Despite reports to the contrary, doubly charged lead monohydrate is a stable species against both proton and charge transfers. [Pb(H(2)O)](2+) has been observed as a minor product in the ligand-exchange reaction of [Pb(CH(3)CN)](2+) with H(2)O after collisional activation. Density functional theory has been used to examine reaction profiles of [Pb(H(2)O)(n)](2+) where n = 1, 2, and 3.

Iminium ions a 2 and a 3 are fragment ions in the gas-phase microsequencing of protonated peptides in proteomics. [1] Protonation of these small a n ions as they are being formed during peptide fragmentation is, in theory, possible but has not hitherto been reported. This absence is presumably because of the Coulombic repulsion encountered by any given mobile proton in the vicinity of the incipient a 2 and a 3 ions, just prior to dissociation.[2] Herein, we report the first observation of a small, dipositive (and hence high-chargedensity) protonated a 3 ion, (a 3 + H) 2+ , and a protonated a 2 ion, (a 2 + H) 2+ , produced through the tandem mass spectrometry of a triply charged lanthanum complex of triglycine (GGG).The fragmentations of metal-ion cationized peptides, predominantly those of monocations (alkali metals, [3] Cu, [4] and Ag [5] ) and dications (Ca, [6] Ni, [7a] Cu, [7a] and Zn [7] ), have aroused much interest. For multicharged complexes, charge reduction by proton transfer is a common channel; [8] consequently, few [M(peptide)] 3+ (M = metal) complexes have been reported.[9] Recently, we observed the ions [La-, in which the peptide had either three or four residues, one of which was an arginine.[9b] The only complexes of peptides having amino acids with hydrocarbon side chains that were observed included solvent molecules, for example, [La(GGG)(CH 3 CN) 2 ] 3+ (Scheme 1). Herein, we exploit the collision-induced dissociation (CID) of this complex, which leads to charge disproportionation and observation of (a 3 + H) 2+ (1), and [LaO(CH 3 CN)] + plus the neutrals CO and CH 3 CN. Ion 1, in turn, eliminates methanimine and CO to give (a 2 + H) 2+ (2). Figure 1 shows , which was mass-shifted to m/z 75 and 73.5, respectively.Density functional theory (DFT) performed by the Gaussian 03 quantum-chemical calculation package [10a] shows that the preferred triglycine conformation in binding to La 3+ is zwitterionic (Scheme 1). The formation of 1 after collisional activation of the complex is facilitated by the high affinity of La for O, which leads to cleavage of the carboxylate moiety and deposition of a second formal positive charge on the imino group of 1. The lowest-energy fragmentation pathway has a barrier of 57.4 kcal mol À1 (Scheme 2). The intermediate ion (b 3 + H) 2+ (3) is not evident in Figure 1, as the barrier to this ion is 52. formed does dissociate efficiently to give 1 (see Figure S1 in the Supporting Information). As anticipated from its high charge density and, therefore, the large intramolecular Coulombic repulsion, the (a 3 + H) 2+ Scheme 1. Lowest-energy structures of [La(GGG)(CH 3 CN) 2 ] 3+ and protonated a 3 (1) and a 2 (2) ions. 3+ (m/z 136.7) at E lab = 30 eV. cps = counts per second.

Ion-molecule reactions between complexes [La(CH3CN)n]3+ (n=6-9) or [La(NC(CH2)4CN)n]3+ (n=3-4) and water were studied at low collision energies in the second quadrupole of a tandem mass spectrometer. The products [La(CH3CN)p(H2O)8-p]3+ (p=6-8) and [La(NC(CH2)4CN)q(H2O)8-2q]3+ (q=3-4) had the highest relative abundances. This strongly suggests that the preferred coordination number of La3+ is eight. Similarly, the coordination number of Ca2+ was re-examined both experimentally and theoretically, and was found to be six, in good agreement with previous observations. Density functional calculations provide strong evidence that the primary solvation shell of [La(L)n]3+ consists of eight ligands; additional ligands reside in a second solvation shell and are hydrogen bonded to one or two water molecules in the first shell.

Gas-phase complexes [La(peptide)] 3+ containing 2-4 amino-acid residues have been investigated by electrospraying solutions containing La 3+ and the peptide; only complexes in which the peptide contained an arginine residue were observed. Using the coordination number of eight for La 3+ [Shi, T.; Hopkinson, A. C.; Siu, K. W. M. Chem. Eur. J. 2007Eur. J. , 13, 1142Eur. J. -1151 and the relative abundances of the hydrates [La(peptide)-(H 2 O) n ] 3+ , the number of binding sites provided by the peptides was deduced: Leu-Trp-Met-Arg, 7; Met-Arg-Phe-Ala, 6; Gly-Arg-Gly, 4; Gly-Gly-Arg 4; and Met-Arg, 4. Density Functional Theory calculations show that the zwitterionic form of Gly-Gly-Arg preferentially binds La 3+ through four coordination sitess the two amide oxygens and the two carboxy oxygens.

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