Direct calculation of cryo-EM and crystallographic model maps for real-space refinement

Area of Science:

• Structural biology
• Biophysical chemistry
• Computational crystallography

Background:

• Accurate comparison between atomic models and experimental maps is essential for real-space refinement in cryo-electron microscopy and crystallography.
• Existing methods for generating limited-resolution maps often struggle with varying resolution across molecular regions and may not fully capture experimental map distortions.

Purpose of the Study:

• To develop a method for calculating limited-resolution maps from atomic models that accurately reflects experimental map distortions.
• To enable more precise real-space refinement by generating model maps comparable to experimental data.

Main Methods:

• Calculates limited-resolution maps by summing "atomic density images" – atomic densities morphed to include resolution loss and distortions.
• Avoids Fourier transforms, instead using analytical functions representing atomic contributions with central peaks and Fourier ripples.
• Determines optimal truncation distances for atomic images, significantly larger than traditional methods, to include necessary Fourier ripples.

Main Results:

• Demonstrates that accurate limited-resolution map calculation requires a large truncation radius, extending beyond the first Fourier ripples.
• Shows that atomic density images can be represented by analytical functions dependent on atomic type, resolution, and displacement parameters.
• Analyzes the impact of atomic displacement parameter accuracy and truncation distance on map accuracy.

Conclusions:

• The proposed method of summing atomic contributions provides a robust way to generate limited-resolution maps that mimic experimental data.
• Accurate map generation necessitates including multiple Fourier ripples within atomic density images, requiring larger truncation radii.
• This approach enhances the reliability of real-space refinement by improving the comparison between atomic models and experimental cryo-electron microscopy and crystallography data.