Macrocyclic effect of auxiliary ligand on the gas-phase dissociation of ternary copper(II)–GGX complexes
Article first published online: 7 FEB 2006
Copyright © 2006 John Wiley & Sons, Ltd.
Rapid Communications in Mass Spectrometry
Volume 20, Issue 5, pages 790–796, 15 March 2006
How to Cite
Lam, C. N. W., Siu, S. O., Orlova, G. and Chu, I. K. (2006), Macrocyclic effect of auxiliary ligand on the gas-phase dissociation of ternary copper(II)–GGX complexes. Rapid Commun. Mass Spectrom., 20: 790–796. doi: 10.1002/rcm.2366
- Issue published online: 7 FEB 2006
- Article first published online: 7 FEB 2006
- Manuscript Accepted: 29 DEC 2005
- Manuscript Revised: 22 DEC 2005
- Manuscript Received: 7 NOV 2005
- University of Hong Kong
- Hong Kong Research Grants Council, Special Administrative Region, China. Grant Numbers: HKU 7019/05P, HKU 7041/03P
Previous studies into the dissociation of [CuII(dien)peptide].2+ ions (dien = diethylenetriamine) have shown that NH-containing auxiliary ligands do not favor the formation of [peptide].+ species; instead, they promote proton-transfer reactions, especially for peptides containing basic amino residues. Formation of radical cationic tripeptides of the form GGX.+ [GGX = glycylglycyl(residue X)] becomes feasible upon substituting the open-chain tridentate ligand dien with its analogous cyclic ligand, 1,4,7-triazacyclononane (9-aneN3); i.e., from [CuII(9-aneN3)GGX].2+ ions. Similar enhancements occur when using 1,4,7,10-tetraoxacyclododecane (12-crown-4) in place of its open-chain analog, 2,5,8,11-tetraoxadecane (triglyme). We have demonstrated that a sterically encumbered auxiliary macrocyclic ligand within [CuII(L)GGX].2+ complex ions [where L = 9-aneN3 or 12-crown-4] facilitates the formation of radical cationic peptides through gas-phase fragmentation. We verified our experimental observations by examining the reactivities of a series of 19 tripeptides of the type GGX that differ only in the identity of their C-terminal residue. The energy of the electron-transfer reaction correlates well with the bond-dissociation energy of the peptide–Cu(II) interaction; the presence of a constrained macrocyclic ligand weakens metal–peptide chelation through steric repulsion between the ligand and the peptide, and this situation may lead to more favorable radical cationic peptide formation. Copyright © 2006 John Wiley & Sons, Ltd.