Analysis of the 1H‐NMR spectra of ferrichrome peptides. I. The non‐amide protons

Abstract

AbstractThe thus far unexplored aliphatic region of the proton magnetic resonance spectra of ferrichrome peptides was investigated at 360 MHz. Six isomorphic diamagnetic analogues of the ferric cyclohexapeptide differing in the coordinated cation (AL3+ or Ga3+) and the amino acid composition were studied in d6‐DMSO solution. By use of a novel resolution enhancement technique which applies a sinusoidal half‐wave window to the free induction decay combined with multiplication by an increasing exponential, the proton chemical shifts and spin‐spin couplings were accurately measured. Homonuclear decoupling combined with Fourier difference spectroscopy was used to selectively extract resonances out of crowded spectral regions. The spectra revealed unique features of fine structure in the proton resonance lines. Thus, the conformation‐dependent geminal coupling constants of glycyl α‐protons were found to be constant throughout the suite of analogue peptides. A similar invariance was observed for the vicinal coupling constants between α‐, β‐, γ‐, and δ‐protons in the ornithyl side chains. Comparison of the proton spin–spin coupling constants with the crstallographic dihedral angles led to a unique stereochemical assignment of the side‐chain resonances. The combined data sets of x‐ray atomic coordinates and 1H‐nmr spin‐spin coupling parameters have been used to calibrate the coefficients for a Karplus curve related to the torsional x angles in amino acid side chains: Structurial information was also obtained for the seryl residues, where the multiplet structures of the OH resonances indicate preferred spatial arrangements of the side chains.