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 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 [y₁₀+ 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 + (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.