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.

Assignment of EPR transitions in a manganese-containing lipoxygenase and prediction of local structure

“…Details of the W-band spectrometer were given in ref. 10. Spectra at 94.1 GHz (W-band) were recorded as the average of increasing and decreasing field scans.…”

“…Matrix-diagonalization computer simulations including only the S e = 5/2 transitions, but no hyperfine splitting, were performed on an SGI Unix system with the FORTRAN program developed by Gaffney and Silverstone, with input from Yang, Doctor and Maguire, as described [ 10,11]. The line shapes for transitions between energy levels that have extended regions of separation equal to the microwave energy took into account the special considerations for "looping" transitions [12].…”

“…The figure legends indicate cases in which calculations were performed with XSophe. The calculations in all other figures were performed with the Gaffney-Silverstone [ 10,11] program.…”

High Resolution EPR

“…EPR spectra at 94 GHz (W-band) of diferric transferrin carbonate show this simplification (Fig. 10, experimental (a) and calculated (b)) [18]. The simulation illustrates the broadening of selected transitions (dotted lines above and below the calculated spectrum) that would result from a distribution in E and D .…”

High-field/high-frequency EPR spectrometer operating in pulsed and continuous-wave mode at 180 GHz

“…continuous-wave (CW) [1][2][3][4][5][6] or in the pulsed mode [7][8][9][10][11], reflecting the potential offered by this technique. Applications of HF-EPR range from semiconductor materials [12] and inorganic coupled spin cluster systems [13,14] to transition metal centers [15][16][17][18][19] and organic radicals in proteins [20][21][22][23][24].…”