Multiwavelength anomalous solvent contrast (MASC): derivation of envelope structure-factor amplitudes and comparison with model values.

A previous article [Fourme et al. (1995). J. Synchrotron Rad. 2, 36-48] presented the theoretical foundations of MASC, a new contrast-variation method using multiwavelength anomalous scattering, and reported the first experimental results. New experiments have been conducted both at the ESRF (Grenoble, France) and at LURE-DCI (Orsay, France), using cryocooled crystals of three proteins of known structures and very different molecular weights. Amplitudes of {GammaT(h)}, the 'normal' structure factors of the anomalously scattering part of the crystal including the solvent zone and the ordered anomalous scattering sites (if any), have been extracted from multiwavelength data. In the very low resolution range (d >/= 20 A), the agreement between experimental {GammaT(h)} and model values calculated from the bulk solvent is all the more satisfactory since the molecular weight of the protein is high. For spacings between 10 and 20 A, the agreement between experimental {GammaT(h)} and model values is also satisfactory if one takes into account ordered anomalous scatterer sites. Such sites have been found in the three cases.