The spectroscopic properties of the lipodepsipeptide, syringomycin E

The spectroscopic properties of syringomycin E, an antibiotic lipodepsinonapeptide associated with pathological states in plants, have been investigated by uv absorbance and CD spectroscopies, and by the synthesis of relevant model compounds. Initial studies [E. Vaillo, A. Ballio, P. L. Luisi, and R. M. Thomas (1990) in Peptides 1990, Giralt, E. & Andreu, D., Eds., Escom Scientific, Leiden, Netherlands] suggested that a significant contribution to the spectra was due to the presence of a zdehydroaminobutyric acid residue in the amino acid sequence. The model peptides N-Boc-L-Phe-delta zAbu-OMe and its analogue, N-Boc-L-Phe-L-Thr-OMe, lacking the unsaturated bond, were synthesized using standard solution chemistry, and a detailed investigation was made in which the spectra of the models and that of syringotoxin (an antibiotic closely related to syringomycin E but without a Phe residue) were compared with those of syringomycin E under a variety of solvent conditions. The uv absorbance spectra of both N-Boc-L-Phe-delta zAbu-OMe and syringomycin E clearly showed the presence of the unsaturated residue while the CD spectra were complex, environmentally sensitive, and contained contributions from both the delta zAbu and Phe residues. In the course of these studies extinction coefficients were obtained for syringomycin E and its dipeptide model. The origins of the uv and CD spectra are discussed in detail, and a comparison is made with the spectra of other, similar lipopeptide antibiotics. Finally, a structural model for syringomycin is proposed in which the changes induced in the spectrum by alterations in the solvent environment are accommodated.