Nitrogen-15 nuclear magnetic resonance spectrum of alumichrome. Detection by a double resonance Fourier transform technique.

Abstract

Alumichrome, the Al/sup 3 +/ analogue of ferrichrome, a ferric cyclohexapeptide, was enriched in /sup 15/N by growing the fungus Ustilago sphaerogena in a medium containing 99.5% /sup 15/N-ammonium acetate as the sole nitrogen source. The magnitude of the amide proton--nitrogen scalar coupling constant, obtained directly from the /sup 1/H NMR spectrum is correlated with the angular deviation from planarity at the peptide bond. The /sup 15/N amide resonanceees were measured indirectly by heteronuclear double resonance methods. The amide /sup 1/H NMR spectrum was obtained by Fourier transform at 220 MHz while the /sup 15/N resonances were irradiated selectively with low power near 22.3 MHz. The chemical shifts of the amide /sup 15/N resonances are sensitive to conformation and the responses to temperature and solvent changes provide excellent criteria for determining the spatial configuration of each -NH--CO-group within the peptide structure. An extension of the Fourier transform double resonance method, Fourier internuclear difference spectroscopy (FINDS), was tested successfully on the amide region of the unenriched peptide thus permitting detection of /sup 15/N resonances at natural abundance. The results suggest a large potential for the method.