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

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Abstract
Some α-amino acids, especially arginine, form protonated clusters when examined by electrospray ionization in an ion trap mass spectrometer. Singly-, doubly-, triply- and quadruply-protonated arginine clusters [(Arg)n + H]+, [(Arg)m + 2H]+2, [(Arg)l + 3H]+3 and [(Arg)k + 4H]+4, were further studied by collision-induced dissociation (CID). The singly-protonated 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 resemblance to the simple salt cluster [(NaCl)4 + Na]+, a 3 × 3 × 1 micro-crystallite. This leads to the suggestion that [(Arg)4 + H]+ is planar, with bonding primarily due to the electrostatic interactions between four zwitterionic arginine molecules. In the doubly-charged ion series, clusters of m = 12–15 have enhanced stability relative to those of immediately smaller size. Drawing on the analogous salt structures, the dication, [(Arg)12 + 2H]+2 might have a structure consisting of three layers of tetramers, two of which are protonated. This structure is analogous to that of the magic number doubly-charged ionic cluster [(NaCl)12 + 2Na]+2 which is a 3 × 3 × 3 micro-crystallite with an internal anion defect.

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