The separations between aromatic residues in the bait region and nitroxide spin labels attached to the thiol ester-forming residues (Cys949 and Gln952) in human α 2-macroglobulin (α 2M) have been determined from paramagnetic broadening effects of the spin labels on bait region 1H NMR signals. We found that both the Cys949 and Gln952 residues are within 11–17 Å of the aromatic residues in the bait region, with closer approach of some residues to the Gln952 spin label than to the spin label attached to Cys949. A model of the location of bait regions and thiol esters within an α 2M half-molecule is proposed that places the bait regions in the central region of α 2M at the interface between the subunits.
