Capillary and Coulombic effects on the gas phase structure of electrosprayed concanavalin A ions and its clusters...

Ion mobility spectrometry (IMS) coupled to mass spectrometry (MS) is used to study the gas phase collision cross section Ω(z, n) in CO(2) of multimers C(n) (n = 1-4, 6) of concanavalin A, whose tetramer C(4) has a crystal structure resembling four tetrahedrically arranged globules. C(n)(+z) ions electrosprayed from aqueous solutions of triethylammonium formate (Et(3)AF) are moderately charged (up to z = 6 and 17 for n = 1 and 6) and produce narrow mobility peaks. Charge states down to z = 1 obtained with a charge-reducing radioactive (63)Ni source are studied for the dimer and the tetramer via pure IMS (no MS). The mobilities are independent of pH in the range 6-8, controlled by addition of triethylamine to the Et(3)AF. The measured compactness group Ω(z, n)/n(2/3) is practically independent of n and z, whereas mobility calculations with clusters of touching spheres show that it should vary with n by 20-30% for a variety of scattering models. This contrast suggests that, irrespective of ambiguities on the scattering model, all multimers adopt globular shapes, precluding in particular a tetrahedral tetramer. Acetic acid solutions (87 mM aqueous) yield ions with substantially higher z, mostly with broad mobility distributions. Exceptionally high z tetramers (z = 25-29) and trimers have narrowly defined mobilities with compact but nonspherical shapes. Addition of 2-4 mM Et(3)AF to the 87 mM aqueous acetic acid solution yields narrowly defined mobilities almost identical at all z values to those from the Et(3)AF buffer, although with higher charge states showing also a transition to nonspherical shapes. We conclude that all gas phase clusters charged below a Rayleigh-like charge, z(R), are globular without regard to solution conditions, some undergoing a sharp shape transition at a critical z = z(R). We confirm that gas phase protein cross sections differ from those expected from the crystal structure, with a trend to compact probably driven by their high surface energy (and opposed by Coulombic stresses). The Rayleigh-like shape transitions seen are similar to those arising in linear homopolymers, although not as sharply defined. They yield a surface energy for protein matter almost as high as the surface tension of water. This quantitative conclusion is corroborated by prior data on cytochrome c and apomyoglobin (also showing a critical shape transition) as well as measurements of the maximum charge versus mass in aggregates of dipeptides.