Single-wavelength anomalous dispersion (abbreviated SAD) is a technique used in X-ray crystallography

X-ray crystallography

X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...

 that facilitates the determination of the structure of proteins or other biological macromolecules by allowing the solution of the phase problem

Phase problem

In physics the phase problem is the name given to the problem of loss of information concerning the phase that can occur when making a physical measurement. The name itself comes from the field of x-ray crystallography, where the phase problem has to be solved for the determination of a structure...

. In contrast to multi-wavelength anomalous dispersion

Multi-wavelength anomalous dispersion

Multi-wavelength anomalous diffraction is a technique used in X-ray crystallography that facilitates the determination of the three-dimensional structure of biological macromolecules via solution of the phase problem...

, SAD uses a single dataset at a single appropriate wavelength. One advantage of the technique is the minimization of time spent in the beam by the crystal, thus reducing potential radiation damage to the molecule while collecting data.

Anomalous Dispersion

• Multi-wavelength anomalous dispersion
  Multi-wavelength anomalous dispersion
  Multi-wavelength anomalous diffraction is a technique used in X-ray crystallography that facilitates the determination of the three-dimensional structure of biological macromolecules via solution of the phase problem...
  
   (MAD)
• Single wavelength anomalous dispersion (SAD)

Isomorphous Replacement

Two methods for providing the needed phasing information by introducing heavy atoms into isomorphous crystals:

• Multiple isomorphous replacement
  Multiple isomorphous replacement
  Multiple isomorphous replacement or MIR is the most common approach of solving the phase problem in X-ray crystallography. This method is conducted by soaking the crystal of a sample to be analyzed with a heavy atom solution or co-crystallization with the heavy atom...
  
   (MIR); and
• Single isomorphous replacement with anomalous signal (SIRAS)

Further reading

• WA 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 Cryst. A45, 303-307.
• A Pahler, JL Smith & WA Hendrickson (1990). "A Probability Representation for Phase Information from Multiwavelength Anomalous Dispersion". Acta Cryst. A46, 537-540.
• TC Terwilliger (1994). "MAD Phasing: Bayesian Estimates of FA" Acta Cryst. D50, 11-16.
• TC Terwilliger (1994). "MAD Phasing: Treatment of Dispersive Differences as Isomorphous Replacement Information" Acta Cryst. 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 & G Bricogne (1997) "Maximum-Likelihood Heavy-Atom Parameter Refinement for Multiple Isomorphous Replacement and Multiwavelength Anomalous Diffraction Methods". Methods in Enzymology 276, 472-494.
• WA Hendrickson and CM Ogata (1997) "Phase Determination from Multiwavelength Anomalous Diffraction Measurements". Methods in Enzymology 276, 494-523.
• J Bella & MG Rossmann (1998). "A General Phasing Algorithm for Multiple MAD and MIR Data" Acta Cryst. D54, 159-174.
• JM Guss, EA Merritt, RP Phizackerley, B Hedman, M Murata, KO Hodgson
  Keith Hodgson
  Keith O. Hodgson is a Professor of Chemistry at Stanford university and the Stanford Synchrotron Radiation Lightsource. He received his B.S. in 1969 from the University of Virginia and his Ph.D. in 1972 from University of California at Berkeley....
  
  , & HC Freeman
  Hans Freeman
  Hans Charles Freeman, AM, FAA, was a German-born Australian bioinorganic chemist, protein crystallographer, and Professor of Inorganic Chemistry who spent most of his career at the University of Sydney...
  
  (1989). "Phase determination by multiple-wavelength X-ray diffraction: crystal structure of a basic blue copper protein from cucumbers". Science 241, 806-811.

External links

• MAD phasing — an in depth tutorial with examples, illustrations, and references.

Computer programs

• The SSRL Absorption Package —
• CHOOCH —
• Shake-and-Bake (SnB) —
• SHELX —