Bis(sulfo-N-succinimidyl) [15N,2H16]doxyl-2-spiro-4'-pimelate, a stable isotope-substituted, membrane-impermeant bifunctional spin label for studies of the dynamics of membrane proteins: application to the anion-exchange channel in intact human erythrocytes.

We have synthesized and characterized an isotopically substituted homologue of the membrane-impermeant bifunctional spin label bis(sulfo-N-succinimidyl) doxyl-2-spiro-4'-pimelate (BSSDP) [Beth et al. (1986) Biochemistry 25, 3824-3832] in which the nitroxide N is substituted with 15N and all of the protons in the doxylpimelate moiety are replaced by deuterons ([15N,2H16]BSSDP). Like its normal isotope homologue, [15N,2H16]BSSDP reacts with the anion-exchange channel in intact human erythrocytes at a site that spans the single extracytoplasmic chymotryptic cleavage site and that overlaps the stilbenedisulfonate site. The narrower line widths in the EPR spectrum of [15N,2H16]BSDP-labeled anion channels allow calculation of a minimum separation of 16 A between spin labels bound at the functionally important stilbenedisulfonate sites on adjacent subunits of an anion channel dimer. The 15N and 2H isotopic substitutions also provide substantial improvement in signal to noise of motionally sensitive regions of the ST-EPR spectrum of [15N,2H16]BSSDP-labeled anion channels in intact erythrocytes. [15N,2H16]BSSDP-labeled anion channels in intact erythrocytes were cross-linked to covalent dimers in the extracytoplasmic domain with the membrane-impermeant cross-linking reagent bis(sulfo-N-succinimidyl) suberate [Staros (1982) Biochemistry 21, 3950-3955], and the saturation-transfer EPR spectrum of these cells was compared with that of cells treated with [15N,2H16]BSSDP but not subsequently cross-linked. The spectra were essentially identical, supporting the hypothesis that anion channel subunits form stable dimers in the membranes of intact erythrocytes.