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

Abstract

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 ®rst 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 fÀ T h g, thè 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 Ê ), the agreement between experimental fjÀ T h jg 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 Ê , the agreement between experimental fjÀ T h jg and model values is also satisfactory if one takes into account ordered anomalous scatterer sites. Such sites have been found in the three cases.

Small-angle scattering and its interplay with crystallography, contrast variation in SAXS and SANS

“…MASC experiments have been performed on three protein structures of different molecular weight (Ramin et al, 1999). In all cases, ordered anomalous scatterers were found in addition to disordered atoms.…”

Hen egg white lysozyme fibrillation: a deep‐UV resonance Raman spectroscopic study

“…It was found that the unfolding mainly involved the melting of a-helix into the disordered structures. The native lysozyme, which consists of 32% of a-helix, [71] unfolded irreversibly into a partially folded intermediate containing 6% of a-helix before it assembled into fibrils. The quantification of the secondary structure using far-UV CD and DUVRR spectra of protein were compared.…”

Better structures from better data through better methods: a review of developments inde novomacromolecular phasing techniques and associated instrumentation at LURE

“…The calculation of the Fourier transform (the molecular envelope) requires the derivation of phases associated with these amplitudes. This is a non-trivial problem, which we are currently addressing by several methods (Ramin, 1999;R. Kahn, unpublished), including the use of maximum entropy and likelihood ranking to test envelope hypotheses (Bricogne, 1993).…”

“…The comparison between experimental and model values has shown that amplitudes from envelope structure factors G(h) derived from MASC data are meaningful (Ramin et al, 1999). The future of this method is strongly dependent on the availability of an ef®cient procedure for phasing these coef®cients.…”