Atomic Detail of Protein Folding Revealed by an Ab Initio Reappraisal of Circular Dichroism.

Circular dichroism (CD) is known to be an excellent tool for the determination of protein secondary structure due to fingerprint signatures of α and β domains. However, CD spectra are also sensitive to the 3D arrangement of the chain as a result of the excitonic nature of additional signals due to the aromatic residues. This double sensitivity, when extended to time-resolved experiments, should allow protein folding to be monitored with high spatial resolution. To date, the exploitation of this very appealing idea has been limited, due to the difficulty in relating the observed spectral evolution to specific configurations of the chain. Here, we demonstrate that the combination of atomistic molecular dynamics simulations of the folding pathways with a quantum chemical evaluation of the excitonic spectra provides the missing key. This is exemplified for the folding of canine milk lysozyme protein.