A LIGAND-FIELD MODEL FOR MCD SPECTRA OF BIOLOGICAL CUPRIC COMPLEXES

A ligand field calculation of magnetic circular dichroism (MCD) spectr a is described that provides new insights into the information contain ed in electronic spectra of copper sites in metalloenzymes and synthet ic analogs. The ligand field model uses metal-centered p- and f-orbita ls to model sigma, pi LMCT mixing mechanism for intensity, allowing th e basic features of optical absorption, MCD, and electron paramagnetic resonance spectra to be simultaneously computed from a single set of parameters and the crystallographically determined ligand coordinates. We have used the model to predict changes in spectra resulting from t he transformation of electronic wavefunctions under systematic variati on in geometry in pentacoordinate ML(5) complexes. The effectiveness o f the calculation is demonstrated for two synthetic copper model compo unds and a galactose oxidase enzyme complex representing limiting coor dination geometries. This analysis permits immediate recognition of ch aracteristic patterns of MCD intensity and correlation with geometry. A complementarity principle between MCD and CD spectra of transition m etal complexes is discussed.