Arginine clusters generated by electrospray ionization and identified by tandem mass spectrometry

Some alpha-amino acids, especially arginine, form protonated clusters when examined by electrospray ionization in an ion trap mass spectrometer, Singl y-, doubly-, triply- and quadruply-protonated arginine clusters [(Arg), + H ](+), [(Arg)(n) + 2H](+2), [(Arg)(l) + 3H](+3) and [(Arg)(k) + 4H](+4), wer e further studied by collision-induced dissociation (CID). The singly-proto nated cluster n = 4 displayed enhanced stability and CID of larger clusters (n > 4) showed fragmentation leading to the preferential formation of n = 4 product ions. The n = 4 Stable cluster is proposed to bear a formal resem blance to the simple salt cluster [(NaCl)(4) + Na](+), a 3 x 3 x 1 micro-cr ystallite. This leads to the suggestion that [(Arg)(4) + H](+) is planar, w ith bonding primarily due to the electrostatic interactions between four zw itterionic arginine molecules. In the doubly-charged ion series, clusters o f m = 12-15 have enhanced stability relative to those of immediately smalle r size. Drawing on the analogous salt structures, the dication, [(Arg)(12) + 2H](+2) might have a structure consisting of three layers of tetramers, t wo of which are protonated. This structure is analogous to that of the magi c number doubly-charged ionic cluster [(NaCl)(12) + 2Na](+2) which is a 3 x 3 x 3 micro-crystallite with an internal anion defect.