NMR (Nuclear Magnetic Resonance) is a spectroscopic technique based on resonant absorption and emission of radio waves by atomic nuclei placed in an intense magnetic field. For peptides, it constitutes the reference method for structural elucidation in solution, complementary to mass spectrometry (identity/purity) and X-ray crystallography (solid-state structure).
Observed nuclei. NMR-active nuclei for peptide analysis are: 1H (proton, 99.98% abundance, maximum sensitivity, direct observation of Hα, HN, side chains), 13C (1.1% abundance, requiring isotopic enrichment for peptides >10 kDa, info on Cα and carbonyls), 15N (0.37% abundance, enrichment needed, info on backbone amide NH), 19F (probe for fluorinated peptides), 31P (phosphorylated peptides). Modern spectrometers operate at 400-950 MHz (field 9.4-22.3 T), resolution increasing with field.
1D and 2D experiments. Typical peptide NMR combines: 1D 1H (global spectrum, 8-12 ppm amide, 7-8 ppm aromatic, 3-5 ppm Hα, 0-3 ppm methyls), 2D COSY/TOCSY (scalar connectivities, amino acid spin system assignment), 2D NOESY/ROESY (dipolar through-space connectivities <5 Å, constraints for 3D structure), 2D HSQC/HMBC (1H-13C or 1H-15N correlations through 1 bond or 2-3 bonds), and for larger peptides 3D HNCA, HNCOCA, HNCACB, CBCACONH experiments for sequential assignment.
Peptide applications. NMR enables: (1) structural confirmation by comparison with standards, (2) de novo sequential assignment, (3) 3D structure determination in solution (NOE constraints + J-coupling dihedral angles, XPLOR/CYANA/CNS software), (4) characterisation of cyclic peptides (turn conformations), (5) dynamics and conformational exchange (R1, R2, heteronuclear NOE, relaxation dispersion). Alternative to cryo-EM and X-ray when crystallisation is impossible. See USP <761> for pharmaceutical NMR guidelines.
Décryptez la caractérisation complète d'un peptide : RP-HPLC préparative et analytique, LC-MS haute résolution Orbitrap/QTOF, RMN 2D (COSY/TOCSY/NOESY), dégradation d'Edman, cartographie peptidique MS/MS, dichroïsme circulaire. Les 10 techniques qui séparent un peptide research-grade d'un peptide douteux, avec seuils de détection et stratégie de QC.
Maitrisez les mecanismes qui detruisent vos peptides : oxydation Met, deamidation Asn, hydrolyse, aggregation. Protocoles de lyophilisation, conditions de stockage, reconstitution optimale et QC shelf-life. Le pilier technique que tout laboratoire doit integrer pour garantir l'integrite de ses peptides en recherche exploratoire.
Principe Merrifield, stratégies Fmoc et Boc, résines (Wang, Rink amide, 2-chlorotrityl), agents de couplage (HBTU, HATU, DIC/HOBt), cycle de synthèse, clivage TFA, purification RP-HPLC, contrôle qualité LC-MS et test Kaiser. Le pilier scientifique de référence sur la fabrication des peptides synthétiques.