Application of magnetic circular dichroism spectroscopy to porphyrins, phthalocyanines and hemes

Abstract

This review of the application of the MCD spectroscopic technique to porphyrinoids aims to illustrate the features and unique information that can be obtained and used in the assignment of the optical absorption spectrum and also the electronic structures of porphyrins and phthalocyanines. By illustrating each of the major and readily available spectral features with simple compounds we hope this review will act to guide new users in formulating the questions that room temperature MCD measurements can answer. The review is in three parts: (i) a brief introduction to the instrumental and theoretical background with an emphasis on the parameters that can be readily extracted without computational or theoretical analyses. (ii) Illustration of each MCD spectral feature observed in room temperature spectra. The MCD spectra Zn -porphyrins and phthalocyanines, and chlorophyll a are described to provide models for measurements of new compounds. (iii) The use of "finger printing" to determine the oxidation and spin states of the central iron in heme proteins, and to identify the proximal (5th) and distal (6th) iron binding ligands from the MCD spectral envelope observed between 250 nm and 1000 nm. The value of this approach is immeasurable because through room temperature measurement of the MCD spectra from dilute solutions under conditions that allow the heme protein to be stabilized it is possible to identify accurately the proximal and distal ligands. Identifying the heme binding environment for heme proteins newly purified and for heme enzymes during their reaction cycle allows function to be considered long before X-ray structural analysis is available.