Abstract

The copper ion, Cu(II), binding sites for amyloid fragment Aβ1-16 (=Aβ16 ) were investigated to explain the biolog- ical activity difference in the Aβ16 aggregation process. The [M+Cu+(z-2)H] z+ (z = 2, 3 and 4, M = Aβ16 monomer) and [D+Cu+(z-2)H] z+ (z = 3 and 5, D = Aβ16 dimer) structures were investigated using electrospray ionization (ESI) mass spectrom- etry (MS) and tandem mass spectrometry (MS/MS). Fragment ions of the [M+Cu+(z-2)H] z+ and [D+Cu+(z-2)H] z+ complexes were observed using collision-induced dissociation MS/MS. Three different fragmentation patterns (fragment “a”, “b”, and “y” ion series) were observed in the MS/MS spectrum of the (Aβ16 monomer or dimer—Cu) complex, with the “b” and “y” ion series regularly observed. The “a” ion series was not observed in the MS/MS spectrum of the [M+Cu+2H] 4+ complex. In the non-covalent bond dissociation process, the [D+Cu+3H] 5+ complex separated into three components ([M+Cu+H] 3+ , M 3+ , and M 2+ ), and the [M+Cu] 2+ subunit was not observed. The {M + fragment ion of [M+Cu+H] 3+ } fragmentation pattern was observed during the covalent bond dissociation of the [D+Cu +3H] 5+ complex. The {M + [M+Cu+H] 3+ } complex geometry was assumed to be stable in the [D+Cu+3H] 5+ complex. The {M + fragment ion of [M+Cu] 2+ } fragmentation pattern was also observed in the MS/MS spectrum of the [D+Cu+H] 3+ complex. The {M + [y 9 +Cu] 1+ } fragment ion was the characteristic fragment ion. The [D+Cu+H] 3+ and [D+Cu+3H] 5+ complexes were likely to form a monomer-monomer-Cu (M-M-Cu) structure instead of a monomer-Cu-monomer (M-Cu-M) structure.