Electron paramagnetic resonance manganese complexes

Guiles (1988) Structure and function of the manganese complex involved in photosynthetic oxygen evolution determined by X-ray absorption spectroscopy and electron paramagnetic resonance spectroscopy. Ph D dissertation, University of California, Berkeley, CA. Lawrence Berkeley Laboratory Report, LBL-25186... 

The coordination structures of enzyme-bound manganese nucleotides can in favorable cases be determined by analysis of electron paramagnetic resonance (EPR) spectra of Mn(II) coordinated to O-labeled nucleotides. When the nucleotide is stereospecifically labeled with O at one diastereotopic position of a prochiral center, either oxygen can in principle be bound to Mn(II) in the coordination complex in an enzymic site. When the coordination bond is between Mn(II) and O, the EPR signals for Mn(II) are broadened and attenuated, owing to unresolved superhyperfine coupling between the nucleus of 0 and the unpaired electrons of Mn(II) (23). No such effect is possible with 0, which has no nuclear spin. The effect is observable in samples in which all the Mn(Il) is specifically bound in one or two defined complexes of the nucleotide with the enzyme. Thus the complex Mg(Sp)-[a- 0]ADP bound at the active site of ere-... 

McDermott E, Yachandra VK, Guiles RD et al. Characterization of the manganese 02-evolving complex and the iron-quinone acceptor complex in photosystem II from a thermophilic cyanobacterium by electron paramagnetic resonance and X-ray absorption spectroscopy Biochemistry 1988 27 4021-4031. 

The structure of this complex has been investigated by electron paramagnetic resonance spectroscopy. The complex has a characteristic signal in the S2 state [1], which has suggested either a bi or tetranuclear arrangement of manganese. Unequivocal structural predictions have not been made. 

The third principal application of the electron spin resonance technique is to the study of paramagnetic transition metal ions in biochemical systems. Most examples are complexes of copper, iron, manganese, chromium, cobalt and molybdenum. Other metals such as titanium, vanadium and nickel are sometimes employed as structural probes. Only four of these ions, Cu ", Mn, Gd " and VO ", are seen in ESR spectroscopy at room temperature under virtually all conditions. Therefore, they are of special importance. 

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