CIF to PDB Converter

Molecular File Converter

Convert between chemical file formats with our web-based tool

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Our CIF to PDB Converter is a simple and efficient web-based tool designed to translate macromolecular coordinate files. It seamlessly converts the modern Crystallographic Information File (CIF/mmCIF) format into the legacy Protein Data Bank (PDB) format.

This conversion is essential for ensuring compatibility with a wide range of molecular visualization and simulation software that may not fully support the comprehensive data structure of CIF files. The tool intelligently generates the biological assembly and restructures the atomic data, allowing you to quickly prepare your structures for tasks like molecular dynamics, docking, or structural analysis.

How to use (step-by-step)

Follow these four simple steps to convert your file in seconds.

Choose Your Formats: Verify that the “Input Format” is set to CIF (Crystallographic Information File) and the “Output Format” is set to PDB (Protein Data Bank format).
Upload Your File: Click the Upload File button to select a file from your device. You can also drag and drop your file directly into the designated area or paste the raw text content using the Paste Content option.
Start the Conversion: Press the Convert File button to begin the process. Our server will process your file instantly.
Download Your File: Once the conversion is complete, a download link for your new PDB file will appear. Click it to save the file to your device.

Tip: If you encounter an error during conversion, please check the Troubleshooting Guide section below for common causes and solutions.

Input, Output, and Key Changes

Understanding the transformation from the comprehensive CIF format to the conventional PDB format is crucial for using structural data effectively. Here’s a breakdown of the formats and the changes that occur during conversion.

Sample Input (CIF Format)

The CIF (Crystallographic Information File), and its macromolecular extension mmCIF, is the current standard archive format for structural data established by the International Union of Crystallography (IUCr). It uses a tag-based structure to store not only atomic coordinates but also extensive metadata, including experimental details, unit cell dimensions, and symmetry operators needed to construct the full crystal lattice.

Example of a CIF file’s content:

_entry.id                                  4HHB
_cell.length_a                           63.150
_cell.length_b                           83.590
_cell.length_c                           53.800
_symmetry.space_group_name_H-M           'P 21 21 21'
#
loop_
_atom_site.group_PDB
_atom_site.id
_atom_site.label_atom_id
_atom_site.label_comp_id
_atom_site.label_asym_id
_atom_site.label_seq_id
_atom_site.Cartn_x
_atom_site.Cartn_y
_atom_site.Cartn_z
_atom_site.occupancy
_atom_site.B_iso_or_equiv
_atom_site.type_symbol
ATOM 1 N VAL A 1 7.378 23.331 16.038 1.00 13.82 N

Sample Output (PDB Format)

The PDB (Protein Data Bank) format is a legacy, fixed-column format that remains widely used due to its compatibility with countless biochemistry and computational biology programs. It primarily stores the 3D Cartesian coordinates of atoms in a biological molecule and has limited capacity for metadata compared to CIF.

Example of the same atom line after conversion to PDB format:

ATOM      1  N   VAL A   1       7.378  23.331  16.038  1.00 13.82           N

Key Changes in the Conversion Process

The conversion from CIF to PDB involves several important data transformations:

Generation of Biological Assembly: The converter uses symmetry information within the CIF file (e.g., _pdbx_struct_assembly_gen) to generate the coordinates for the molecule’s functional form, known as the biological assembly. This is critical because the raw coordinates in a CIF file often represent only the smallest unique portion of the crystal (the asymmetric unit).
Loss of Crystallographic Metadata: The PDB format cannot store rich crystallographic data. Information like unit cell dimensions (_cell.length_a, etc.) and space group symmetry (_symmetry.space_group_name_H-M) is discarded during conversion. The output focuses solely on the atomic coordinates of the biological unit.
Data Restructuring: The tool parses the flexible tag-value structure of the CIF file’s _atom_site loop and reformats it into the rigid, 80-column ATOM/HETATM records required by the PDB specification.
Handling of Alternate Conformations: If a residue has multiple observed positions (alternate locations), the converter typically retains only the conformation with the highest occupancy (usually labeled ‘A’ or the first one listed), simplifying the structure for subsequent analysis.

Compatible Software

The generated PDB files are ready for immediate use with the most popular molecular modeling, visualization, and simulation software, including:

PyMOL
UCSF Chimera/ChimeraX
VMD (Visual Molecular Dynamics)
GROMACS
AMBER
Schrödinger Maestro

Troubleshooting Guide

Encountering an error can be frustrating, but most issues are easy to fix. Here are the most common problems you might face and how to resolve them.

General Tool Errors

Error: “File size exceeds the limit”

Why it happens: Your uploaded file is larger than the maximum allowed size. Our server has this limit to ensure quick processing for all users.
How to fix: Ensure your CIF file is for a single structure. If processing exceptionally large complexes, please contact us for custom solutions.

Error: “Processing timed out”

Why it happens: The conversion is taking too long to complete, which can occur with extremely large or complex biological assemblies.
How to fix: Verify that the input file is correct. If the structure is inherently large and complex, the conversion may require more resources. Please contact us to discuss options for handling larger computations.

Error: “CAPTCHA validation failed”

Why it happens: Our system uses a CAPTCHA to prevent automated bots. This error occurs if the CAPTCHA was not solved correctly or timed out.
How to fix: Simply reload the page and solve the new CAPTCHA. If you continue to have trouble, please get in touch with our support team.

Conversion-Specific Errors

These errors typically relate to the scientific data within your CIF file.

Error: “Invalid or corrupt CIF file syntax”

Why it happens: The input file does not adhere to the strict formatting rules of the Crystallographic Information File. This can be caused by mismatched quotes, incorrectly defined loop_ blocks, or missing values.
How to fix: Validate your file using an external tool or carefully inspect the file for syntax errors. Ensure all data blocks and loops are correctly formatted according to IUCr standards.

Error: “Failed to parse atom records”

Why it happens: The converter could not find or interpret the _atom_site loop, which contains the essential atomic coordinate information. The file may be a different type of CIF (e.g., for small molecules without this block) or the block may be malformed.
How to fix: Ensure your file is a standard mmCIF file from a source like the RCSB PDB. Check that the _atom_site loop and its associated tags (_atom_site.Cartn_x, etc.) are present and correctly defined.

Error: “Missing symmetry information to build biological assembly”

Why it happens: The tool is designed to build the biological assembly, but the input CIF file lacks the necessary _pdbx_struct_assembly and _pdbx_struct_oper_list records. This can happen with older or non-standard CIF files.
How to fix: If possible, obtain a more recent mmCIF file for your structure from the RCSB PDB, which typically includes this information. Alternatively, you can generate the biological unit first using software like PyMOL or UCSF Chimera and then use the resulting file.

If your problem isn’t listed here, we want to know about it! Please help us improve the tool by reporting the issue.

Support Our Work

We are committed to keeping our scientific tools free and accessible for everyone. If this tool has been helpful in your work, please consider supporting our mission with a donation. Your support directly helps us cover server costs and fund the development of new, powerful tools for the scientific community.

FAQ

How does this converter work?

The tool parses the input CIF file to read atomic coordinates, residue information, and biological assembly definitions. It then uses the symmetry operators specified in the CIF file to generate the complete biological unit and formats this 3D coordinate data into the fixed-column PDB file format.

Can I perform batch conversions?

No, the free online version of this tool processes one file at a time. For inquiries about automated or bulk conversion solutions, please contact our team.

What is the maximum file size?

The maximum allowed file size is 25 MB. If you need to process larger files, please get in touch for a custom solution.

Are my uploaded files stored on the server?

No. Your privacy and data security are our top priorities. Files are uploaded for the sole purpose of conversion and are permanently deleted from our servers immediately after the process is complete.

What’s the next step after converting my file?

Your new PDB file is ready for a wide range of applications. You can use it for 3D visualization in programs like PyMOL or ChimeraX, or as an input for molecular dynamics simulations, docking studies, or other computational chemistry workflows.

How should I cite or mention this tool?

Thank you for asking! If our tool contributes to your research or you feature it on your website, the best way to give credit is to link back to this page. This helps other scientists find our resources and supports our goal of keeping these tools free. A mention on your acknowledgments page or a simple link is greatly appreciated.

References & Suggested Reading

This tool was developed using established structural biology and bioinformatics principles for accurate, reliable results. The resources listed below are the foundational research and key papers that define these standards, and we highly recommend them for a deeper understanding of the scientific principles.

Berman, H. M., Kleywegt, G. J., Nakamura, H., & Markley, J. L. (2014). The Protein Data Bank at 40: Reflecting on the past to prepare for the future. Structure, 22(12), 1687–1698. https://doi.org/10.1016/j.str.2014.10.008
Berman, H. M., Westbrook, J., Feng, Z., Gilliland, G., Bhat, T. N., Weissig, H., Shindyalov, I. N., & Bourne, P. E. (2000). The Protein Data Bank. Nucleic Acids Research, 28(1), 235–242. https://doi.org/10.1093/nar/28.1.235
O’Boyle, N. M., Banck, M., James, C. A., Morley, C., Vandermeersch, T., & Hutchison, G. R. (2011). Open Babel: An open chemical toolbox. Journal of Cheminformatics, 3(1), 33. https://doi.org/10.1186/1758-2946-3-33
Krissinel, E., & Henrick, K. (2007). Inference of macromolecular assemblies from crystalline state. Journal of Molecular Biology, 372(3), 774–797. https://doi.org/10.1016/j.jmb.2007.05.022
Pettersen, E. F., Goddard, T. D., Huang, C. C., Meng, E. C., Couch, G. S., Croll, T. I., Morris, J. H., & Ferrin, T. E. (2021). UCSF ChimeraX: Structure visualization for researchers, educators, and developers. Protein Science, 30(1), 70–82. https://doi.org/10.1002/pro.3943
The PyMOL Molecular Graphics System, Version 2.0 Schrödinger, LLC.

Meet the Authors

Mahdi Morshedi Yekta

Founder & Bioinformatics Developer

Mahdi is the founder of ScienceCodons and a Medical Biotechnologist with a deep passion for computational biology. Holding an M.Sc. in Medical Biotechnology, he specializes in transforming complex biological algorithms into accessible, high-performance web tools, bridging the gap between laboratory sciences and software engineering.

Fatemeh Faryadras

Medical Biotechnologist & Researcher

Fatemeh is a Medical Biotechnologist and researcher. With extensive expertise in genetic engineering, molecular cloning, and cancer biology, she combines her rigorous laboratory background with intuitive design principles to create reliable, user-centered scientific calculators and tools.

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