PDB to CIF Converter

Molecular File Converter

Convert between chemical file formats with our web-based tool

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Supports PDB, SDF, MOL2 and many more

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Our PDB to CIF Converter is an essential web-based tool for structural biologists and computational chemists. It seamlessly converts legacy Protein Data Bank (PDB) files into the modern macromolecular Crystallographic Information File (mmCIF) format. This conversion is crucial for compatibility with modern analysis software and is the standard required for deposition to the Worldwide Protein Data Bank (wwPDB). The tool ensures your structural data adheres to current standards, overcoming the limitations of the older PDB format, such as atom and chain count limits, thereby preparing your files for validation, analysis, and publication.

How to use (step-by-step)

Follow these simple steps to convert your file in seconds.

Choose Your Formats: Use the “Input Format” and “Output Format” dropdown menus for your conversion. Ensure PDB (Protein Data Bank format) is selected as the input and CIF (macromolecular Crystallographic Information File) as the output.
Upload Your File: Click “Upload File” or drag and drop your file directly into the designated area. You can also paste the file’s content using the “Paste Content” option.
Start the Conversion: Press the “Convert File” button to begin the process. The tool will process your file instantly.
Download Your File: Once the conversion is complete, a download link for your new mmCIF file will appear. Click it to save the file to your device.

Tip: If you see an error during conversion, check the Troubleshooting Guide below—common causes and fixes are listed.

Input, Output, and Key Changes

Understanding the transformation from the legacy PDB format to the modern mmCIF standard is key to accurate data representation. Here’s a breakdown of the formats and the changes that occur.

Sample Input (PDB Format)

The PDB format is a long-standing fixed-column format for storing 3D coordinates of biological macromolecules. While historically significant, it has limitations, particularly with very large structures (over 99,999 atoms) and complex assemblies with many chains.

Example of a PDB file:

ATOM      1  N   ALA A   1      27.222  18.238  35.532  1.00 28.80           N  
ATOM      2  CA  ALA A   1      27.981  19.349  35.038  1.00 28.80           C  
ATOM      3  C   ALA A   1      27.218  20.539  34.469  1.00 28.80           C  
ATOM      4  O   ALA A   1      26.002  20.505  34.238  1.00 28.80           O  
ATOM      5  CB  ALA A   1      29.029  18.995  34.015  1.00 28.80           C  
ATOM      6  H   ALA A   1      26.619  18.421  36.315  1.00 28.80           H  
ATOM      7  HA  ALA A   1      28.423  19.648  35.986  1.00 28.80           H

Sample Output (mmCIF Format)

The mmCIF format is a flexible, key-value data format that has replaced PDB as the archive standard. It is more robust, extensible, and capable of representing much larger and more complex molecular systems without ambiguity.

Example of the same data in an mmCIF file:

# 
loop_
_atom_site.group_PDB 
_atom_site.id 
_atom_site.type_symbol 
_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   1    N   N   ALA   A   1   27.222   18.238   35.532   1.00   28.80 
# ... (subsequent lines follow this format)

Key Changes in the Conversion Process

The conversion from PDB to mmCIF is a structural reformatting to meet modern data standards:

Shift to Key-Value Structure: The primary change is the move from PDB’s rigid, fixed-column format to mmCIF’s self-describing loop_ and key-value pair structure. This makes the data easier to parse programmatically and less prone to formatting errors.
Removal of Limitations: mmCIF resolves the inherent limitations of the PDB format. It supports structures with more than 99,999 atoms and assemblies with more than 62 unique chain identifiers.
Enhanced Data Representation: The mmCIF format can store a much richer set of metadata, including detailed information about experimental conditions, refinement statistics, and sample source, which is often lost or stored in non-standard REMARK fields in PDB files.
Official Deposition Standard: The Worldwide Protein Data Bank (wwPDB) mandates mmCIF as the sole acceptable format for the deposition of macromolecular structures. Converting your file ensures it is ready for submission.

Compatible Software

The generated mmCIF files are fully compatible with all modern molecular graphics and analysis software, including:

UCSF Chimera/ChimeraX
PyMOL
Coot
Phenix
CCP4

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: If your PDB file is exceptionally large, try removing non-essential elements like solvent molecules (HOH) or splitting a multi-model NMR file into separate models. For processing larger files, please contact us for custom solutions.
Error: “Processing timed out”
- Why it happens: The conversion for your molecule is taking too long, which can happen with extremely large or complex structures.
- How to fix: Simplify your input file where possible. If the issue persists because your structure is inherently complex, 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.

Conversion-Specific Errors

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

Error: “Invalid PDB format” or “Parsing error”
- Why it happens: The input file does not strictly adhere to the PDB format specification. This can be caused by incorrect column spacing, missing atoms, or corrupted lines of text.
- How to fix: Use a structure validation tool or a text editor to check for formatting issues. Ensure that all ATOM and HETATM records follow the official fixed-column layout.
Error: “Unrecognized residue or atom name”
- Why it happens: The PDB file contains non-standard residue or atom names that do not exist in the standard chemical component dictionary used for conversion.
- How to fix: Ensure all residue and atom names conform to PDB standards. You may need to manually edit the file to correct these names or use modeling software to standardize the structure first.
Error: “Incomplete or missing header information”
- Why it happens: While not always a fatal error, a PDB file missing HEADER, CRYST1, or SCALE records may be processed incorrectly, especially if symmetry or crystal packing information is important.
- How to fix: If this information is critical for your analysis, add the necessary records to the PDB file header before uploading. Most visualization software can help generate or correct this information.

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 atomic coordinate records and other standard fields from your input PDB file. It then reorganizes this information into the structured, key-value format defined by the macromolecular Crystallographic Information File (mmCIF) dictionary, ensuring compliance with modern data standards.

Can I perform batch conversions?

The free web version of this tool processes one file at a time. For automated, high-throughput, or batch-processing solutions, please contact our team to discuss our API and custom service options.

What is the maximum file size?

The maximum allowed file size for our online converter is 25 MB. If you need to process larger macromolecular complexes, please get in touch with our support team for a custom solution.

How should I cite or mention this tool?

If you use our tool for your research or feature it on a website, the best way to give credit is to link back to this page. Proper citation helps us maintain these tools as a free resource for the scientific community. For a publication, you can mention the tool’s name and URL in your methods section. A donation is also a great way to show support.

References & Suggested Reading

This tool was developed in line with established principles in structural biology 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., 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
Bourne, P. E., Berman, H. M., Watenpaugh, K., Westbrook, J. D., & Fitzgerald, P. M. (1997). The macromolecular Crystallographic Information File (mmCIF). Methods in Enzymology, 277, 571–590.
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
Goddard, T. D., Huang, C. C., Meng, E. C., Pettersen, E. F., Couch, G.S., Morris, J. H., & Ferrin, T. E. (2018). UCSF ChimeraX: Meeting modern challenges in visualization and analysis. Protein Science, 27(1), 14-25. https://doi.org/10.1002/pro.3235
wwPDB consortium. (2019). Protein Data Bank atomic coordinate entry format description Version 3.30. wwPDB. http://www.wwpdb.org/documentation/file-format-content/format33/v3.3.html

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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