We present a method
to simulate ultrafast pump–probe time-resolved
circular dichroism (TRCD) spectra based on time-dependent density
functional theory trajectory surface hopping. The method is applied
to simulate the TRCD spectrum along the photoinduced ring-opening
of provitamin D. Simulations reveal that the initial decay of the
signal is due to excited state relaxation, forming the rotationally
flexible previtamin D. We further show that oscillations in the experimental
TRCD spectrum arise from isomerizations between previtamin D rotamers
with different chirality, which are associated with the helical conformation
of the triene unit. We give a detailed description of the formation
dynamics of different rotamers, playing a key role in the natural
regulation of vitamin D photosynthesis. Going beyond the sole extraction
of decay rates, simulations greatly increase the amount of information
that can be retrieved from ultrafast TRCD, making it a sensitive tool
to unravel details in the subpicosecond dynamics of photoinduced chirality
changes.