Aggregates of singly protonated peptides formed with a nanoelectrospray ion source have been observed in the gas phase using Fourier transform ion cyclotron resonance (FT-ICR). Employment of 'soft' ion sampling conditions in the source, which were developed previously to generate water clusters of biomolecules, provides significant yields of aggregates of singly protonated GGDPG ([2GGDPG + 2H]2+), GGEPG ([2GGEPG + 2H]2+), and VEPIPY (2VEPIPY + 2H]2+). With peptide mixtures, heteroaggregates, e.g., [GGDPG + GGEPG + 2H]2+ have also been observed along with the homoaggregates. These weakly bound noncovalent complexes undergo facile exothermic dissociation into the corresponding singly protonated monomer species with normal operation of the electrospray ion source. For example, the aggregates were not observed in FT-ICR experiments utilizing a conventional electrospray ionization (ESI) or fast atom bombardment source or with a quadrupolar ion trap mass spectrometer equipped with a conventional ESI source. The formation and metastability of these aggregates are dependent on highly specific intermolecular hydrogen bonding between the monomers. The amino acid sequence (DPG) of GGDPG mimics the well-known β reverse turn of proteins and semiempirical calculations show that it provides excellent hydrogen bonding sites for a protonated N-terminus amino group. Support for this conjecture is provided by the failure to observe aggregate formation of singly protonated peptides with several larger peptides, including hexaglycine and hexaalanine.
|Number of pages
|Journal of the American Society for Mass Spectrometry
|Published - 1999
Bibliographical noteFunding Information:
This work was supported in part by the National Science Foundation under Grant CHE-9727566. Funds for instrument development have been provided by ARPA and the DOD-URI program (ONR-N0014-92-J-1901). We are also indebted to the Beckman Foundation and Institute for the initial funding and continuing support of the research facilities.
ASJC Scopus subject areas
- Structural Biology