Final answer:
Amyloid shows metachromatic staining due to the unique structural properties of its Ab peptide fibers, which cause the dye to change color upon binding. This property is essential for Alzheimer's disease diagnosis through visualization of amyloid plaques in the brain.
Step-by-step explanation:
Amyloid, composed largely of the amyloid beta (Ab) peptide, shows metachromatic staining because of its unique structural properties. Metachromasia occurs when a dye changes color as it binds to certain materials, and in the context of amyloid, this happens due to the dense packing of polymorphic Ab fibers. When subjected to certain stains, such as Congo red or thioflavin, the amyloid plaques, which aggregate from the misfolded Ab peptides that shift from α-helix to β-pleated sheet structures, exhibit a color change. This property is crucial for the diagnosis of diseases such as Alzheimer's disease, where extracellular amyloid plaques are characteristic pathological features. Scans using compounds like Pittsburgh compound B also exploit these properties, binding to plaques in the brain and allowing for visualization via PET scans.