Applications of MCD spectroscopy

It was quickly recognized that a nonrelativistic formulation of MCD led to predictions that were qualitatively wrong for some paramagnetic metal halides (18-20). The introduction of spin-orbit coupling created addition contributions to the C term parameters and produced predictions in qualitative agreement with experiment. It was not until some time later that general formulations of the new terms caused by spin-orbit coupling appeared (21-23). The spin-orbit-induced C terms have played an important role in modem applications of MCD spectroscopy as they have proven to be extremely useful in the study of metalloen-zymes (15,24-26). 

In recent years, some of the most impressive applications of MCD spectroscopy have come in the area of metalloenzymes... 

Another spectroscopic application of MCD spectroscopy is important for highly symmetrical molecules. The presence of nonzero A terms reveals clearly which transitions are degenerate and which are not. This distinction is in genera] not possible from ordinary absorption spectra, even if no bands overlap. The magnetic moment of the excited state may be derived from the A, value by means of the relationship... 

The technique of X-ray crystallography has been, and will remain, indispensable for the determination of the unusual structures of S—N compounds. A more recent development is the application of N NMR spectroscopy in S—N chemistry. Despite the necessity to employ N-enriched materials for these studies, the judicious application of this technique in both structural determinations and in monitoring the progress of reactions will undoubtedly accelerate the progress of the subject. The advent of MCD spectroscopy and the use of the perimeter model have also enhanced our understanding of the electronic structures of cyclic S—N molecules. Rapid advances in this area are to be expected. 

This pedagogical account is intended to provide a brief introduction for the non-specialist, to the theoretical and experimental aspects of variable temperature MCD spectroscopy that are applicable in the study of metallopro-teins. This is followed by some individual examples of MCD studies of metallo-proteins that have been chosen to illustrate the utility of the technique and the type of information that is available. 

With the advances in experimental techniques and theory, MCD spectroscopy became a viable analytical technique and found application in many areas of chemistry (11-17). The distinct spectral forms of A, B, and C terms and the fact that MCD intensity can be positive or negative make an MCD measurement a useful complement to an absorption spectrum. MCD has been found to be rather more useful than MOR as the latter type of spectrum is generally more difficult to interpret (12). 

Little information was obtained from ORD and MORD measurements concerning structural aspects of erythrocuprein. The application of CD and MCD spectroscopy promised to be a better tool (Figs. 11 and 12). 

The wide range of values of the ratio of MCD to absorption forms the basis of another practical, albeit rarely used, application in biophysical spectroscopy. Molecules with a characteristic MCD spectrum and high MCD/absorption anisotropy in a limited spectral region (heme proteins are a good example) can be detected and quantified in complex mixtures that exhibit continuous absorption and scattering of light across the spectrum, such as crude cell or tissue extracts.This use of MCD is similar to one-beam, two-wavelength absorption difference spectroscopy, which is widely used in studies of metabolism and the biochemistry of heme proteins. ... 

MCD spectroscopy combines the CD experiment with a longitudinal magnetic field, where the application of the magnetic field induces optical activity in any material so that all substances exhibit MCD activity. MCD probes the Zeeman splittings in the ground and excited states and the field-induced mixing between states. 

Magnetic circular dichroism (MCD) spectroscopy is a type of electronic spectroscopy, also called the Faraday effect or the Zeeman effect, that can be a particularly useful and effective method for structural analysis. For example, MCD can be used to assign the transitions in the electronic absorption spectrum (UV-visible), with respect to details such as the molecular orbital origins of the transitions. Often, such transitions are not clearly observed in the UV-visible spectra, because they are spin-forbidden and weak, but upon application of the magnetic field, Hq, they can be detected. MCD spectroscopy can also be used to determine not only the spin state for a metal such iron, but also the coordination number at the metal. 

The analysis of the metal content in MTs includes determination of the number and nature of the bound metal ions and elucidation of the coordination geometry around the different metal centers enfolded by the polypeptide chain. To this end, optical spectroscopies (UV-Vis, c.d. MCD and luminescence) have played a significant role, despite providing information on the predominant species present in solution. Conversely, electrospray mass spectrometry (ES MS) allows determination of the molecular distribution of the various complex species coexisting within the sample. However, in ES MS, the formation of artifacts due to operational features should not be disregarded. A summary of the techniques more commonly used in the study of the metal-binding features of MTs can be found elsewhere. Additionally, relevant information on the application and possible limitations of the X-ray absorption spectroscopy within the study of MTs has been reported. ... 

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