Accurate geometrical restraints for Watson-Crick base pairs.

Area of Science:

• Structural Biology
• Biochemistry
• Computational Chemistry

Background:

• Geometrical restraints are crucial for determining biomolecular structures using experimental methods like crystallography and cryo-electron microscopy, especially at lower resolutions.
• Existing restraint libraries, such as Parkinson et al. (1996), have been standard for over two decades but may benefit from updates.

Purpose of the Study:

• To compare and evaluate restraint targets for nucleic acid bases derived from three distinct sources: the Cambridge Structural Database (CSD), ultrahigh-resolution Protein Data Bank (PDB) structures, and quantum-mechanical (QM) calculations.
• To identify the optimal source for generating accurate geometrical restraints for nucleic acid bases.

Main Methods:

• Extraction and comparison of geometrical restraint data for nucleic acid bases from the CSD, PDB, and QM calculations.
• Validation of CSD-derived restraints against QM results for isolated and paired bases.
• Assessment of the accuracy of CSD and QM derived targets using root-mean-square deviations from ultrahigh-resolution PDB structures.

Main Results:

• Geometrical restraints derived from CSD small-molecule crystal structures provide the most accurate parameters.
• CSD-derived geometry is fully compatible with Watson-Crick base pairing, as confirmed by QM calculations.
• QM calculations, while capable of distinguishing single from paired bases, show approximately two times lower accuracy than CSD targets when compared to ultrahigh-resolution PDB structures.
• QM accuracy is sufficient for generating targets for synthetic base pairs lacking experimental data.

Conclusions:

• The Cambridge Structural Database (CSD) is the superior source for deriving accurate geometrical restraints for nucleic acid bases.
• Updated CSD-derived restraints offer improvements over existing libraries and are essential for precise biomolecular structure determination.
• Quantum-mechanical calculations are valuable for assessing base pairing and for designing novel synthetic base pairs.