Solution conformation of ferrichrome, a microbial iron transport cyclohexapeptide, as deduced by high resolution proton magnetic resonance

Abstract

The proton magnetic resonance spectra of the cyclohexapeptides deferriferrichrome and its Al³⁺ chelate (alumichrome) dissolved in water and in dimethyl sulfoxide are compared. The spin-spin coupling constants (J_NC) between the amide NH and C_αH protons suggest that the metal affects the conformation of the δ-N-acetyl-δ-N-hydroxy-l-ornithine sidechains more than that of the peptide backbone. The data are consistent with a structure containing two transannular H-bonds in both deferriferrichrome and alumichrome as in the Schwyzer model of cyclohexapeptides. Metal chelation induces chemical shifts which are pronounced for all of the amide NH's and markedly reduces the solvent interactions with four of the NH's. The solution conformation of the chelate is in good agreement with the X-ray structure of crystalline ferrichrome A, a closely related compound.