ELECTRONIC ABSORPTION-SPECTROSCOPY OF NONHEME IRON PROTEINS

Electronic absorption spectroscopy is a technique that has been used extensively to examine nonheme iron proteins. Two types of electronic transitions can, in principle, be expected to be observed in any given system, namely, charge transfer and ligand field transitions. The former typically results from a photo-induced transfer of an electron from a ligand to iron (ligand-to-metal charge transfer or LMCT) and is characterized by extinction coefficients greater than 1000 M-1 cm-1. The latter arises from transitions within the d-orbital manifold (d-d transitions) and are typically much less intense (e < 500 M-1 cm-1), owing to their intrinsically forbidden character. As a result, d-d transitions are much more difficult to observe than LMCT transitions, and they usually require the use of specialized techniques, such as low temperature optical spectroscopy, circular dichroism, or magnetic circular dichroism, often also at low temperature. Successful application of these techniques can, however, result in a detailed picture of the geometric and electronic structure of the iron site in a given protein—for example, studies by Solomon on the binuclear iron protein hemerythrinl-3. Because of the general need for specialized techniques to observe d-d transitions, the focus of this chapter is on the LMCT transitions that are readily observed using simple electronic absorption spectroscopy. Examination of the LMCT bands in a particular system can provide a substantial amount of information about the iron chromophore. © 1993, Academic Press, Inc.