Chaperonin-10s possess a highly flexible segment of approximately 10 residues that covers their dome-like structure and closes the central cavity of the chaperonin assembly. The dome loop is believed to contribute to the plasticity of their oligomeric structure. We have exploited the presence of a single tryptophan residue occurring in the dome loop of Mycobacterium tuberculosis chaperonin-10 (cpn-10), and through intrinsic fluorescence measurements show that in the absence of metal ions, the tryptophan is almost fully solvent exposed at neutral pH. The dome loop, however, assumes a closed conformation in the presence of metal ions, or at low pH. These changes are fully reversed in the presence of chelating agents such as EDTA, confirming the role of cations in modulating the metastable states of cpn-10.