Three-to-one converter
v1.0.0Convert three-letter amino acid codes (Ala, Arg) to single-letter format (A, R)
Convert three-letter amino acid codes (Ala, Arg) to single-letter format (A, R)
Paste three-letter residue codes into the Three-to-one converter, then select Convert. Codes such as Cys-Tyr-Ile become CYI. The conversion runs in your browser, preserves FASTA headers by default, and returns a copyable preview, a text download, and a run log. Use One-to-three converter for the reverse direction.
For the nine residues of oxytocin, use the default settings:
Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-GlyOutput:
CYIQNCPLGMultiple peptides stay separate when each has a FASTA header. This example also uses the defaults:
>oxytocin
Cys Tyr Ile Gln Asn Cys Pro Leu Gly
>vasopressin
Cys Tyr Phe Gln Asn Cys Pro Arg GlyOutput:
>oxytocin
CYIQNCPLG
>vasopressin
CYFQNCPRGThese examples describe residue identities. One-letter notation does not encode terminal amidation, disulfide bonds, or stereochemistry.
| Input | Accepted content |
|---|---|
Protein sequence input | Three-letter amino acid codes, with optional FASTA headers. Paste text or upload .txt, .fasta, .fa, .fas, or .seq files. .docx and text-based .pdf uploads have their text extracted. The file limit is 50 MB. |
| Code spelling | Matching ignores case: Ala, ALA, ala, and aLa all become A. |
| Separators | Auto-detect accepts hyphens, commas, tabs, and other whitespace, including mixed separators. Concatenated codes such as AlaArgAsn are also accepted. Choose Semicolon (;) or Custom... for other delimiters. |
| Wrapped or copied text | Wrapped lines form one sequence until the next FASTA header. Position numbers at the start and end of a line are removed when separated by whitespace. Typographic dashes, invisible formatting characters, and leading document bullets are normalized. |
Put record names on lines beginning with >. A plain name line, spreadsheet header, or prose is not recognized as a sequence label and can become unknown codes. Provide only the residue column from a spreadsheet. Scanned PDFs require text recognition before uploading.
| Setting | Behavior and default |
|---|---|
Input delimiter | Auto-detect is the default. Explicit choices are Hyphen (-), Space, Comma (,), Tab, Semicolon (;), and Custom.... Space accepts repeated whitespace. Other explicit delimiters split on their literal value, with surrounding whitespace trimmed. |
Custom delimiter | Appears for Custom.... Defaults to /; accepts literal characters, including multi-character separators. An empty value reads concatenated three-letter codes. |
Unknown residues | Convert to X is the default. Skip (omit from output) removes unrecognized codes and shortens the sequence. Keep original code includes the original text in brackets. All three policies produce a warning when unrecognized codes occur. |
Output case | Uppercase (A, R, N) is the default; Lowercase (a, r, n) changes converted letters. Bracketed unknown codes retain their original spelling. |
Preserve FASTA format | On by default. Keeps header text and record order. Turning it off removes headers while keeping records on separate lines. Empty records are retained as headers when preservation is on; header-only input returns an error. |
Characters per line | Defaults to 0, with no wrapping. Values from 0 to 200 wrap sequence text at that character width. Headers are never wrapped. With bracketed unknown codes, wrapping counts all characters, including brackets. |
With Input delimiter set to Custom..., Custom delimiter set to /, and Unknown residues set to Keep original code:
Ala/Foo/ArgOutput:
A[Foo]RThe warning reports one unrecognized code retained in brackets. This is readable text, but it is not a valid one-letter sequence for most analysis tools. With Skip, the result is AR; if every code would be skipped, the converter returns an error instead of an empty download. An incomplete final triplet follows the same unknown-code policy.
| Result | Contents |
|---|---|
| Preview | Converted sequence, with a summary showing processed records, output residues, and unrecognized codes. The record count includes empty FASTA records. |
| Result warnings | Notices above the preview describe whether unrecognized codes were replaced, skipped, or retained. |
Converted .txt file | The exact preview text. Uploaded files keep their base filename; pasted input downloads as converted.txt. |
run.log | Counts, formatting cleanup, selected conversion settings, and warnings. Available in Files and the Logs tab. It does not repeat the sequence or headers. |
Inspect warnings before using the result. Xaa is a recognized unknown-residue symbol and converts to X without an unrecognized-code warning. Modified residues such as MSE follow the selected unknown policy; they are not automatically changed to their parent amino acid.
The standard mappings follow the IUPAC-IUB amino acid nomenclature. The INSDC amino acid abbreviations also list the extended codes, including Sec, Pyl, and Xle.
| Amino acid | Three-letter | One-letter |
|---|---|---|
| Alanine | Ala | A |
| Arginine | Arg | R |
| Asparagine | Asn | N |
| Aspartic acid | Asp | D |
| Cysteine | Cys | C |
| Glutamic acid | Glu | E |
| Glutamine | Gln | Q |
| Glycine | Gly | G |
| Histidine | His | H |
| Isoleucine | Ile | I |
| Leucine | Leu | L |
| Lysine | Lys | K |
| Methionine | Met | M |
| Phenylalanine | Phe | F |
| Proline | Pro | P |
| Serine | Ser | S |
| Threonine | Thr | T |
| Tryptophan | Trp | W |
| Tyrosine | Tyr | Y |
| Valine | Val | V |
| Selenocysteine | Sec | U |
| Pyrrolysine | Pyl | O |
| Asparagine or aspartic acid | Asx | B |
| Glutamine or glutamic acid | Glx | Z |
| Leucine or isoleucine | Xle | J |
| Unspecified amino acid | Xaa | X |
| Starting point | Tool |
|---|---|
| Three-letter residue codes | This converter |
| One-letter protein sequence | One-to-three converter |
| PDB or mmCIF structure file | PDB to FASTA extracts its sequence |
| FASTQ reads with quality scores | FASTQ to FASTA removes the quality section |
| One-letter sequence needing FASTA headers | TXT to FASTA adds or preserves names |
Yes, when position numbers are separated from the codes by whitespace. For example, 1 Ala-Arg-Asn 3 becomes ARN. Numbers attached to a residue, such as Ala1, are not removed and follow the unknown-code policy.
It changes amino acid notation only. It does not translate nucleotides, infer protein sequence from prose, or preserve chemical modifications that cannot be represented by the supported one-letter alphabet. FASTA headers keep descriptions intact, but the converted residue codes do not encode those modifications.
Convert CSV or TSV sequence tables to FASTA with automatic delimiter detection and column mapping.
Paste or upload a sequence file in any common format and download FASTA. The format is detected automatically, and nothing leaves your browser.
Paste or upload FASTA and download a CSV or TSV table for spreadsheets, or a plain text file with one sequence per line.
Convert one-letter amino acid sequences to three-letter codes with customizable formatting.
Extract observed or full SEQRES sequences from PDB structures with chain selection and RCSB fetching.
Paste or upload DNA, RNA, or protein text and download FASTA. Sequence names, wrapped lines, and tables are detected automatically, and nothing leaves your browser.
Translate DNA sequences into proteins across reading frames and genetic code tables.
Convert PDB coordinates to mmCIF with model selection and warnings for omitted records.
Convert PDB structures up to 20,000 atoms with Open Babel and clear chemistry caveats.
Reverse translate protein sequences into DNA with codon optimization and GC-content controls.