Analysis of dimer and trimer complexes of the non-amyloidogenic rat islet amyloid polypeptide 21-37 by electrospray ionization-tandem mass spectrometry.

The dimer and trimer structures of the non-amyloidogenic rat islet amyloid polypeptide 21-37 peptide, formed in an H2O/CH3OH (1% CH3COOH) solution were investigated using electrospray ionization-tandem mass spectrometry (ESI-MS/MS). The dissociation of monomers, dimers, and trimers was investigated by MS/MS using collision-induced dissociation. The peptide bond dissociation between L7 and P8 was mainly observed in the tandem mass spectra of the monomers and oligomers, regardless of the parent ion charge state. The fragment ions were observed as a series of bu (u = 3-4, 6-7, 12) or yn (n = 10-11, 13-14) in the [Mono + 2H]2+ (=[monomer + 2H]2+) tandem mass spectrum. MS/MS analysis of the [Di + 3H]3+ (=[dimer + 3H]3+) complex indicated that [Di + 3H]3+ comprised [Mono + H]1+ and [Mono + 2H]2+ subunits. During covalent bond dissociation of the [Di + 3H]3+ complex, a fragmentation pattern was observed in the form of {mono + (fragment ion of [Mono + 2H]2+)}, resulting from the collision energy dissociation of the [Mono + 2H]2+ peptide. The [(C-terminal)-(C-terminal)] interaction geometry was proposed for the [Di + 3H]3+ complex based on the observation of [y10 + yn]2+ (n = 10-11, 13-16) fragment ions in the [Di + 3H]3+ tandem mass spectrum. MS/MS analysis of the [Tri + 4H]4+ (=[trimer + 4H]4+) complex indicated that [Tri + 4H]4+ comprised [Mono + H]1+ and [Di + 3H]3+ subunits. The (monomer-[Di + 3H]3+)4+ complex geometry was assumed to be stable based on the presence of {mono + (fragment ion of [Di + 3H]3+)} ions in the tandem mass spectrum of the [Tri + 4H]4+ complex. The two [Mono + (y10 + y10)]2+ and [Mono + (Mono + y10)]3+ fragment ions also supported the (monomer-[Di + 3H]3+)4+ complex geometries of the [Tri + 4H]4+ complex. The [(C-terminal)-(C-terminal)] interaction geometry of the [Di + 3H]3+ subunit is thought to be conserved in the [Tri + 4H]4+ complex geometries.