FIGURE 6. MALDI-TOF-Q-TOF sequencing of Asp-N digestion fragments.

A, m/z 1906 ion from Aβ42 (Fig. 4A) yielded product ions consistent with Aβ-(7–22), including a His immonium ion at m/z 110. B, m/z 2200 ion from HNE-treated Aβ42 (Fig. 4B) yielded an ion at m/z 266 as expected for an HNE-His immonium ion. There is no evidence that ions smaller than b₇ or y₈ are shifted, but larger ions apparently shifted by 138 atomic mass unit are marked with an “a,” by 156 atomic mass unit with a “b,” and by 294 (= 138 + 156) with a “c.” These data suggest that His-13 and His-14 are both modified by HNE. The nature of these modifications is discussed in the text.