Why do GroEL ions exhibit two gas phase conformers?

The reasons for the existence of two gas phase conformers for electrosprayed ions of the large tetradecameric protein complex GroEL are considered. Key features are that: (1) both conformers extrapolate to very similar cross sections in the limit of zero charge; (2) both conformers supercharge above the maximum value permitted by the Rayleigh criterion; and (3) one of the conformers supercharges substantially more than the other. We hypothesize that the supercharging observed is associated to the approximately cylindrical shape of GroEL in aqueous solution, enabling accumulation of extra charge on either one or two of its bases when they intersect the evaporating drop surface. The two conformers would then correspond to cases when either one or the two bases of the cylinder carry extra charge. Apparently, the conformer symmetrically (doubly) supercharged on both ends is Coulombically stretched, therefore exhibiting a mobility different from the conformer asymmetrically supercharged only on one end. Several general consequences follow. First, non-spherical proteins may generally be charged above the usual Rayleigh limit. Second, we confirm the previously advanced but contentious notion that gas phase protein ions are readily compacted or stretched away from their crystal structure by capillary and Coulombic forces during the electrospraying process.