Protein single-crystal diffraction with 5 A synchrotron X-rays at the sulfur K-absorption edge

Area of Science:

• Crystallography
• Structural Biology
• X-ray Diffraction

Background:

• Sulfur atoms are crucial components of many proteins.
• Anomalous scattering using sulfur is a potential method for protein phase determination.
• A key challenge is the need for long wavelengths (around 5 Å) to detect sulfur's anomalous signal, which causes significant absorption.

Purpose of the Study:

• To investigate the feasibility of using sulfur atoms for anomalous scattering in protein phase determination.
• To overcome the challenges posed by large absorption effects at the required wavelengths.
• To demonstrate the observability of intensity changes in Bragg reflections around sulfur's absorption edge.

Main Methods:

• Utilized a synchrotron X-ray source for high-intensity radiation.
• Employed evacuated beam tubes and a diffractometer within a vacuum chamber to minimize air scattering and absorption.
• Designed a special sample holder and scattering geometry to optimize data collection.
• Measured Bragg reflections and their intensity variations around the sulfur absorption edge.

Main Results:

• Successfully measured Bragg reflections from protein crystals containing sulfur.
• Observed measurable changes in reflection intensities around the sulfur absorption edge.
• Demonstrated the potential for obtaining anomalous scattering data from sulfur atoms.

Conclusions:

• The experiments show encouraging results for using sulfur anomalous scattering in protein crystallography.
• Despite significant absorption effects, measuring Bragg reflections and their intensity changes is achievable.
• This work supports the development of multiple-wavelength anomalous diffraction (MAD) phasing using sulfur.