2D Correlation Deep UV Resonance Raman Spectroscopy of Early Events of Lysozyme Fibrillation:  Kinetic Mechanism and Potential Interpretation Pitfalls

The early stages of hen egg white lysozyme (HEWL) fibrillation were quantitatively characterized by two-dimensional correlation deep UV resonance Raman spectroscopy (2D-DUVRR) in terms of the sequential order of events and their characteristic times. The evolution of individual secondary structural elements was established through the correlation between Amide I, Amide III, and Cα−H bending Raman bands. The temporal order of tertiary and individual secondary structural changes was probed through the cross-correlation of phenylalanine and Amide Raman bands. Both the sequential order and the characteristic times of tertiary and secondary structural changes allowed for reconstructing the molecular mechanism of lysozyme structural changes at the early stages of fibrillation. The 2D-DUVRR analysis of our data indicated that melting of the α-helix happened after the formation of the disordered structure, which was termed as apparent inverse order of secondary structural changes. We demonstrated that this apparent inverse order of events is typical for all chemical reactions involving the formation of intermediate(s), which may lead to the serious misinterpretation of 2D correlation results. We proposed a new simulation-aided approach for reconstructing and quantitatively characterizing the reaction mechanism of a (bio)chemical reaction that accounts for the apparent inverse order of events.