FIGURE 5. MALDI-TOF-Q-TOF sequencing of Asp-N digestion fragments.

A, m/z 774 ion from Aβ42 (Fig. 4A) yielded product ions consistent with Aβ-(1–6), including a His immonium ion at m/z 110. B, m/z 930 ion from HNE-treated Aβ42 (Fig. 4B) yielded product ions suggesting that HNE has modified one residue in Aβ-(1– 6) by Michael addition and added 156 atomic mass unit to its mass. Ions that appear to be shifted by 156 atomic mass units for this reason are marked with an asterisk. The presence of an unmodified b₅ ion and the appearance of a new ion at m/z 266 as expected for an HNE-His immonium ion indicate that the residue modified by HNE is His-6. The origin of the y₁ + 138 ion is discussed in the text.