Fig. 5.

Nitroxide line shape and local side chain environment. A plot of the reciprocal of the central linewidth (ΔH₀⁻¹) versus the reciprocal of the second moment (〈H²〉⁻¹) of the EPR spectrum for sites in the FhuA Ton box (●), the FecA Ton box without (■) and with (□) ferric citrate, and in the switch helix of FhuA without (▴) and with (▵) ferrichrome. Highly mobile regions having narrow line widths, and low second moments are unstructured. Other regions are defined based on earlier work on spin-labeled annexins and T4 lysozyme (23, 29) and are defined as follows. Loop/surface sites are on flexible regions of proteins. Loop/contact sites are on flexible loops that are in tertiary contact. Contact sites are in regions of defined secondary structure with some surface exposure but in tertiary contact. Buried sites are within the protein interior having no surface exposure. Selected residues are indicated. Embossed numbers indicate sites in the presence of substrate.