Ultrafast Structural Dynamics of Biomolecular Complexes Probed by Broadband Time-Resolved Circular Dichroism

Spectroscopic probing of ultrafast chiral molecular dynamics gives deep insight into the mechanism of biomolecular reactions, since the three-dimensional molecular structure plays crucial roles in the selectivity and efficiency of the reactions. Here, we demonstrate a practical method for tracking the change of the absolute configuration of photoexcited biomolecules by combining ultrafast and broadband time-resolved circular dichroism (TRCD) spectroscopy with exciton coupling theory. A proof-of-principle experiment on bilirubin-human serum albumin (BR-HSA) complexes reveals that the excited-state CD spectra show sign inversion with a time constant of ∼5 ps. Instead of a complete chirality inversion across a high energy barrier, we attribute this sign reversal to a structural change from the stable ridge-tile conformation to a stretched conformation based on exciton coupling theory. These results show that the present method is a useful tool for probing excited-state molecular dynamics accompanying structural changes on a real-time basis.