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 H 2 O/CH 3 OH (1% CH 3 COOH) 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 L 7 and P 8 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 b u (u = 3-4, 6-7, 12) or y n (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 [y 10  + y n ] 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 + (y 10  + y 10 )] 2+ and [Mono + (Mono + y 10 )] 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.