NMR structure determination of a synthetic analogue of bacillomycin Lc reveals the strategic role of l-Asn1 in the natural iturinic antibiotics

Iturins are a group of antifungal produced by Bacillus subtilis. All are cyclic lipopeptides with seven α-amino acids of configuration LDDLLDL and one β-amino fatty acid. The bacillomycin L is a member of this family and its NMR structure was previously resolved using the sequence Asp-Tyr-Asn-Ser-Gln-Ser-Thr. In this work, we carefully examined the NMR spectra of this compound and detected an error in the sequence. In fact, Asp1 and Gln5 need to be changed into Asn1 and Glu5, which therefore makes it identical to bacillomycin Lc. As a consequence, it now appears that all iturinic peptides with antibiotic activity share the common β-amino fatty acid 8-l-Asn1-d-Tyr2-d-Asn3 sequence. To better understand the conformational influence of the acidic residue l-Asp1, present, for example in the inactive iturin C, the NMR structure of the synthetic analogue SCP [cyclo (l-Asp1-d-Tyr2-d-Asn3-l-Ser4-l-Gln5-d-Ser6-l-Thr7-β-Ala8)] was determined and compared with bacillomycin Lc recalculated with the corrected sequence. In both cases, the conformers obtained were separated into two families of similar energy which essentially differ in the number and type of turns. A detailed analysis of both cyclopeptide structures is presented here. In addition, CD and FTIR spectra were performed and confirmed the conformational differences observed by NMR between both cyclopeptides.