Studies on the selective trifluoroacetolytic scission of native peptaibols and model peptides using HPLC and ESI-CID-MS.

Representative members of a group of linear, N-acylated polypeptide antibiotics (peptaibols) containing alpha-aminoisobutyric acid (Aib) and, in part, isovaline (Iva), as well as proteinogenic amino acids and a C-terminal-bonded 2-amino alcohol, were treated with anhydrous trifluoroacetic acid (TFA) at 37 degrees for 0.5-26 h. The resulting fragments were separated by HPLC and characterized by electrospray ionization collision-induced dissociation mass spectrometry (ESI-CID-MS). The following 16-20-residue peptaibols were investigated: natural, microheterogeneous mixtures of antiamoebins and alamethicin F50, uniform paracelsin A, and synthetic trichotoxin A50/E. In the natural peptides, bonds formed between Aib (Iva) and Pro (Hyp) were rapidly and selectively cleaved within 0.5 h. Furthermore, TFA esters of the C-terminal amino alcohols were formed. Depending on time, release of C-terminal tri- and tetrapeptides as well as amino acids from the major fragments was observed. Synthetic homooligopeptides, namely Z- and Ac-(Aib)(10)-O(t)Bu and Z-(Aib)(7)-O(t)Bu, were analyzed for comparison. On treatment with TFA, a regular series of Z-(Aib)(10-5)-OH from Z-(Aib)(10)-O(t)Bu were detected within 0.5 h, and, after 3 h, release of a regular series of Z-(Aib)(7-3)-OH from Z-(Aib)(7)-O(t)Bu were observed. Moreover, concomitant release of the series of H-(Aib)(10-3)-OH from the decapeptide occurred. From these data, a repetitive cleavage mechanism via intermediate formation of C-terminal oxazolones on trifluoroacetolysis is proposed. Furthermore, their formation and stability in native peptaibols are correlated with subtle structural differences in protein amino acids linked to Aib. From the conspicuous concordance of the formation and abundance of regular series of trifluoroacetolytic fragments and of positive ions of the b-series in CID-MS, the generation of intermediate oxazolonium ions in both gas and liquid phase is concluded.