Paramagnetic Resonance of Metallobiomolecules

X-ray magnetic circular dichroism XMCD is the difference in absorption of leftand right-circularly polarized x-rays by a magnetized sample. Although MCD with x-rays is only about 15 years old, the physics is essentially the same as for UV-visible MCD that has been known since 1897. For (bio)inorganic chemists and materials scientists, XMCD has the advantage of elemental specificity that comes with all core electron spectroscopies. Thanks to simple sum rules, XMCD can provide quantitative information about the distribution of spin and orbital angular momentum. Other strengths include the capacity to determine spin orientations from the sign of the XMCD signal, to infer spin states from magnetization curves, and the ability to separate magnetic and non-magnetic components in heterogeneous samples. With new synchrotron radiation sources and improved end stations, XMCD measurements on biological samples are, if not routine, at least no longer heroic. One goal of this review is to encourage chemists, materials scientists, and biologists to consider XMCD as an approach to understanding the electronic and magnetic structure of their samples.