Sequence information, distinction and quantitation of C-terminal leucine and isoleucine in ternary complexes of tripeptides with Cu(II) and 2,2-' bipyridine

Tripeptides form ternary complexes with Cu2+ and 2,2 ' -bipyridine (bpy) th at self-assemble upon mixing the components in aqueous methanol solution. E lectrospray ionization (ESI) of the complex solutions provides abundant sin gly charged [Cu(peptide - H)bpy](+) and doubly charged [Cu(peptide)bpy](2+) ions. Collision-induced dissociation (CID) at low ion kinetic energies of several tripeptides, AGG, GGA, LGG, GGL, GGI, FGG, GGF, LGF, GLF, GFL, GYA and GAY, showed fragments that were indicative of the amino acid sequence i n the peptide. In addition, CID of single and doubly charged complexes of i someric tripeptides GGL and GGI provided unambiguous distinction of the iso meric leucine and isoleucine residues. Leucine peptides eliminated C3H7 rad icals from the amino acid side-chain whereas isoleucine eliminated C2H5 rad icals. CID of gas-phase doubly charged peptide complexes in a quadrupole io n trap produced a series of singly charged sequence fragments that followin g isolation and further CID furnished distinct fragments that allowed quant itation of leucine and isoleucine-containing peptides in mixtures. Copyrigh t (C) 2001 John Wiley & Sons, Ltd.