Conformational changes within the subunit b-dimer of the E. coli ATP synthase occur upon binding to the F1 sector. ESR spectra of spin-labeled b at room temperature indicated a pivotal point in the b-structure at residue 62. Spectra of frozen b ± F1 and calculated interspin distances suggested that where contact between b2 and F1 occurs (above about residue 80), the structure of the dimer changes minimally. Between b-residues 33 and 64 inter-subunit distances in the F1-bound b-dimer were found to be too large to accommodate tightly coiled coil packing and therefore suggest a dissociation and disengagement of the dimer upon F1-binding. Mechanistic implications of this “bubble” formation in the tether domain of ATP synthase b2 are discussed.