A 1:1 ratio of Aβ16–22/Aβ*16–22 or Aβ*16–22/Aβ40 (40 μM total peptide concentration) in 100 mM ammonium bicarbonate buffer (pH 7.4) with a final concentration of 1% (v/v) DMSO was incubated in Eppendorf tubes for either 5 min or 24 hours. Samples were then irradiated for 30 s using a homebuilt light-emitting diode lamp at 365 nm, then removed, lyophilized overnight, taken up in HFIP for at least 2 hours, and vortexed to ensure that any aggregates were disrupted. HFIP was then removed under a stream of N2, and the sample was resuspended in 50:50 (v/v) MeCN/H2O + 0.05% formic acid to a final concentration of ~40 μM. Any cross-links were then analyzed using the method previously described and the ESI-IMS-MS system as described above (16).