Abundant dipositively charged protonated a2 and a3 ions from diproline and triproline

Abundant (a(2) + H)(2+) from diproline and (a(3) + H)(2+) from triproline were observed via collisionally activated charge disproportionation of [La(peptide)(CH(3)CN)(1,2)](3+). These small, dipositive ions with the charges formally located on the peptide backbone are stabilized by charge delocalization onto the pyrrolidine-derived rings, making their formation competitive against other monopositive ions. The (a(2) + H)(2+) and (a(3) + H)(2+) ions from diproline and triproline are major products in contrast with those derived from triglycine, whose unprecedented and surprising observations were recently reported ( Shi et al. Angew. Chem., Int. Ed. 2008 , 47 , 8288 - 8291 ).