Comparing the gas-phase fragmentation reactions of protonated and radical cations of the tripeptides GXR

Abstract

Electrospray ionization (ESI) mass spectrometry of methanolic solutions of mixtures of the copper salt (2,2′:6′,2″-terpyridine)copper(II) nitrate monohydrate ([Cu(II)(tpy)(NO 3) 2]·H 2O) and a tripeptide GXR (where X = 1 of the 20 naturally occurring amino acids) yielded [Cu(II)(tpy)(GXR)] 2+ ions, which were then subjected to collision induced dissociation (CID). In all but one case (GRR), these [Cu(II)(tpy)(GXR)] 2+ ions fragment to form odd electron GXR + radical cations with sufficient abundance to examine their gas-phase fragmentation reactions. The GXR + radical cations undergo a diverse range of fragmentation reactions which depend on the nature of the side chain of X. Many of these reactions can be rationalized as arising from the intermediacy of isomeric distonic ions in which the charge (i.e. proton) is sequestered by the highly basic arginine side chain and the radical site is located at various positions on the tripeptide including the peptide back bone and side chains. The radical sites in these distonic ions often direct the fragmentation reactions via the expulsion of small radicals (to yield even electron ions) or small neutrals (to form radical cations). Both classes of reaction can yield useful structural information, allowing for example, distinction between leucine and isoleucine residues. The gas-phase fragmentation reactions of the GXR + radical cations are also compared to their even electron [GXR+H] + and [GXR+2H] 2+ counterparts. The [GXR+H] + ions give fewer sequence ions and more small molecule losses while the [GXR+2H] 2+ ions yield more sequence information, consistent with the ‘mobile proton model’ described in previous studies. In general, all three classes of ions give complementary structural information, but the GXR + radical cations exhibit a more diverse loss of small species (radicals and neutrals). Finally, links between these gas-phase results and key radical species derived from amino acids, peptides and proteins described in the literature are made.