NMR structure of active and inactive forms of the sterol‐dependent antifungal antibiotic bacillomycin L

The antifungal antibiotic lipopeptide bacillomycin  L [cyclo‐( l ‐Asp1‐ d ‐Tyr2‐ d ‐Asn3‐ l ‐Ser4‐ l ‐Gln5‐ d ‐Ser6‐ l ‐Thr7‐β‐amino fatty acid)] from Bacillus subtilis belongs to the iturinic family of antifungal agents and acts with a strict sterol‐phospholipid dependence on biomembranes. This antibiotic has been analysed using solution NMR spectroscopy in its native active form and its inactive ( l ‐Asp1,  d ‐Tyr2) di‐O‐methylated form. The structures were calculated under NMR‐derived restraints using molecular‐dynamic simulated‐annealing protocols starting from a random array of atoms. The structure of the native antibiotic is spread over different conformers in which two families are recognized. It was found that most structures have dihedral φ and ψ angles defining a type‐II′β‐turn including amino acids 5–8, in certain cases stabilized by a 8H N ‐5CO hydrogen bond, whereas a minority of structures adopt an inverse γ‐turn including amino acids 6–8, stabilized in all cases by an 8H N ‐6CO hydrogen bond. The di‐O‐methylation of l ‐Asp1 and d ‐Tyr2, an amino acid strictly conserved within the iturinic group of antibiotics, does not induce major differences in the NMR spectra and in the NMR structures. The results are discussed in relation to the specific loss of interaction with sterols when the native antifungal bacillomycin  L is methylated on the conserved d ‐Tyr2 position.