Formation and dissociation of phosphorylated peptide radical cations.

In this study, we generated phosphoserine- and phosphothreonine-containing peptide radical cations through low-energy collision-induced dissociation (CID) of the ternary metal-ligand phosphorylated peptide complexes Cu(II)(terpy)(p)M and Co(III)(salen)(p)M [(p)M: phosphorylated angiotensin III derivative; terpy: 2,2':6',2''-terpyridine; salen: N,N'-ethylenebis(salicylideneiminato)]. Subsequent CID of the phosphorylated peptide radical cations ((p)M(·+)) revealed fascinating gas-phase radical chemistry, yielding (1) charge-directed b- and y-type product ions, (2) radical-driven product ions through cleavages of peptide backbones and side chains, and (3) different degrees of formation of M - H(3)PO(4) species through phosphate ester bond cleavage. The CID spectra of the (p)M(·+) species and their non-phosphorylated analogues featured fragment ions of similar sequence, suggesting that the phosphoryl group did not play a significant role in the fragmentation of the peptide backbone or side chain. The extent of neutral H(3)PO(4) loss was influenced by the peptide sequence and the initial sites of the charge and radical. A preliminary density functional theory study, at the B3LYP 6-311++G(d,p) level of theory, of the neutral loss of H(3)PO(4) from a prototypical model--N-acetylphosphorylserine methylamide--revealed several factors governing the elimination of neutral phosphoryl groups through charge- and radical-induced mechanisms.