We have tested the role of the polar loop of subunit c of the Escherichia coli ATP synthase in stabilizing the hairpin structure of this protein. The structure of the c(32-52) peptide corresponding to the cytoplasmic region of subunit c bound to the dodecylphosphocholine micelles was solved by high-resolution NMR. The region comprising residues 41-47 forms a well-ordered structure rather similar to the conformation of the polar loop region in the solution structure of the full-length subunit c and is flanked by short alpha-helical segments. This result suggests that the rigidity of the polar loop significantly contributes to the stability of the hairpin formed by the two helices of subunit c. This experimental system may be useful for NMR studies of interactions between subunit c and subunits gamma and epsilon, which together form the rotor of the ATP synthase.