Single-wavelength anomalous diffraction (SAD) is a technique used in X-ray crystallography that facilitates the determination of the structure of proteins or other biological macromolecules by allowing the solution of the phase problem. In contrast to multi-wavelength anomalous diffraction (MAD), SAD uses a single dataset at a single appropriate wavelength.
Compared to MAD, SAD has weaker phasing power and requires density modification to resolve phase ambiguity.[1] This downside is not as important as SAD's main advantage: the minimization of time spent in the beam by the crystal, thus reducing potential radiation damage to the molecule while collecting data. SAD also allows a wider choice of heavy atoms and can be conducted without a synchrotron beamline.[1] Today, selenium-SAD is commonly used for experimental phasing due to the development of methods for selenomethionine incorporation into recombinant proteins.
SAD is sometimes called "single-wavelength anomalous dispersion", but no dispersive differences are used in this technique since the data are collected at a single wavelength.
See also
References
- 1 2 "Dictionary of common terms used in PHENIX". phenix-online.org.
MAD: [...] The differences in anomalous scattering around the edge allow calculation of phase angles without the phase ambiguity present in SAD experiments, although density modification will usually still be necessary to obtain an easily interpretable map. [...] Although very powerful, MAD phasing has declined somewhat in popularity relative to SAD because of the more limited choice of heavy atoms, the difficulty of avoiding radiation damage, and the requirement for a synchrotron beamline. [...] SAD: [...] SAD is often performed with selenomethionine-incorporated protein, but any anomalously scattering atom (including sulfur, if the data are of very high quality) may be used.
Further reading
- W. A. Hendrickson (1985). "Analysis of Protein Structure from Diffraction Measurement at Multiple Wavelengths". Trans. ACA Vol 21.
- J Karle (1980). "Some Developments in Anomalous Dispersion for the Structural Investigation of Macromolecular Systems in Biology". International Journal of Quantum Chemistry: Quantum Biology Symposium 7, 357–367.
- J. Karle (1989). "Linear Algebraic Analyses of Structures with One Predominant Type of Anomalous Scatterer". Acta Crystallogr. A45, 303–307.
- A. Pahler, JL Smith & WA Hendrickson (1990). "A Probability Representation for Phase Information from Multiwavelength Anomalous Dispersion". Acta Crystallogr. A46, 537–540.
- T. C. Terwilliger (1994). "MAD Phasing: Bayesian Estimates of FA" Acta Crystallogr. D50, 11–16.
- T. C. Terwilliger (1994). "MAD Phasing: Treatment of Dispersive Differences as Isomorphous Replacement Information" Acta Crystallogr. D50, 17–23.
- R. Fourme, W. Shepard, R. Kahn, G l'Hermite & IL de La Sierra (1995). "The Multiwavelength Anomalous Solvent Contrast (MASC) Method in Macrocolecular Crystallography". J. Synchrotron Rad. 2, 36–48.
- E. de la Fortelle and G. Bricogne (1997) "Maximum-Likelihood Heavy-Atom Parameter Refinement for Multiple Isomorphous Replacement and Multiwavelength Anomalous Diffraction Methods". Methods in Enzymology 276, 472–494.
- W. A. Hendrickson and CM Ogata (1997) "Phase Determination from Multiwavelength Anomalous Diffraction Measurements". Methods in Enzymology 276, 494–523.
- J. Bella & M. G. Rossmann (1998). "A General Phasing Algorithm for Multiple MAD and MIR Data" Acta Crystallogr. D54, 159–174.
- J. M. Guss, E. A. Merritt, R. P. Phizackerley, B. Hedman, M. Murata, K. O. Hodgson, and H. C. Freeman (1989). "Phase determination by multiple-wavelength X-ray diffraction: crystal structure of a basic blue copper protein from cucumbers". Science 241, 806–811.
- B. Vijayakumar and D. Velmurugan (2013). "Use of europium ions for SAD phasing of lysozyme at the Cu Kα wavelength" Acta Crystallogr. F69, 20–24.
- J. P. Rose & B-C Wang (2016) "SAD phasing: History, current impact and future opportunities" Archives Biochem Biophys 602, 80-94.
External links
- MAD phasing — an in depth tutorial with examples, illustrations, and references.
Computer programs
- The SSRL Absorption Package — Brennan S, Cowan PL (1992). "A suite of programs for calculating x-ray absorption, reflection and diffraction performance for a variety of materials at arbitrary wavelengths" (PDF). Rev. Sci. Instrum. 63 (1): 850. Bibcode:1992RScI...63..850B. doi:10.1063/1.1142625.
- CHOOCH — Evans G, Pettifer RF (2001). "CHOOCH: a program for deriving anomalous-scattering factors from X-ray fluorescence spectra". J. Appl. Crystallogr. 34 (1): 82–86. doi:10.1107/S0021889800014655.
- Shake-and-Bake (SnB) — Smith GD, Nagar B, Rini JM, Hauptman HA, Blessing RH (1998). "The use of Snb to determine an anomalous scattering substructure". Acta Crystallogr D. 54 (Pt 5): 799–804. doi:10.1107/S0907444997018805. PMID 9757093.
- SHELX — Sheldrick GM (1998). "SHELX: applications to macromolecules". In S Fortier (ed.). Direct methods for solving macromolecular structures. Dordrecht: Kluwer Academic Publishers. pp. 401–411. ISBN 0-7923-4949-0.
Tutorials and examples
- Evans, Gwyndaf (October 1994). "The method of Multiple wavelength Anomalous Diffraction using Synchrotron Radiation at optimal X-ray energies: Application to Protein Crystallography". PhD Thesis. University of Warwick.