Sensing Tryptophan Microenvironment of Amyloid Protein Utilizing Wavelength-Selective Fluorescence Approach

Structural transition among various forms of proteins involves subtle interplay between structure and dynamics and is crucial in human diseases. Red edge excitation shift (REES) represents a suitable approach to explore the environmental organization and dynamics surrounding tryptophan residues in proteins. Although REES from tryptophan residues has been reported for native, molten globule and denatured states of proteins, such data on the amyloid form of proteins is lacking. κ-casein is one of the most important constituents of casein micelles in milk and has a tendency to form amyloid fibril. We report here REES of the sole tryptophan residue for native, acid-denatured and urea-denatured forms of κ-casein. More importantly, we show that the amyloid form of κ-casein displays REES of 4 nm. We analyze these results in terms of tryptophan microenvironment in various forms of κ-casein, particularly the amyloid form. We conclude that REES is a sensitive tool to monitor structural plasticity in proteins.