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. 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. Together, the SAXS/WAXS scattering patterns provide fingerprints that can be correlated with protein structure via molecular models. Time-dependent changes of the SAXS/WAXS fingerprint can therefore be used to assess which models best describe the reaction pathway in solution. Thus, progress in this area requires close connections between experiment and theory. Our time-resolved SAXS/WAXS methodology is based on the pump-probe method, in which a laser pulse triggers a structural change in the protein, and a delayed X-ray pulse probes the proteins structure through its scattering pattern. We initially pursued time-resolved WAXS studies at the ESRF, but our studies there suffered from a lack of sufficient beamtime. Thus, we invested much effort to develop the infrastructure required to pursue time-resolved X-ray scattering studies on the BioCARS beamline at the APS, and expanded our goals to access the SAXS region as well. We reported in 2010 the ability to acquire, for the first time, time-resolved SAXS/WAXS patterns with 100 ps time resolution. Numerous innovations made this demonstration experiment possible. For example, our diffractometer design allows us to acquire both SAXS and WAXS data on the same detector at the same time over a range of q (momentum transfer) spanning 0.02 to 2.6 -1. This large dynamic range of q includes the water ring, which can be used to scale images before calculating their differences. Accurate scaling is crucial when computing time-resolved scattering differences and when subtracting buffer scatter from protein scatter in static SAXS/WAXS measurements. Instead of flowing the protein solution through a capillary during X-ray exposure, we employ a rapid translation stage capable of more than 1g acceleration, and translate the sample capillary after each pump-probe pair in a move-stop-acquire data collection protocol. The translation stage moves at a repetition frequency up to 41 Hz, a frequency that allows acquisition of time-resolved scattering patterns spanning time delays from 100 ps to 10 ms. Data acquired at longer time delays requires reducing the repetition rate below 41 Hz. After integrating the X-rays from many pump-probe pairs, the detector is read, and a pump introduces fresh protein solution into the interaction region of the capillary. We have used this infrastructure to investigate the photocycle of photoactive yellow protein (PYP) in solution, a protein that has long served as a model system for investigating light-induced signaling. Exploiting the principle of photoselection, we partially overcame the orientational average intrinsic to solution scattering methods, and observed for the first time an anisotropic change in the protein size and shape following photoactivation. Indeed, we observed protein compaction of approximately 0.3% along the direction defined by the electronic transition dipole moment of the pCA chromophore. The strain induced by this compaction persists out to milliseconds, and presumably provides the force required to drive the structural transition to the signaling state. These results were published in """"""""Probing Anisotropic Structure Changes in Proteins with Picosecond Time-Resolved Small-Angle X-ray Scattering"""""""", Hyun Sun Cho, Friedrich Schotte, Naranbaatar Dashdorj, John Kyndt, and Philip A. Anfinrud, J Phys Chem B, 117, 15825-15832 (2013). In our time-resolved SAXS/WAXS studies, X-ray scattering from the protein typically accounts for less than 3% of the total number of scattered photons detected on the X-ray camera. To isolate the static protein scattering pattern from that arising from the surrounding buffer, capillary, and air, it is crucial to independently measure scattering from a buffer-filled capillary, and subtract the correct magnitude of this relatively uninteresting scattering pattern from the total scattering observed with the protein-containing solution. This subtraction requires knowledge of the volume fraction occupied by the protein, as well as the X-ray fluence at the time the protein and buffer scattering images were acquired. The scale factor for subtraction must be determined with very high precision to avoid contamination by buffer/capillary/air scattering. To estimate the X-ray fluence, we set out to record traces of every X-ray pulse that impinges on the sample, and integrate the area under the pulse profiles. Though this approach is still being refined, it appears we can now estimate the X-ray fluence to a tenth of a percent precision, if not better. The new, high-speed X-ray detector recently installed on the BioCARS beam line is two times bigger than the detector used in past time-resolved SAXS/WAXS studies, which required us to design and fabricate a new helium cone to mount in front of the detector. This large detector approximately doubled the range of q accessible in our time-resolved studies. This expanded range not only improves the resolution of our measurements by approximately a factor of two, but also helps facilitate accurate subtraction of the buffer and capillary scattering from the protein. When acquiring scattering data over a broad range of q, the scattering curves recovered can be distorted due to sample absorption and the angle-dependent responsivity of the X-ray detector. We have developed a method to correct for these distortion effects, which allows us to generate accurate protein scattering curves over a broad range of q that can be compared with theory. Further refinements have been made in our sample cell design. In the past, the intense laser pulse caused ablation of the aluminum sample cell housing, with the by-products condensing on the glass capillary and causing small angle scattering that interfered with our measurement. We developed a new sample cell design based on BeO, a ceramic material that doesnt ablate. Moreover, its thermal conductivity is somewhat higher than aluminum, ensuring we can precisely control the temperature of the capillary with a TEC bonded directly to one side of the BeO sample cell. 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 |
