Time-resolved Laue crystallography, as its name implies, can only be performed on crystalline samples. The intermolecular forces that maintain crystalline order constrain large amplitude conformational motion, and this loss of flexibility may perturb or even inhibit the function of a protein. Nonetheless, Laue crystallography stands alone in its ability to track structural changes in proteins on ultrafast time scales with near-atomic spatial resolution. X-rays can also extract structural information from molecules in solution where the full range of conformational motion is permitted. However, without external alignment forces, the protein molecules are randomly oriented, and the structural information contained in its orientationally-averaged diffuse scattering pattern is one dimensional. Our time-resolved SAXS/WAXS methodology employs the pump-probe method in which a laser pulse triggers a structural change in the protein and a delayed X-ray pulse probes the protein's structure through its scattering pattern. It is well known that the Small-Angle X-ray Scattering (SAXS) region of the scattering pattern reports on the size and shape of the protein, while the Wide-Angle X-ray Scattering (WAXS) region is sensitive to secondary and tertiary structure. The time-resolved SAXS/WAXS scattering patterns therefore provide 'fingerprints' that can be correlated with protein structure via molecular models, and can assess which models best describe reaction pathways in solution. Progress in this area requires close connections between experiment and theory. The capacity to respond to environmental changes is crucial to an organisms survival. Halorhodospira halophila is a photosynthetic bacterium that swims away from blue light, presumably in an effort to evade photons energetic enough to be genetically harmful. The protein responsible for this response is believed to be photoactive yellow protein (PYP), whose chromophore photoisomerizes from trans to cis in the presence of blue light. We investigated the complete PYP photocycle by acquiring time-resolved small and wide-angle X-ray scattering patterns over 10 decades of time spanning from 100 ps to 1 s. Using a sequential model, global analysis of the time-dependent scattering differences recovered four intermediates (pR0/pR1, pR2, pB0, pB1), the first three of which can be assigned to prior time- resolved crystal structures. The 1.8 ms pB0 to pB1 transition produces the PYP signaling state, whose radius of gyration (Rg = 16.6 Angstroms) is significantly larger than that for the ground state (Rg = 14.7 Angstroms) and is therefore inaccessible to time-resolved protein crystallography. The shape of the signaling state, reconstructed using GASBOR, is highly anisotropic and entails significant elongation of the long axis of the protein. This structural change is consistent with unfolding of the 25 residue N-terminal domain, which exposes the beta-scaffold of this sensory protein to a potential binding partner. This mechanistically detailed description of the complete PYP photocycle, made possible by time-resolved crystal and solution studies, provides a framework for understanding signal transduction in proteins and for assessing and validating theoretical/computational approaches in protein biophysics. We continue to upgrade our time-resolved infrastructure and refine methods used to acquire and analyze the scattering data. For example, our latest generation time-resolved SAXS/WAXS diffractometer employs a K-B mirror pair to focus the x-ray beam onto the sample capillary with independent control of the the vertical and horizontal dimensions, a very small beamstop ( 0.5 mm), and a large area (340x340 mm), high-speed (up to 10 Hz) x-ray detector. With the sample-detector distance set at 186 mm, scattering data can be acquired over a broad range of q (momentum transfer) spanning 0.02 to 5.2 inverse Angstroms, which corresponds to spatial resolution down to 1.2 Angstroms. When the x-ray source operates in hybrid mode, nearly ten billion, 12-keV photons are delivered to the sample in each 100-ps x-ray pulse. With this infrastructure, integrating the signal from 240 x-ray pulses is sufficient to produce high dynamic range scattering images. To mitigate the adverse effects of radiation damage during x-ray exposure, the capillary containing the protein solution is rapidly translated over a 20-mm span using a 1-um resolution linear motor translation stage capable of more than 1g acceleration. Thanks to a closed-loop circulation system, about 150 uL of protein solution is sufficient to acquire a high S/N data set. A stepper-motor-driven peristaltic pump pushes a fresh volume of solution into the capillary during each return stroke of the linear translation stage. The flow generated by peristaltic pumps vary as a function of rotation, and is interrupted several times times per revolution (4 in our peristaltic pump setup). This variable and interrupted flow can lead to systematic errors in the scattering data acquired. To mitigate this problem, we developed a novel means that linearizes the flow and minimizes the pulsation effects that plague peristaltic pumps. We also recently developed a new capillary holder and high-precision temperature controller that allow us to characterize protein structural changes over a broad range of temperatures spanning from -20 to over 100 degrees Celsius. As our time-resolved SAXS/WAXS methodology becomes more precise and easier to use, we expect it to become an ever more important complement to time-resolved Laue studies and time-resolved optical spectroscopy studies of proteins, and will help provide a structural basis for understanding how proteins function.
| Cho, Hyun Sun; Schotte, Friedrich; Dashdorj, Naranbaatar et al. (2016) Picosecond Photobiology: Watching a Signaling Protein Function in Real Time via Time-Resolved Small- and Wide-Angle X-ray Scattering. J Am Chem Soc 138:8815-23 |
| Cho, Hyun Sun; Schotte, Friedrich; Dashdorj, Naranbaatar et al. (2013) Probing anisotropic structure changes in proteins with picosecond time-resolved small-angle X-ray scattering. J Phys Chem B 117:15825-32 |
| Cho, Hyun Sun; Dashdorj, Naranbaatar; Schotte, Friedrich et al. (2010) Protein structural dynamics in solution unveiled via 100-ps time-resolved x-ray scattering. Proc Natl Acad Sci U S A 107:7281-6 |
| Cammarata, Marco; Levantino, Matteo; Schotte, Friedrich et al. (2008) Tracking the structural dynamics of proteins in solution using time-resolved wide-angle X-ray scattering. Nat Methods 5:881-6 |
