Peptide derivatization as a strategy to form fixed-charge peptide radicals

Authors

  • Asimo Karnezis,

    1. School of Chemistry, The University of Melbourne, Victoria 3010, Australia
    2. ARC Centre of Excellence – Centre for Free Radical Chemistry and Biotechnology, The University of Melbourne, Victoria 3010, Australia
    3. Bio21 Institute of Molecular Science and Biotechnology, The University of Melbourne, Victoria 3010, Australia
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  • Christopher K. Barlow,

    1. School of Chemistry, The University of Melbourne, Victoria 3010, Australia
    2. ARC Centre of Excellence – Centre for Free Radical Chemistry and Biotechnology, The University of Melbourne, Victoria 3010, Australia
    3. Bio21 Institute of Molecular Science and Biotechnology, The University of Melbourne, Victoria 3010, Australia
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  • Richard A. J. O'Hair,

    Corresponding author
    1. School of Chemistry, The University of Melbourne, Victoria 3010, Australia
    2. ARC Centre of Excellence – Centre for Free Radical Chemistry and Biotechnology, The University of Melbourne, Victoria 3010, Australia
    3. Bio21 Institute of Molecular Science and Biotechnology, The University of Melbourne, Victoria 3010, Australia
    • School of Chemistry, The University of Melbourne, Victoria 3010, Australia.
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  • W. David McFadyen

    1. School of Chemistry, The University of Melbourne, Victoria 3010, Australia
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  • Part 50 of the series ‘Gas Phase Ion Chemistry of Biomolecules’.

Abstract

As a means of generating fixed-charge peptide radicals in the gas phase we have examined the collision-induced dissociation (CID) chemistry of ternary [CuII(terpy)(TMPP-M)]2+ complexes, where terpy = 2,2′:6′2″-terpyridine and TMPP-M represents a peptide (M) modified by conversion of the N-terminal amine to a [tris(2,4,6-trimethoxyphenyl)phosphonium]acetamide (TMPP-) fixed-charge derivative. The following modified peptides were examined: oligoglycines, (Gly)n (n = 1–5), alanylglycine, glycylalanine, dialanine, trialanine and leucine-enkephaline (YGGFL). The [CuII(terpy)(TMPP-M)]2+ complexes are readily formed upon electrospray ionization (ESI) of a mixture of derivatized peptide and [CuII(terpy)(NO3)2] and generally fragment to form transient peptide radical cations, TMPP-M+., which undergo rapid decarboxylation for the simple aliphatic peptides. This is contrasted with the complexes containing the unmodified peptides, which predominantly undergo fragmentation of the coordinated peptide. These differences demonstrate the importance of proton mobility in directing fragmentation of ternary copper(II) peptide complexes. In the case of leucine-enkephaline, a sufficient yield of the radical cation was obtained to allow further CID. The TMPP-YGGFL+. ion showed a rich fragmentation chemistry, including CO2 loss, side-chain losses of an isopropyl radical, 2-methylpropene and p-quinomethide, and *a1 and *a4 sequence ion formation. In contrast, the even-electron TMPP-YGGFL+ ion fragments to form *an and *bn sequence ions as well as the [*b4 + H2O]+ rearrangement ion. Copyright © 2006 John Wiley & Sons, Ltd.

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