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, but since there is no long-range order in protein solutions, scattering from randomly oriented proteins is diffuse, and its structural information is one dimensional. 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 biomolecular models. Time-dependent changes of the SAXS/WAXS fingerprint can therefore be used to assess which models best describe the reaction pathway in solution. 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 adequate 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 5g acceleration, and translate the sample capillary after each pump-probe pair in a novel move-stop-acquire approach. The translation stage moves at a repetition frequency up to 41 Hz, allowing 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 further. Up to 100 pump-probe pairs have been acquired over the 24-mm stroke of the translation stage. After many pump-probe pairs, often up to 1100, the detector is read, and a syringe pump introduces fresh protein solution into the interaction region of the capillary. We found that temperature instabilities can produce a thermal difference signal in the WAXS region that is as large or larger than the signal generated by laser-induced changes in the protein structure. To address this problem, we have developed a sample cell that is temperature stabilized using a TEC heater/cooler. We adapted a high-precision diode laser temperature controller for this task, whose specifications cite a stability of 3 mK rms. Air currents in the laser hutch degrade the temperature stability of our sample, but the stability achieved is nonetheless sufficient to suppress the thermal instability problem. Moreover, this sample cell design is capable of rapidly changing the temperature of the protein solution (3 K s-1) over a large range from below 0 to 120 C. Our time-resolved SAXS/WAXS studies of proteins in solution are typically conducted at concentrations of 50 mg/ml. At this concentration, the protein contribution to the scattering pattern is only a few percent of the scattering power of the capillary and buffer. Moreover, pump-induced changes in the protein structure typically alter the protein scattering intensities by only a few percent. Therefore, the structural changes we seek to characterize require precision and accuracy down to the 10-4 level. Since the time-resolved difference signals are very small, we are very sensitive to beamline instability and scaling inaccuracy. To mitigate this problem, we alternately acquire laser ON and laser OFF scattering data, and thereby reduce our sensitivity to longer-term beamline instability (long compared to the time between ON and OFF images). Recent achievements using this infrastructure include time-resolved SAXS/WAXS characterization of the complete photocycle of PYP in solution, including its crystal-inaccessible, long-lived signaling state. PYP has long served as a model system for investigating signaling in proteins, and is described in another report. We have characterized structural transitions in PYP spanning 10 decades of time, from 100 ps to 1 s, and identified 4 intermediates, the last of which corresponds to the signaling state. Photoactivation of PYP triggers trans-to-cis isomerization of p-coumaric acid (pCA), which shortens the distance between its sulfur and phenolate oxygen by 0.7 , and like a winch, shortens the distance between the helices to which it and its hydrogen bonding partners are attached. This contraction, which is aligned parallel to the polarization direction of the laser beam, should generate an anisotropic distribution of PYP molecules in the solution and should produce an anisotropic SAXS scattering pattern. Indeed, anisotropy in the SAXS scattering pattern appears promptly with the pump pulse (following the integral of the pump-probe cross-correlation) and decays with a 12 ns time constant, which corresponds to the rotational diffusion time for PYP in water. The time-resolved scattering patterns acquired were analyzed with a global fitting procedure we developed for solving the set of rate equations required to model the time-dependent populations of the intermediates, and to properly account for the duration of the pump and probe pulses. This procedure recovers difference scattering patterns for each intermediates in the model, as well as their time-dependent amplitudes. Given the power density used to photoexcite the PYP, the protein solution experiences a laser-induced temperature jump of 0.5 K. The pR to pB0 transition inflates the protein radius of gyration (RG) only modestly, but RG changes dramatically during the pB0 to pB1 transition. Clearly, formation of pB0 is the triggering event that produces the signaling state. This observation is also made clear in our GASBOR structure calculations, in which the protein size and shape clearly inflates along the long axis of the protein during the pB0 to pB1 transition. The time-resolved SAXS/WAXS methodology developed thus far on the BioCARS 14-IDB beamline has proven capable of producing structurally-sensitive scattering patterns of proteins in solution spanning 10 decades of time with 100 ps time resolution. These data are being used to assess the validity of structural models for protein intermediates and transitions between them. As this methodology becomes 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 |
