cupric complexes provide information on the environment through the ESR parameters, /4 , g,  

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