--- title: "Sanger’s reagent: N-terminal labelling Template | MolDraw" url: https://www.moldraw.com/templates/sanger-fdnb-n-terminal/ description: "Sanger’s reagent: N-terminal labelling template: The N-terminal amine of a peptide attacks 1-fluoro-2,4-dinitrobenzene through a Meisenheimer complex and…" lang: en publisher: Scidart Academy generated: 2026-11-13 --- [MolDraw](https://www.moldraw.com/) › [Templates](https://www.moldraw.com/templates/) › [Biochemistry](https://www.moldraw.com/templates/biochemistry/) › Sanger’s reagent: N-terminal labelling [Biochemistry](https://www.moldraw.com/templates/biochemistry/)Premium # Sanger’s reagent: N-terminal labelling template ![Sanger’s reagent: N-terminal labelling template: The N-terminal amine of a peptide attacks 1-fluoro-2,4-dinitrobenzene through a Meisenheimer complex and fluoride leaves; 6 M HCl breaks every peptide bond but not the aryl C–N bond, so the DNP-amino acid names the N-terminal residue.](https://www.moldraw.com/templates/sanger-fdnb-n-terminal.webp) [Edit in MolDraw](https://www.moldraw.com/)Download with Pro Premium template. Preview and edit it free in MolDraw; downloading and exporting it is part of [MolDraw Pro](https://www.moldraw.com/pricing). The N-terminal amine of a peptide attacks 1-fluoro-2,4-dinitrobenzene through a Meisenheimer complex and fluoride leaves; 6 M HCl breaks every peptide bond but not the aryl C–N bond, so the DNP-amino acid names the N-terminal residue. ## Mechanism step by step FDNB labels the free α-amino group by SNAr; acid hydrolysis then releases the yellow DNP-amino acid 1. **Addition (slow)** – The free α-NH₂ attacks the carbon bearing F (R = the rest of the peptide) 2. **Meisenheimer complex** – The negative charge spreads onto the ortho nitro group 3. **Loss of fluoride** – Aromaticity returns 4. **Deprotonation** – Hydrogencarbonate removes H⁺ from nitrogen: the DNP-peptide ## Key points - SNAr: the ortho and para NO₂ groups hold the negative charge of the Meisenheimer complex. Addition is slow, so fluoride is the best leaving group. - The aryl C–N bond survives 6 M HCl, which cuts every peptide bond. The yellow DNP-amino acid is identified by chromatography. - The method is destructive: one end residue per sample. Lys ε-NH₂ and Tyr OH are labelled too. - Sanger used FDNB to find the N-termini of insulin’s two chains (1945), on the way to the first protein sequence. Drawn with 184 atoms, 14 arrows, 8 shapes – every one editable in MolDraw. ## Sources - [Nelson & Cox, Lehninger Principles of Biochemistry, 8th ed., 2021, ch. 3](https://openlibrary.org/isbn/9781319228002) Textbook - [Berg, Tymoczko, Gatto & Stryer, Biochemistry, 9th ed., 2019](https://openlibrary.org/isbn/9781319114657) Textbook - [Sanger, Biochem. J. 1945, 39, 507](https://doi.org/10.1042/bj0390507) Original paper ## How to use this template 1. Press **Edit in MolDraw**. The template opens on the canvas of the free MolDraw editor. 2. Change anything: move structures, edit atoms and labels, recolour highlights or delete what you do not need. 3. Add your own structures, arrows and text next to it, or combine several templates on one page. 4. Export the drawing as PNG, SVG or PDF for slides, worksheets, theses and papers. ## Frequently asked questions ### Is the Sanger’s reagent: N-terminal labelling template free? You can preview the Sanger’s reagent: N-terminal labelling template here and open and edit it in MolDraw for free. Downloading or exporting it is part of MolDraw Pro, together with the other premium templates. ### Can I edit the Sanger’s reagent: N-terminal labelling template? Yes. It opens as an ordinary MolDraw drawing, so every structure, arrow, label and colour can be changed, and you can add your own content around it. ### Is the chemistry checked? Every structure is checked with RDKit and every curly arrow passes MolDraw’s electron-flow audit. The sources used are listed on this page. ### Are the biochemical pathways drawn with structures? Yes. 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