Laser-Induced Liquid Bead Ion Desorption Mass Spectrometry: An Approach to Precisely Monitor the Oligomerization of the β-Amyloid Peptide

In the present work, the recently developed laser-induced liquid bead ion desorption mass spectrometry (LILBID MS) is applied as a novel technique to study A beta oligomerization, thought to be crucial in Alzheimer's disease (AD). The characterization of the earliest nucleation events of this peptide necessitates the application of several techniques to bridge the gap between small oligomers and large fibrils. We precisely monitored in time the transformation of monomeric A beta (1-42) into oligomeric A beta(n) (n < 20) and its dependence on concentration and agitation. The distribution shows signs of the hexamer being crucial in the assembly process. The intensity of the monomer decreases in time with a time constant of about 9 h. After a lag time of around 10 h, a phase transition occurred in which the total ion current of the oligomers goes to nearly zero. In this late stage of aggregation, protofibrils are formed and mass spectrometry is no longer sensitive. Here fluorescence correlation spectroscopy (FCS) and transmission electron microscopy (TEM) are complementary tools for detection and size characterization of these large species. We also utilized the oligomers of A beta (1-42) as a model of the corresponding in vivo process to screen the efficacy and specificity of small molecule inhibitors of oligomerization. The LILBID results are in excellent agreement with condensed phase behavior determined in other studies. Our data identified LILBID MS as a powerful technique that will advance the understanding of peptide oligomerization in neurodegenerative diseases and represents a powerful tool for the identification of small oligomerization inhibitors.