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

Abstract

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.

Gas‐phase fragmentation of deprotonated tryptophan and its clusters [Trpn–H]– induced by different activation methods

“…Negative ion ESI of a 1 mM solution of tryptophan (solvent: MeOH) yields a series of cluster ions of the type [Trp n –x H] x – , where x = 1–4, as observed in a recent study . However, the present work focuses on singly charged tryptophan cluster, i.e.…”

“…Non‐covalent amino acid clusters have been the subject of intense research over the past 20 years in diverse areas of interest, e.g. the fragmentation of proton‐bound dimers to determine gas‐phase proton affinities or methylating abilities, peptide bond formation within amino acid clusters, formation and ionisation of neutral non‐covalent amino acid complexes, or the formation and fragmentation reactions of multiply charged clusters using various activation methods . Most of the research has focused on positive ions and there have been only a few reports on the study of anionic clusters of amino acids …”

The Solution Assembly of Biological Molecules Using Ion Mobility Methods: From Amino Acids to Amyloid β-Protein

“…This observation stimulated great interest with other research groups focusing on the determination of the structure of the serine octamer (38,86–88) but the detailed structure remains undetermined to the present time. There have been reports of various levels of assembly of other AAs including arginine (89), tryptophan (90), proline (91), tyrosine (92) and phenylalanine (93–95). However, until recently there had been little effort to understand the non-covalent assembly of AA oligomers based on their chemical and physical properties, nor had this information been applied to help understand the aggregation of peptides and proteins (96).…”

Collisional Activation of [14Pro+2H]2+ Clusters: Chiral Dependence of Evaporation and Fission Processes

“…Multiply charged serine 7,21-23 and serine-containing clusters, 24 and clusters of arginine, 25 tryptophan, 26 and the amino acid analog betaine 27-30 show predominant loss of neutrals for fragmentation of larger clusters and fission of smaller clusters with higher charge densities. The fission of multiply charged clusters is used to probe the structure and stability 8-9,30 of amino acid clusters and to distinguish doubly-charged clusters from singly charged clusters with identical m/z .…”