Editing Magnetic circular dichroism (section)

== Applications ==
MCD can be used as an optical technique for the detection of electronic structure of both the ground states and excited states. It is also a strong addition to the more commonly used absorption spectroscopy, and there are two reasons that explain this. First, a transition buried under a stronger transition can appear in MCD if the first derivative of the absorption is much larger for the weaker transition or it is of the opposite sign. Second, MCD will be found where no absorption is detected at all if ΔA > (ΔA<sub>min</sub>) but A < A<sub>min</sub>, where (ΔA)<sub>min</sub> and A<sub>min</sub> are the minimum of ΔA and A that are detectable. Typically, (ΔA<sub>min</sub>) and A<sub>min</sub> are of the magnitudes around 10<sup>−5</sup> and 10<sup>−3</sup> respectively. So, a transition can only be detected in MCD, not in the absorption spectroscopy, if ΔA/A > 10<sup>−2</sup>. This happens in paramagnetic systems that are at lower temperature or that have sharp lines in the spectroscopy.<ref name=steph>{{cite journal|author=Stephens, P. J.|journal=Annu. Rev. Phys. Chem.|date=1974|volume=25|pages=201–232|doi=10.1146/annurev.pc.25.100174.001221|title=Magnetic Circular Dichroism|bibcode = 1974ARPC...25..201S }}</ref>

In [[biology]], [[metalloprotein]]s are the most likely candidates for MCD measurements, as the presence of [[metals]] with degenerate energy levels leads to strong MCD signals. In the case of ferric heme proteins,<ref>{{cite journal|author=G. Zoppellaro|display-authors=etal|journal=Biopolymers|date=2009|volume=91|pages=1064–82|doi=10.1002/bip.21267|title=Review: Studies of ferric heme proteins with highly anisotropic/highly axial low spin (S = 1/2) electron paramagnetic resonance signals with bis-Histidine and histidine-methionine axial iron coordination|issue=12|pmid=19536822|pmc=2852197}}</ref> MCD is capable of determining both oxidation and spin state to a remarkably exquisite degree. In regular proteins, MCD is capable of [[stoichiometry|stoichiometrically]] measuring the [[tryptophan]] content of [[protein]]s, assuming there are no other competing absorbers in the spectroscopic system.
In addition, the application of MCD spectroscopy greatly improved the level of understanding in the ferrous non-heme systems because of the direct observation of the d–d transitions, which generally can not be obtained in optical absorption spectroscopy owing to the weak extinction coefficients and are often electron paramagnetic resonance silent due to relatively large ground-state sublevel splittings and fast relaxation times.<ref name=solomon>{{cite journal|author=E.I. Solomon|display-authors=etal|author-link=Edward I. Solomon|journal=Coordination Chemistry Reviews|volume=144|date=1995|pages=369–460|doi=10.1016/0010-8545(95)01150-N|title=Magnetic circular dichroism spectroscopy as a probe of the geometric and electronic structure of non-heme ferrous enzymes|doi-access=free}}</ref>