Magnetic circular dichroism

From Academic Kids

Magnetically induced circular dichroism (MCD), is the differential absorption of left and right circularly polarized light in the presence of a magnetic field. In general, this means wrapping a circular dichrometer in a large electromagnet. If the magnet is water cooled, field strength of a couple of teslas are common. With a superconducting magnet, field strengths can reach 5 teslas.

Some superconducting magnets have a small sample chamber, far too small to contain the entire optical system. Instead, the magnet sample chamber has windows on two opposite sides. Light from the source enters one side, interacts with the (usually also temperature controlled) sample in the magnetic field, and exits through the opposite window to the detector. Optical relay systems are typically employed that allow the source and detector each to be about a meter from the sample. This arrangement avoids many of the difficulties that would be encountered if the optical apparatus had to operate in the high magnetic field, and also allows for a much less expensive magnet.

Although there is much overlap in the requirements and use, ordinary CD instruments are usually optimized for operation in the ultraviolet, approximately 170–300 nm, while MCD instruments are typically required to operate in the visible to near infrared, approximately 300–2000 nm.

MCD measurements are made by taking a circular dichroism, or CD measurement, with the magnetization oriented in the positive sense, and then a CD measurement with the magnetization oriented in the negative sense. The two signals are then subtracted:

Signal 1 = CD + MCD
Signal 2 = CD - MCD
Signal 1 - Signal 2 = 2 MCD

MCD signals tend to be small, so the measurements often take time. To get a reasonable signal to noise ratio, the experiements can often take a few hours. Multiple spectra are often taken, digitized, accumulated and stored via computer.

The physical processes that lead to MCD are substantively different than those of CD. However, like CD, it is dependent on the differential absorption of left and right hand circularly polarized light. At any given wavelength, MCD will not exist unless the molecule has an optical absorption at that wavelength. This is distinctly different than the related phenomenon of Optical Rotatory Dispersion, which can be observed at wavelengths far from any absorption band.

In the most common kind of MCD (the so called C spectra), the magnetic field breaks the degeneracy of excited state optical energy levels differing only in magnetic spin. This separation is small, leading to a tiny shift in frequency in otherwise nearly identical spectrum, that, because of different signs of the spin state, are opposite in sign. As a consequence, this kinds of MCD absorption (so called C spectra) are shaped like a frequency derivative of the underlying optical spectra, having a peak on one side of the related optical maximum and a trough on the other side.

In type A spectra, MCD absorption is a consequence of the magnetic field breaking the degeneracy of the ground state of the optical absorption. Unlike C spectra, the intensity of A spectra are affected by temperature, with stronger signals being observed as the sample is cooled. For samples exhibiting both A and C class spectra, taking the MCD as a function of temperature is a way to separate the relative contributions of these two effects.

In biology, metalloproteins 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 hemoproteins, MCD is capable of determining both oxidation and spin state to a remarkably exquisite degree. In regular proteins, MCD is capable of stoichiometrically measuring the tryptophan content of proteins, assuming there are no other competing absorbers in the spectroscopic system.

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