Ultrafast time-resolved circular dichroism in a pump-probe experiment

Circular dichroism (CD) is known to be a very sensitive probe of molecular conformation, and it is in particular widely used in biochemistry. Measuring the CD as a function of time is therefore very appealing to access information on the dynamics of conformational changes in molecules or biomolecules. An idea developed in the last decade is to implement the measurement of circular dichroism in a pump-probe experiment, which should allow one to access changes of CD with an unprecedented time resolution, down to the subpicosecond range. The most straightforward way to do so is to modulate the probe polarization form left to right circular and to measure the CD as a function of the pump-probe delay. This technique is however prone to many artifacts and one must be very careful when carrying out measurements. An alternative technique relies on the measurement of the pump-induced probe ellipticity. Although less direct, this technique is much more robust and very well fitted for measurements in the ultraviolet. Both techniques are described in details in this article. We then present several results obtained with these techniques. First, demonstration of the technique is made on the dynamics of the binaphthol dihedral angle after photoexcitation in the ultraviolet. We then present a complete study of the dynamics of conformational changes following photolysis of carboxy-myoglobin. Combining time-resolved CD in the visible and in the far-ultraviolet with classical calculations of CD based on coupled oscillators, we can assign the 100 ps dynamics that we measure to a transient deformation of the proximal histidine following the heme doming. Extension of these techniques to longer timescales, and particularly to the protein folding problem will be addressed in conclusion.