Metalloproteins spin states

Raman (R) and resonance Raman (RR) spectroscopy detects vibrational modes involving a change in polarizability. For RR, enhancement of modes is coupled with electronic transition excited by a laser light source. This technique is complementary to IR and is used for detection of v(O-O) and v(M-0), especially in metalloproteins. In porphyrins, one may identify oxidation and spin states. 

The purpose of this article is to review studies carried out on hemes incorporated inside the micellar cavity, and examine the effect of micellar interaction on the electronic and structural properties of the heme. A comparison of these results with those on the metalloproteins is clearly in order to assess their suitability as models. The article begins with a general introduction to micellar properties, the incorporation of hemes in the micellar cavity, and then discusses results on hemes inside the micelles with different oxidation and spin states, and stereochemistry. The experimental techniques used in the studies on these aqueous detergent micelles are mostly NMR and optical spectroscopy. The present article has therefore a strong emphasis on NMR spectroscopy, since this technique has been used very extensively and purposefully for studies on hemes inside micellar cavities. 

The association of sulfur and iron into simple to more complex molecular assemblies allows a great flexibility of electron transfer relays and catalysis in metalloproteins. Indeed, the array of different structures, the interactions with amino-acid residues and solvent and their effect on redox potential and spectroscopic signatures is both inspiring for chemists and electrochemists, and of paramount importance for the study of these centers in native conditions. Most of the simpler natural clusters have been synthesized and studied in the laboratory. Particularly, the multiple redox and spin states can be studied on pure synthetic samples with electrochemical and spectroscopic techniques such as EPR or Fe Mossbauer spectroscopy. More complex assembhes still resist structural... 

Paramagnetism in a metalloprotein does not have to be associated with only a single metal site but can arise from metal clusters. In these clusters the spins of the different metal sites couple either ferromagnetically or antiferromagnetically to produce a net spin state of the cluster. 

Perhaps the most important area of biochemistry in which ESR is used is the study of metalloproteins. Transition metals in certain oxidation and spin states have unpaired electrons, are paramagnetic, and in many cases are amenable to ESR spectroscopy. The most commonly found transition metals in biological systems are iron, copper, molybdenum, cobalt, and manganese. The remainder, including metals such as vanadium and... 

By virtue of unoccupied d-orbitals, iron binds to many ligands - preferably to their oxygen, nitrogen, and sulfur atoms. In enzymes and other metalloproteins, iron participates in a large number of biochemical reactions. Its chemical reactivity changes due to the oxidation state, electron spin state and redox potential, the latter ranging from +1000 mV in some heme proteins to —550 mV in some bacterial ferredoxins (Cammack et al. 1990). 

In metalloproteins, the paramagnet is an inseparable part of the native biomacromolecule, and so anisotropy in the metal EPR is not averaged away in aqueous solution at ambient temperatures. This opens the way to study metalloprotein EPR under conditions that would seem to approach those of the physiology of the cell more closely than when using frozen aqueous solutions. Still the number of papers describing metalloprotein bioEPR studies in the frozen state by far outnumbers studies in the liquid state. Several additional theoretical and practical problems are related to the latter (1) increased spin-lattice relaxation rate, (2) (bio)chemical reactivity, (3) unfavorable Boltzmann distributions, (4) limited tumbling rates, and (5) undefined g-strain. 

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