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Statistical quality indicators for electron-density maps.

Ian J Tickle1

  • 1Astex Pharmaceuticals, 436 Science Park, Milton Road, Cambridge CB4 0QA, England. ian.tickle@astx.com

Acta Crystallographica. Section D, Biological Crystallography
|April 17, 2012
PubMed
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Current validation metrics like real-space R (RSR) and real-space correlation coefficient (RSCC) fail to distinguish model accuracy from precision. New metrics, real-space difference density Z score (RSZD) and real-space observed density Z score (RSZO), are proposed for accurate structure validation.

Area of Science:

  • Crystallography
  • Structural Biology
  • Biochemistry

Background:

  • Real-space R (RSR) and real-space correlation coefficient (RSCC) are commonly used for validating local agreement between structure models and observed electron density.
  • These metrics, however, struggle to differentiate between model accuracy and precision, which is crucial for effective validation.
  • Current validation metrics often lack statistical significance indicators.

Purpose of the Study:

  • To review the limitations of existing real-space validation metrics (RSR, RSCC).
  • To propose improved metrics for assessing local model accuracy and precision in crystallographic structure validation.
  • To introduce statistically robust measures for evaluating electron density maps and structure models.

Main Methods:

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  • Review of statistical properties of electron-density maps.
  • Development of a likelihood-based metric for model accuracy.
  • Introduction of a chi-squared (χ(2)) significance test for difference density using the real-space difference density Z score (RSZD).
  • Proposal of a new real-space observed density Z score (RSZO) for model precision.

    • Main Results:

    • Demonstration that RSR and RSCC cannot reliably distinguish model accuracy from precision.
    • Identification of the lack of statistical significance reporting in RSR and RSCC.
    • Introduction of RSZD as a metric purely for local model accuracy.
    • Introduction of RSZO as a metric purely for model precision, serving as an alternative to B-factor.

    Conclusions:

    • Existing real-space validation metrics (RSR, RSCC) are insufficient for accurate crystallographic model validation due to their inability to separate accuracy from precision.
    • The proposed real-space difference density Z score (RSZD) provides a statistically significant measure of local model accuracy.
    • The proposed real-space observed density Z score (RSZO) offers a precise measure of model precision, improving upon traditional metrics like the B factor for structure validation and optimization before PDB submission.