¹³C NMR Spectroscopic Control of the Synthesis of Alamethicin F 30 and its Segments

Abstract

All segments built up for the total synthesis of the a-helical antibiotic alamethicin F 30 (1) were investigated by l3C NMR spectroscopy of rnethanolic solutions. With a few exceptions the signals were unequivocally assignable by comparison with spectra of related peptides, by coupled spectra and J-modulated spin-echo experiments. For the first time a synthetic and highly pure alamethicin F 30 preparation could be studied by I3C NMR spectroscopy at 100.6 MHz. All results were in full agreement with the synthesized intermediates and the final product, and with their conformations as derived from circular dichroism and X-ray analysis. Partially there is a striking retention of conformation from smaller to larger segments. '3GNMR-Spektroskopishe Synthesekontrolle von Alamethicin F 30 und seinen SegmentenAlle zur Totalsynthese des Alamethicin F 30 (1) aufgebauten Segmente wurden mittels 13C-NMRSpektroskopie methanolischer LXisungen untersucht. Mit wenigen Ausnahmen waren alle Signale eindeutig durch Vergleich mit anderen Peptidspektren, gekoppelte Spektren und vor allem durch J-modulierte Spin-Echo-Experimente zuzuordnen. Erstmals konnte ein synthetisches und hochreines Alamethicin-Praparat durch '3C-NMR-Spektroskopie bei 100.6 MHz untersucht werden. Alle Resultate sind in voller Ubereinstirnmung mit den synthetisierten Zwischenstufen und dem Endprodukt sowie deren Konformationen, die aus dem Circulardichroismus und aus Rontgenstrukturanalysen entnommen werden konnen. Es wurde eine zum Teil iiberraschende Erhaltung der Konformation beim Ubergang von kleineren zu groReren Segmenten beobachtet.Our total synthesis of the linear eicosapeptide antibiotic alamethicin F 30 (1) has been described in the preceding publication'). In order to support the common analyses used for control of this peptide synthesis we report in the following a complete I3C NMR study of all segments and the final product alamethicin F 30 (1).The results are also interpreted in terms of conformational assignments, particulary for those segments where X-ray analyses or CD spectra were available.The first I3C NMR study of natural alamethicin was reported in 1974, and it included already "C NMR longitudinal relaxation time (q) measurementsz), an interpretation in terms of a-helical conformationz), and data of some dipeptide derivatives. In the following years a considerable variety of synthetic Aib-containing oligo-and polypeptides were measured by 13C NMR3-m.These spectroscopic studies dealt with the con formational development during stepwise synthesis of sequential oligomers bound to the polyoxyethylene and they included the N-terminal natural undecapeptide sequence of alamethicin'). 13C NMR was of particular value for the conformational assignments of @-helices such as the undecapeptide Boc-Ala-(Aib-Ala),-Glu-0 VCH Verlagsgesellschaft mbH, D-6940 Weinheim, 1985

The Interactions with Solvent, Heat Stability, and 13C‐Labelling of Alamethicin, an Ion‐Channel‐Forming Peptide

“…1D I3C spectrum of alamethicin in CD,OD is similar to that previously reported for the synthetic peptide with Glu in position 18 (Schmitt and Jung, 1985b). The a-carbons and the sidechain3 of all alamethicin non-Ala(Me) residues were assigned previously using natural abundance 'H-''C correlation experiments (Kelsh et al, 1992;Yee et al, 1995).…”

The chemistry of peptides related to metabolites of Trichoderma spp. 2. An improved method of characterization of peptides of 2-methylalanine

“…The structures of many of the intermediates in the syntheses have been established by detailed assignment of 'H and I3C nmr spectra (see e.g., ref. 18), by elemental analysis, and, in some cases, by X-ray crystallographic studies. However, the structures of several other intermediates are open to doubt.…”

Conformational studies of Aib‐rich peptides containing lactam‐bridged side chains: Evidence of 310‐helix formation

“…This is shown by Cβ‐selective HMQC experiments (Figure 4). A difference in 13 C chemical shifts of the two pro‐chiral methyl groups of 2 ppm or higher has been ascribed to the presence of a stable helical conformation in solution 26–28. The Δδ values of the three peptides, determined by Cβ‐selective HMQC experiments are reported in Table I.…”