Fluorescence spectroscopy is a frequently used and versatile technique for the analysis of higher order structures of proteins.
Intrinsic fluorescence of proteins in solution is commonly measured in order to detect changes in protein conformation, e.g., due to different solution conditions (pH, excipients, etc.), elevated temperature and storage. Upon excitation at 280 nm, a protein’s fluorescence signal arises primarily from the presence of tryptophan and tyrosine residues. Selective excitation of tryptophan residues, e.g., at 295 nm, is also possible and commonly applied to proteins, because tryptophan fluorescence is particularly sensitive to subtle conformational changes. Tyrosine and tryptophan residues are typically buried in the core of a folded protein (not solvent exposed). Partial unfolding and aggregation lead to changes in the local environment of these residues (e.g., solvent exposure), leading to changes in fluorescence intensity and emission maximum, amongst others.
Extrinsic fluorescence can also be used to detect changes in protein structure and the formation of aggregates. Various fluorescent dyes are available, such as Nile Red, ANS and Bis-ANS, which can be used as hydrophobic probes. If a protein unfolds or aggregates, e.g., due to destabilizing solution conditions or increased temperature, the dye binds to hydrophobic patches on the protein (aggregates) and fluoresces with much greater intensity than free dye in solution.