Are the radical centers in peptide radical cations mobile? The generation, tautomerism, and dissociation of isomeric alpha-carbon-centered triglycine radical cations in the gas phase.
The mobility of the radical center in three isomeric triglycine radical cations[G(*)GG](+), [GG(*)G](+), and [GGG(*)](+) has been investigated theoretically via density functional theory (DFT) and experimentally via tandem mass spectrometry. These radical cations were generated by collision-induced dissociations (CIDs) of Cu(II)-containing ternary complexes that contain the tripeptides YGG, GYG, and GGY, respectively (G and Y are the glycine and tyrosine residues, respectively). Dissociative electron transfer within the complexes led to observation of [Y(*)GG](+), [GY(*)G](+), and [GGY(*)](+); CID resulted in cleavage of the tyrosine side chain as p-quinomethide, yielding [G(*)GG](+), [GG(*)G](+), and [GGG(*)](+), respectively. Interconversions between these isomeric triglycine radical cations have relatively high barriers (> or = 44.7 kcal/mol), in support of the thesis that isomerically pure [G(*)GG](+), [GG(*)G](+), and [GGG(*)](+) can be experimentally produced. This is to be contrasted with barriers < 17 kcal/mol that were encountered in the tautomerism of protonated triglycine [Rodriquez C. F. et al. J. Am. Chem. Soc. 2001, 123, 3006-3012]. The CID spectra of [G(*)GG](+), [GG(*)G](+), and [GGG(*)](+) were substantially different, providing experimental proof that initially these ions have distinct structures. DFT calculations showed that direct dissociations are competitive with interconversions followed by dissociation.
Department of Chemistry, University of Hong Kong, Pokfulam Road, Hong Kong, China. ivankchu@hkucc.hku.hk
Ivan K
IK
Chu
Junfang
J
Zhao
Minjie
M
Xu
Shiu On
SO
Siu
Alan C
AC
Hopkinson
K W Michael
KW
Siu
D002244:Carbon / Q000737:chemistry*; D002412:Cations; D005609:Free Radicals / Q000737:chemistry; D005740:Gases / Q000737:chemistry*; D009842:Oligopeptides / Q000737:chemistry*; D010455:Peptides / Q000737:chemistry*; D044367:Phase Transition; D013237:Stereoisomerism
D002412:Cations; D005609:Free Radicals; D005740:Gases; D009842:Oligopeptides; D010455:Peptides; D002244:Carbon; C035647:glycyl-glycyl-glycine