Magnetic circular dichroism

Abstract

Circular dichroism (CD) induced in an electronic transition by an external magnetic field has recently been employed to investigate the nature of chromophores found in biological systems, most notably the tryptophanyl chromophores of proteins and the Co(II), Fe and Fe-porphyrin chromophores of several metalloproteins. Magnetic circular dichroism (MCD) might better be referred to as the molecular Zeeman effect or Faraday effect, since MCD invites direct comparison to natural CD, a comparison which in most respects is misleading. MCD was first observed by Michael Faraday in 1845 and arises from the splitting by the field of magnetically degenerate ground or excited states. Much of the modern theory has been presented by Buckingham and Stephens [ 1,2]. MCD is generated by several mechanisms and one transition can potentially give rise to a complex, often biphasic CD spectrum. This is in marked contrast to natural CD where one gaussian CD band is associated with each optically active transition. When the Zeeman splitting is small compared to the linewidth and at temperatures where the Zeeman energies are < kT, MCD bands can be shown to arise from contributions to net ellipticity by three separate terms, the so-called A, B and C terms. A brief qualitative description of these is appropriate before discussing specific applications of MCD to biologically important chromophores. Splitting of a magnetically degenerate ground state by the magnetic field results in a population asymmetry between the new ground states, since the lower energy state will be preferentially occupied. Hence the transition from the lower state will be more intense and absorption of circularly polarized light of one sense by the lower energy transition will not be completely balanced by absorption of light of the opposite polarization by the adjacent transition of slightly higher energy. Neglecting for the moment the frequency shift, a net gaussian CD band will result, the C term, positive or negative depending on the selection rules. The population asymmetry is temperature dependent, hence the magnitude of bands due to C terms will increase at lower temperatures. Even in the absence of the-population asymmetry (e.g. when only the excited state is split by the field) cancellation of equal and opposite ellipticity from two magnetically degenerate transitions cannot occur, since there is a frequency shift between them. A net biphasic CD will remain, the A term. The frequency shift is proportional to the field strength but independent of temperature; hence, A terms are not temperature dependent. A third mechanism by which the magnetic field can induce ellipticity is by field-induced mixing of the zero-Iield states of the chromophore, the B term. This depends on the details of the particular energy levels characterizing the chromophore and delineation of this term requires a complete spectral analysis. B terms are gaussian and temperature independent, but the magnitude may vary inversely with field strength. Significant contributions from all three terms need not be present in a particular MCD spectrum and before detailed analysis can be made, it is important to sort out the contributions of the various terms, a potentially complex problem. Determination of the temperature dependence is essential and will reveal the contribution of C terms, Applications of MCD to biological systems from which useful information can be derived will depend in great measure on the specific nature of the chromophore and on what type of information is sought. The molecular Zeeman effect is insensitive to conformation. In contrast natural CD is highly sensitive to conformation and derives its great usefulness as well as its complexity of interpretation from its dependence on the dissymmetric potential N 159