MAD

Multiwavelength anomalous diffraction(MAD) has been used to solve several crystal structures. What is MAD and how does MAD do to solve the crystal structures? The idea is from the resonant absorption of x-rays that occurs when the frequency of the incident radiation approaches the frequency of oscillations in a bound electronic orbital. Therefore different x-ray wavelength can obtain different absorption from the same atom and the absorption strength is proportion to the atomic number, that is, heavy atoms have much strong absorption which can be used to solve the phase problems. The method is to obtain phases from heavy-atom derivatives. The heavy atom is capable to absorb x-rays of specified wavelength. As a result of this absorption, Friedel’s law does not hold, and the reflections hkl and –h-k-l are not equal in intensity. This inequality of symmetry-related reflections is called anomalous scattering or anomalous dispersion. This effect can be exploited in x-ray crystallographic studies on biological macromolecules by making diffraction measurements at selected wavelengths associated with a particular resonant transition.

Determination of Macromolecular Structures from Anomalous Diffraction of Synchrotron Radiation

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		Multiwavelength anomalous diffraction(MAD) has been used to solve several crystal structures. What is MAD and how does MAD do to solve the crystal structures? The idea is from the resonant absorption of x-rays that occurs when the frequency of the incident radiation approaches the frequency of oscillations in a bound electronic orbital. Therefore different x-ray wavelength can obtain different absorption from the same atom and the absorption strength is proportion to the atomic number, that is, heavy atoms have much strong absorption which can be used to solve the phase problems. The method is to obtain phases from heavy-atom derivatives. The heavy atom is capable to absorb x-rays of specified wavelength. As a result of this absorption, Friedel’s law does not hold, and the reflections hkl and –h-k-l are not equal in intensity. This inequality of symmetry-related reflections is called anomalous scattering or anomalous dispersion. This effect can be exploited in x-ray crystallographic studies on biological macromolecules by making diffraction measurements at selected wavelengths associated with a particular resonant transition.

			The total scattering factor is then : f= fo + \|fD\|eid = fo + f’ + if’’ where f’ and f’’ are the real and imaginary components of the anomalous scattering, respectively.