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.
Chu
Ivan K
IK
Department of Chemistry, University of Hong Kong, Pokfulam Road, Hong Kong, China. ivankchu@hkucc.hku.hk
Zhao
Junfang
J
Xu
Minjie
M
Siu
Shiu On
SO
Hopkinson
Alan C
AC
Siu
K W Michael
KW
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