HIGH-ORDER STRUCTURE AND DISSOCIATION OF GASEOUS PEPTIDE AGGREGATES THAT ARE HIDDEN IN MASS-SPECTRA

Injected-ion mobility and high-pressure ion mobility; techniques have been used to examine the conformations of bradykinin, insulin chain A, and several other peptide ions in the gas phase. Under the experiment al conditions employed, evidence for multimer formation in the mass sp ectra of peptides is minimal or absent altogether. However, ion mobili ty distributions show that aggregates of peptides (containing a single charge per monomer unit) are observed at the same mass-to-charge rati os as the singly charged parent ions. Collision cross sections for the se clusters show that they have tightly packed roughly spherical confo rmations. We have bracketed the average density as 0.87 < p < 1.00 g c m(-3). In some cases, specific stable aggregate forms within a cluster size can be distinguished indicating that some high order structures are favored in the gas phase. Multimer formation between different siz es of polyalanine peptides shows no evidence for size specificity in a ggregate formation. Collisional and thermal excitation studies have be en used to examine structural transitions and dissociation of the mult imers. Aggregates appear to dissociate via loss of singly charged mono mers. The observation that peptide multimers can be concealed in mass spectral data requires that fragmentation patterns and reactivity stud ies of singly charged monomers be undertaken with care. (C) 1998 Ameri can Society for Mass Spectrometry.