Structural basis for the polymorphism of β-lactoglobulin amyloid-like fibrils

The whey protein β-lactoglobulin (β-lg) forms amyloid fibrils upon hydrolysis or partial unfolding. The morphology of these fibrils depends on the environmental conditions during formation, including pH, ionic strength, protein concentration and reaction time. Long and straight, as well as short, worm-like β-lg fibrils are observed. The molecular basis of this polymorphism remains poorly understood. In this chapter we review the relationship between the fibril morphology and the peptide molecular structure of amyloids formed from β-lg. Fibril morphologies are typically measured by atomic force microscopy and electron microscopy, and the molecular structure is measured by a variety of spectroscopic techniques, such as Raman and FT-IR spectroscopy. Recent studies have begun to combine spectroscopy with imaging techniques to correlate fibril morphology with the underlying molecular structure. It was shown that straight and rigid fibrils have a substantially higher β-sheet content compared to worm-like fibrils. However, in the future, to elucidate the local molecular structure of peptides within fibrils, newly developed techniques, such as tip-enhanced Raman spectroscopy, will be necessary.