We aim to investigate structural dynamics of proteins in solutions with time-resolved Small- and Wide-Angle-X-ray-Scattering (SAXS/WAXS). By probing structural changes in solution, where the full range of conformational motion is permitted, the dynamics recovered are expected to be authentic. The time resolution of this technique is as short as 100 ps, which should be sufficient to follow the primary motions associated with global structure rearrangement. These results complement those obtained from time-resolved Laue crystallography studies. Summary: 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. We have developed a novel time-resolved SAXS/WAXS diffractometer that employs 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 -1, which corresponds to spatial resolution down to 1.2 . With an incident flux of approximately four billion, 12 keV photons per 100 ps duration x-ray pulse, a high dynamic range scattering image is typically acquired by integrating the signal from 984 x-ray pulses. To mitigate the adverse effects of radiation damage, the capillary containing the protein solution is rapidly translated along its length during x-ray exposure with a 1-um resolution linear motor translation stage capable of more than 1g acceleration. We have also developed a small volume, closed-loop protein circulation system that can operate with as little as 125 uL of protein solution. After reading each x-ray scattering image, a stepper-motor driven peristaltic pump pushes a fresh volume of solution into the capillary. The circulation system is routinely pressured to about 3 atm, which allows us to acquire scattering data at temperatures up to 120 C without boiling the solution. The capillary is shrouded by a temperature controlled support with slots through which laser and x-ray pulses can pass. The temperature range typically accessed with this sample cell design spans -15 to 100 C. Time-resolved SAXS/WAXS scattering patterns of photoactive yellow protein (PYP) were acquired following laser excitation on time scales ranging from 100 ps to 1 s. These data are rich in structural information, as reported in Cho et al., 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 (2016). PYP is found in Halorhodospira halophila, a photosynthetic bacterium that swims away from blue light, presumably in an effort to evade photons energetic enough to be genetically harmful. PYP is believed to be the protein responsible for this response, whose chromophore photoisomerizes from trans to cis in the presence of blue light. Using a sequential model, global analysis of the time-dependent scattering differences recovered 4 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 ) is significantly larger than that for the ground state (Rg = 14.7 ), and is therefore inaccessible to time-resolved protein crystallography. This transition is also accompanied by a significant increase in I0, which implies a significant increase in the solvent-accessible surface area of the protein. Both of these observations are consistent with the view that the protein partially unfolds during the pB0 to pB1 transition. The shape of the pB1 signaling state, reconstructed using GASBOR, was found to be highly anisotropic with 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 -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. 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 |
