Biological X-Ray Absorption Spectroscopy (XAS)
X-ray absorption spectroscopy (XAS) is a technique that provides a probe of the local geometric and electronic structure surrounding a specific element. It can be employed for materials in a wide range of states, temperatures, and pressure conditions, and is thus especially useful for circumstances where X-ray diffraction cannot be used to obtain structural insights, e.g., highly reactive intermediate states or amorphous materials. Bending magnet and wiggler sources at synchrotrons produce smooth broadband X-ray spectra that are suitable for measurement of XAS spectra over energies ranging from <100 eV to >100 keV, allowing element-specific coverage of all elements ranging from hydrogen to the transuranic actinides.
Generally, XAS spectra comprise two regions, providing distinct but complementary information about the element of interest. The first, termed X-ray Absorption Near Edge Spectroscopy (XANES), consists of the rising x-ray absorption edge, and provides insight into symmetry, oxidation state, and covalency. The second region, the Extended X-ray Absorption Fine Structure (EXAFS), is the region of the spectrum above the absorption edge. EXAFS gives information on the local coordination environment around the element of interest, answering the question of “how many of what kind at what distance.” XANES and EXAFS can provide a complete picture of the local geometric and electronic structure of a metal center, and are complementary to other synchrotron based methods such as macromolecular crystallography, as well as non-synchrotron spectroscopies such as EPR, Mössbauer, and vibrational methods. As an example, XAS can be used to track changes at the catalytic center of a metalloenzyme in the as-isolated form, substrate-bound forms, in one or more isolated reactive intermediates along the catalytic pathway, and in the final product-bound state, providing key insight into the role of the metal center throughout the reaction chemistry.
XAS at CSB: Facilities + User Support
Building upon over 20 years of successful (bio)XAS beamline operations at the original National Synchrotron Light Source, CSB is developing an XAS endstation capability at our recently completed NSLS-II XFP beamline. The planned facility will provide monochromatic X-rays over an energy range of 4.5 – 16 keV (K edges of Ti - Br, and L edges of Cs – Bi). Standard endstation apparatus will consist of a fully motorized sample alignment stage with low/variable-temperature capabilities using a He Displex cryostat (10K sample temperatures). A Canberra 31-element solid state germanium fluorescence detector with XIA DXP-XMAP electronics will be standard for dilute biological samples, while ion chambers and a PIPS detector will be available for higher concentrations.
The CSB XAS Core provides comprehensive beamline user support for research projects incorporating XAS techniques in the biological and environmental sciences. This includes consultation on experimental design and sample preparation, assistance with the development of beamtime proposals, and training in XAS data collection and data analysis methods.
Additional Opportunities at NSLS-II
In addition to the capabilities planned for XAS at XFP, beamlines for X-ray spectroscopy are available at NSLS-II across a wide range of energy scales. These include CSX-2 for soft X-ray absorption spectroscopy (250-2000 eV), TES for tender XAS and imaging (1-8 keV), and ISS for conventional hard x-ray absorption spectroscopy at energies above 5 keV. ISS also offers access to both resonant and non-resonant X-ray emission spectroscopy. Additional beamlines emphasizing XAS in the materials sciences (BMM, QAS, and SST-1/SST-2) will be available in the near future. The NSLS-II Users Guide provides information on currently available capabilities at each of these beamlines, and CSB staff can also answer questions about the NSLS-II X-ray spectroscopy beamline suite.