Ultrafast pump-induced differences are generally isomorphous in the case of protein crystals, which allows the generation of Fourier-difference analysis of electron density changes. We must apply lessons learned from decades of ultrafast spectroscopy to this new capability and address the details of the nonlinear optical response and the control and preparation of vibrational coherence. 1–3 Now that time resolved X-ray crystallography is possible in the coherent time domain, fundamental questions regarding the control, assignment, and analysis of ultrafast motion have arisen. Enabled by the advent of X-ray Free Electron laser (XFEL) sources operating at Angstrom wavelengths and beamline technology, it has become possible to perform femtosecond time resolved pump-probe experiments revealing ultrafast structural dynamics. This is the key point and focus of this contribution, to discuss their complementary nature and identify opportunities for future developments. Ultrafast X-ray crystallography and ultrafast structural optical crystallography provide access to molecular transformations on the coherent time scale and are both highly selective in their observations. The definition of a true molecular movie should reference the ultrafast single-molecule dynamics and is not formally applicable to measurements of ensembles such as those that are obtained from ultrafast structural dynamics methods which are discussed in this contribution. The desire to create “molecular movies” of protein function has driven rapid technological advances in the area of ultrafast crystallography. Ultrafast structural optical crystallography of photosynthetic energy transfer has been demonstrated, and the theory of two-dimensional structural optical crystallography has shown a method for accessing the structural selection of electronic coherence. Specific selection of electronic coherence requires optical probes, which can provide real-space structural information through photoselection of oriented samples and specifically in birefringent crystals. The current and future high repetition rate capabilities provided by X-ray free electron lasers for ultrafast diffraction studies provide opportunities for optical control and optical selection of nuclear coherence which may develop to access higher frequency dynamics through improvements of sensitivity and time resolution to reveal coherence directly. A discussion of methods of analysis using ultrafast macromolecular X-ray crystallography and ultrafast nonlinear structural optical crystallography is presented. Ultrafast pump-probe applications of protein dynamics in crystals provide real-space information through direct X-ray crystallographic structure analysis or through structural optical crystallographic analysis. Real-space information obtained from the measurement of electron density dynamics by X-ray crystallography provides aspects of both, while the molecular physics of coherence parameters and frequency-frequency correlation needs spectroscopy methods. Both nuclear and electronic dynamics contribute to protein function and need multiple and complementary techniques to reveal their ultrafast structural dynamics response.
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