Fourier transform IR and NMR studies of hydrogen bonding in Helminthosporium carbonum toxin

HC toxin, a metabolite of Helminthosporium carbonum is a host-specific phytotoxin affecting certain varieties of corn. It is a cyclic tetrapeptide containing two alanyl residues, one proline, and the unusual 2-amino-9,10-epoxy-8-oxodecanoic acid (AEO). The sequence and the configuration at the alpha carbon of the four amino acids have been reported. Although there has been some discrepany the sequence cyclo-(Ala super(1)-Ala super(2)-AEO super(3)-Pro super(4)) is now generally accepted. Walton et al. proposed a D configuration for the prolyl residue and L for the other three residues, a proposal inconsistent with subsequent chemical and NMR findings. The authors report here a study of hydrogen bonding of the native HC toxin 1 and its diacetyl derivative 2 using five different IR and NMR parameters. The assignment of the super(1)H spectrum of 1 and 2 was accomplished by 1D and 2D NMR at 200 and 600 MHz.