Diffractive optics-based heterodyne-detected grating spectroscopy: Application to ultrafast protein dynamics

The initial structural evolution of carboxymyoglobin (MbCO) following photo dissociation of CO is measured using a recently developed method of heterod yne detection. This method increases the sensitivity, generates tilted puls e fronts which exactly cancel temporal broadening in noncollinear beam geom etries, and provides an inherent acoustic reference that enables unambiguou s separation of the real and imaginary components to the nonlinear suscepti bility. With the latter advance, both the absorption anisotropy and its rea l counterpart, the phase anisotropy, can be measured with this technique. A ccess to the real part of the material anisotropy provides new information that can be correlated to the bath dynamics. In the MbCO studies, the phase anisotropy is found to develop on picosecond time scales and is much great er than can be attributed to the symmetry of the heme dipole transition. Th is provides direct evidence that the shape of the protein changes in the fi rst few picoseconds following photodissociation. The asymmetric nature of t he protein structure presumably plays an integral role in the development o f the nonuniform displacements. The magnitude of the phase anisotropy and o bserved dynamics give strong evidence that the low-frequency collective mod es of the protein are involved in transducing the reaction forces of the Fe ligation site into directed motions associated with the initial oxy to deo xy tertiary structure change.