Disorder at metal sites in proteins: A high-frequency-EMR study

Abstract

High-frequency EMR of metalloprotein samples offers considerable potential for providing improvements in characterization of metal centers. For those metal ions that are studied at low temperature, line widths may be frequency-dependent, depending on the terms that limit the widths. Here we examine apparent EMR line widths and line shapes in some representative metalloprotein samples at two frequencies: ∼9.2 GHz (X-band) and ∼94 GHz (W-band). Samples of the same size were measured at both frequencies. The samples include cupric ion in lactoferrin, high-spin ferric ion in diferric transferrin and high-spin ferric heme in catalase. For the approximately tenfold increase in EMR frequency, the observed line widths increased tenfold or less for the samples chosen. Distributions in one or more spin Hamiltonian parameters can account for these line width dependences on EMR frequency in the lactoferrin and transferrin samples, while molecular heterogeneity is a likely contributor to the catalase frequency-dependent line width. These measurements over frequencies differing by a factor of ten also contributed new insight for simulation of the EMR spectra of diferric transferrin.