title: Sub-millisecond time-resolved SAXS using a continuous-flow mixer and X-ray microbeam.
abstract: Small-angle X-ray scattering (SAXS) is a well established technique to probe the nanoscale structure and interactions in soft matter. It allows one to study the structure of native particles in near physiological environments and to analyze structural changes in response to variations in external conditions. The combination of microfluidics and SAXS provides a powerful tool to investigate dynamic processes on a molecular level with sub-millisecond time resolution. Reaction kinetics in the sub-millisecond time range has been achieved using continuous-flow mixers manufactured using micromachining techniques. The time resolution of these devices has previously been limited, in part, by the X-ray beam sizes delivered by typical SAXS beamlines. These limitations can be overcome using optics to focus X-rays to the micrometer size range providing that beam divergence and photon flux suitable for performing SAXS experiments can be maintained. Such micro-SAXS in combination with microfluidic devices would be an attractive probe for time-resolved studies. Here, the development of a high-duty-cycle scanning microsecond-time-resolution SAXS capability, built around the Kirkpatrick-Baez mirror-based microbeam system at the Biophysics Collaborative Access Team (BioCAT) beamline 18ID at the Advanced Photon Source, Argonne National Laboratory, is reported. A detailed description of the microbeam small-angle-scattering instrument, the turbulent flow mixer, as well as the data acquisition and control and analysis software is provided. Results are presented where this apparatus was used to study the folding of cytochrome c. Future prospects for this technique are discussed.
authors.affiliation: BioCAT, CSRRI and Department BCS, Illinois Institute of Technology, 3101 South Dearborn, Chicago, IL 60616, USA.
authors.forename: Rita
authors.initials: R
authors.lastname: Graceffa
authors.forename: R Paul
authors.initials: RP
authors.lastname: Nobrega
authors.forename: Raul A
authors.initials: RA
authors.lastname: Barrea
authors.forename: Sagar V
authors.initials: SV
authors.lastname: Kathuria
authors.forename: Srinivas
authors.initials: S
authors.lastname: Chakravarthy
authors.forename: Osman
authors.initials: O
authors.lastname: Bilsel
authors.forename: Thomas C
authors.initials: TC
authors.lastname: Irving
mesh_terms: D011506:Proteins / Q000737:chemistry*; D012313:RNA / Q000737:chemistry*; D053838:Scattering, Small Angle; D014961:X-Ray Diffraction
chemical_list: D011506:Proteins; D012313:RNA
keywords: micro-SAXS; protein folding; time-resolved