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Nmr Of Proteins Notes

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Chemical Shift for Amino Acids in Proteins: Every peak in the 1D spectrum is associated with a single proton in the protein. E.g. Alanine: N-H = ~9 ppm (deshielded), CH3 (side chain) = ~0.9 ppm (least deshielded), Hα = ~4.5 ppm (similar in all amino acids). Putting an amino acid in a protein changes the chemical shift slightly as amino acids experience a slightly different chemical environment. Aromatic ring currents Ring currents arise because the 6 π-electrons are delocalised, creating an even larger local magnetic field. Protons above and below the ring (usually from other amino acids) will be deshielded up to 4 ppm. Protons to the side of the ring (usually from other amino acids) will be shielded.

Secondary Structures from Chemical Shift Deviation: Chemical shift deviations from random coil shifts for Hα, Cα and CO are all characteristic of the backbone conformation. Deviation of <1ppm in Hα, or <3ppm in Cα = irregular, unstructured residues, no significant difference from the random coil. Hα positive deviation of >1 ppm =
β-Sheet (regular secondary structures) Hα negative deviation of >1 ppm =
α-Helix

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