Simulation of charge effects on density maps obtained by high-resolution electron crystallography

Area of Science:

• Crystallography
• Biophysics
• Materials Science

Background:

• Atomic scattering factors are sensitive to an atom's charge state.
• This charge sensitivity is particularly notable below 5 Å resolution in electron crystallography.
• Charged amino acid residues (glutamate, aspartate) exhibit different electron densities compared to neutral forms.

Purpose of the Study:

• To mathematically simulate and validate the impact of atomic charge status on electron crystallographic density maps.
• To investigate the charge effect on density maps, corroborating experimental observations in bacteriorhodopsin.
• To quantify the influence of Gaussian approximations for atomic scattering factors.

Main Methods:

• Mathematical simulations of charge effects on electron density maps.
• Evaluation of Gaussian approximations for neutral and charged atomic scattering factors.
• Analysis of charge compensation effects in polarized atom pairs.

Main Results:

• Charged atoms, especially negatively charged ones, lead to weaker densities in low-resolution electron crystallographic maps.
• The charge effect is pronounced for charged glutamate or aspartate residues.
• Simulations confirmed experimental observations of charge effects in bacteriorhodopsin.
• Polarization effects from charged pairs were smaller than individual charge effects due to compensation.
• Density maps show slight elongation towards positively charged atoms.

Conclusions:

• Atomic charge status significantly influences electron crystallography density maps, particularly at lower resolutions.
• Mathematical simulations support experimental findings regarding charge effects.
• Understanding these charge effects is vital for accurate interpretation of electron crystallographic data.