High spins metalloproteins

Seamlessly covers all important bioEPR applications, including low-spin and high-spin metalloproteins, spin traps and spin lables, interaction between active sites, and redox systems... 

Hagen, W.R. 2007. Wide zero field interaction distributions in the high-spin EPR of metalloproteins. Molecular Physics 105 2031-2039. 

Metals at the active sites of metalloproteins display special properties and are generally impressively efficient in their functional roles. The metal or metals are said to be poised for catalytic action or in an entatic state, an expression first used by Vallee and Williams in their seminal paper of 1968 (2). The fine tuning or control of the peptide in many cases results in previously unobserved aspects of the coordination chemistry of the metal in question. There are many examples, almost as many as there are metalloproteins, from (in the case of Fe) the five-coordinate high-spin Fe(II) in myoglobin and hemoglobin... 

Much of this work has been discussed earlier (Section 2.3.3) in the context of ENDOR spectroscopy of high-spin S > 1/2) systems, as the electronic spin ground state of nitrogenase FeMo-cofactor has S = 3/2. From a historical perspective, this work was one of the earlier applications of ENDOR to metalloprotein systems by the Hoffman laboratory, and owed a great deal of its success to the biochemical expertise of the late William H. Orme-Johnson, one of the pioneers not only of nitrogenase research but of bioinorganic chemistry in general. ... 

Nonheme ferrous centers in some metalloproteins react reversibly with NO forming nitrosyl complexes with S = 312 characterized by the g values of about 4.0 and 2.0 [51]. The EPR spectrum of the nitrosylated NorR (abacterial NO-responsive transcription factor, the enhancer binding protein) is typical of a d high-spin Fe NO", where the S = 5/2 iron is antiferromagnetically coupled to the NO (Fig. 5, [52]). This is confirmed by the X-ray, resonance Raman, MCD, Mossbauer spectroscopies, and DFT calculations. Similar structures were proposed for the classical complexes, [Fe(NO)(l-isopropyl-4,7-(4-ferf-butyl-2mercaptobenzyl)-l,4, 7-triazacyclononane)], [53], Fe(EDTA)NO [54—56], the brown-ring compound, Fe(H20)5N0 [57], and for the Fe(N/V ,N -trimethyl-l,4,7-triazacyclononane) (N3)2N0 [54]. Interestingly, for the latter a spin equilibrium between the valence tautomers 5=1/2 and 3/2 in the solid state was observed. 

It should be mentioned that the work described earlier on MaMaMb systems represents an extension of the conceptual design first described by Mashima for complexes containing phosphine derivative ligands and mainly second- or third-row transition metals [66-77]. By changing from P-donor ligands to N-donors, the new EMAC complexes of first-row metals can exist in high-spin configurations and are more relevant to the types of heterometallic active sites observed in metalloproteins. 

One of the major anticipated applications of high-field EPR was the study of integer-spin metalloproteins whose (expected) zero-field splittings made them either very difficult to characterise or EPR silent at X-band. Mn(III), Ni(II), Fe(II), Co(I), Mo(IV) and W(IV) are all redox states of important enzymes and in principle are advantageously measured at high frequencies. However, for bio-... 

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