The Multiwavelength Anomalous Solvent Contrast (MASC) Method in Macromolecular Crystallography

Area of Science:

• Crystallography
• Biophysics
• Structural Biology

Background:

• Anomalous scattering of X-rays offers a method for contrast variation in macromolecular crystallography.
• Tuning X-ray wavelengths near absorption edges maximizes contrast variations.

Purpose of the Study:

• To introduce and describe the MASC (MArking by Solvent Contrast) method for macromolecular crystallography.
• To demonstrate MASC's applicability for determining molecular envelopes and low-resolution structure-factor phases.
• To compare MASC with the multiwavelength anomalous-dispersion (MAD) method.

Main Methods:

• MASC utilizes wavelength-dependent anomalous scattering from randomly dispersed atoms in the solvent.
• The method involves tuning X-ray wavelengths near absorption edges of anomalous scatterers.
• Data analysis yields the modulus of the molecular envelope and normal structure factors, along with phase information.

Main Results:

• MASC provides an extended uniform electron density for the anomalous partial structure, unlike MAD's punctual scatterers.
• The method allows for the determination of molecular envelopes and low-resolution structure-factor phases.
• Preliminary experiments using selenium and ytterbium demonstrated the expected anomalous ratios and effects.

Conclusions:

• MASC is a viable technique for solvent-density contrast variation in macromolecular crystallography.
• The method is particularly useful for determining molecular envelopes and low-resolution phases.
• MASC offers an alternative or complementary approach to existing phasing methods in crystallography.