Amyloid
fibrillation is closely associated with a series of neurodegenerative
diseases. According to that, the intermediate soluble oligomers and
protofibrils are more toxic; reducing their concentrations in protein
solutions by accelerating fibrillation is believed as a feasible strategy
for treatment or remission of the diseases. Using hen egg-white lysozyme
(HEWL) as a model protein, the promotion effect of succinimide was
revealed by a series of experiments, e.g., atomic force microscopy
(AFM), thioflavin T (ThT) fluorescence assay, Far-UV circular dichroism
(CD) and Raman spectroscopy, and modeling the effect of succinimide-like
derivative intermediates of intramolecular deamidation of the backbone
during amyloid fibrillation. The AFM measurement confirmed that succinimide
effectively accelerated the morphological changes of HEWL, while at
the molecular level, the accelerative transformation of protein secondary
structures was also clarified by ThT fluorescence assay and Far-UV
CD spectroscopy. The incubation time-dependent Raman spectroscopy
further revealed that the direct transformation from α-helices
to organized β-sheets occurred upon skipping the intermediate
random coils under the action of succinimide. This “bridge”
effect of succinimide was attributed to its special influence on disulfide
bonds. In the presence of succinimide in protein solutions, the native
disulfide bonds of lysozyme could be broken more efficiently and quickly
within hydrolysis, resulting in exposure of the buried hydrophobic
residues and accelerating the formation of cross β-sheet structures.
The present investigation provides very useful information for understanding
the effect of intramolecular deamidation on the whole amyloid fibrillation.