Unboiling an Egg: An Introduction to Circular Dichroism and Protein Refolding

A signature of life is that biological molecules occur as a single stereoisomer with respect to each chiral center, and this is echoed in the structures of biological polymers. Thus, circular dichroism (CD) has emerged as a hallmark of biomolecular structures. The tangible and familiar phenomenon of raw egg white's conversion from transparent fluid to rubbery white solid, upon boiling, was exploited to relate protein molecular structure measured by CD, to macroscopic material properties. Specifically, students employed a denaturant solution to dissolve hard-boiled egg white back into solution, and then upon dilution of the denaturant recovered CD signatures of structure. This experiment is adaptable to students from high school to early in graduate school, and provides an accessible introduction to the use of CD for characterizing protein secondary structure. It is a class favorite due to its demonstration of the surprising possibility of reversing the protein aggregation that underlies the solidity of hard-boiled egg white.