Fragmentation of the tryptophan cluster [Trp9-2H]2- induced by different activation methods

Electrospray ionization (ESI) of tryptophan gives rise to multiply charged, non-covalent tryptophan cluster anions, [Trp(n)-xH](x-), in a linear ion trap mass spectrometer, as confirmed by high-resolution experiments performed on a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The smallest multiply charged clusters that can be formed in the linear ion trap as a function of charge state are: x = 2, n = 7; x = 3, n = 16; x = 4, n = 31. The fragmentation of the dianionic cluster [Trp(9)-2H](2-) was examined via low-energy collision-induced dissociation (CID), ultraviolet photodissociation (UVPD) at 266  nm and electron-induced dissociation (EID) at electron energies ranging from >0 to 30  eV. CID proceeds mostly via charge separation and evaporation of neutral tryptophan. The smallest doubly charged cluster that can be formed via evaporation of neutral tryptophans is [Trp(7)-2H](2-), consistent with the observation of this cluster in the ESI mass spectrum. UVPD gives singly charged tryptophan clusters ranging from n = 2 to n = 9. The latter ion arises from ejection of an electron to give the radical anion cluster, [Trp(9)-2H](-·). The types of gas-phase EID reactions observed are dependent on the energy of the electrons. Loss of neutral tryptophan is an important channel at lower energies, with the smallest doubly charged ion, [Trp(7)-2H](2-), being observed at 19.8  eV. Coulomb explosion starts to occur at 19.8  eV to form the singly charged cluster ions [Trp(x)-H](-) (x = 1-8) via highly asymmetric fission. At 21.8  eV a small amount of [Trp(2)-H-NH(3)](-) is observed. Thus CID, UVPD and EID are complementary techniques for the study of the fragmentation reactions of cluster ions.