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 inverse angstroms. 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. 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. The scale factor for subtraction must be determined with very high precision to avoid contamination by buffer/capillary/air scattering. To measure the X-ray fluence impinging on the capillary, we have developed a transmissive beamstop that allows the direct beam intensity and position to be recorded on the same detector used to record the SAXS/WAXS scattering pattern. Anumber of different designs were tried, and our current beamstop consists of a cylinder of high-purity aluminum inside a thin-wall tantalum sleeve. X-ray absorption by the aluminum attenuates the direct beam by many orders of magnitude, and the tantalum sleeve prevents aluminum diffraction from hitting the detector. Scaling of the scattering data is often compromised by X-ray beam drift, which can affect the relative scattering intensities arising from the protein, buffer, and glass capillary. With a transmissive beamstop, we can not only record the intensity of the direct beam, but can also track changes in the beam position to a precision of a few microns, which is small compared to the 89 micron pixel size. In the future, we aim to use this position information to tweak the mirrors controlling the beam alignment in real time, and thereby maintain proper alignment throughout the duration of the experiment, which can span many hours. The new, high-speed X-ray detector installed on the BioCARS beam line in 2014 is two times bigger than the detector used in past studies, and approximately doubled the range of q accessible in our time-resolved studies. This dimensional change required us to design and fabricate a new helium chamber to mount in front of the detector. We experimented with several different designs and settled on a relatively small pyramidal chamber that can be attached with high positional repeatability to the high-speed X-ray detector. 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. In the past, the X-ray detector readout time was rate limiting. With the new, high-speed, large area X-ray detector, network-based control of the motion control system as well as timing jitter in the motion control system limited the rate at which we could acquire data. To address these issues, we have developed new approaches that facilitate far faster data collection. Key to these efforts, we developed a novel method for phase locking our motion control system timing to the X-ray source. With that innovation, we are able to boost the maximum pulse repetition rate 6-fold from 41 to 246 Hz, which speeds significantly the rate of data collection achievable for time delays less than 10 microseconds. In the past, acquiring data at time delays beyond 10 ms required slowing the pulse repetition rate accordingly. To speed the acquisition of data at long time delays, we are developing an exotic mode of operation that allows us to first pump a series of spots in the capillary with laser pulses arriving at 246 Hz, and then transmit X-ray pulses through the same spots at a time delay controlled by the motion control system. The timing precision required for this exotic mode of operation was beyond the capabilities of the motion control system, but is now possible thanks to our ability to phase lock the motion control system timing to the X-ray source. These improvements will allow us to study more protein systems at a greater level of precision in less time than ever before. 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 |
