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

A previous article [Fourme er al. (1995). J. Synchrotron Rad. 2, 36-48] pre sented the theoretical foundations of MASC, a new contrast-variation method using multiwavelength anomalous scattering, and reported the first experim ental results. New experiments have been conducted both at the ESRF (Grenob le, France) and at LURE-DCI (Orsay, France), using cryocooled crystals of t hree proteins of known structures and very different molecular weights. Amp litudes of {Gamma(T)(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 multiwaveleng th data. In the very low resolution range (d greater than or equal to 20 An gstrom), the agreement between experimental {\Gamma(T)(h)\} and model value s calculated from the bulk solvent is all the more satisfactory since the m olecular weight of the protein is high. For spacings between 10 and 20 Angs trom, the agreement between experimental {\Gamma(T)(h)\} and model values i s also satisfactory if one takes into account ordered anomalous scatterer s ites. Such sites have been found in the three cases.