The occurrence of ansamers in the synthesis of cyclic peptides

Abstract

α-Amanitin is a bicyclic octapeptide composed of a macrolactam with a tryptathionine cross-link forming a handle. Previously, the occurrence of isomers of amanitin, termed atropisomers has been postulated. Al-though the total synthesis of α-amanitin has been accomplished this aspect still remained unsolved. We performed the synthesis of amanitin analogues, accompanied by in-depth spectroscopic, crystallographic and molecular dynamics studies. The data unambiguously confirmed the synthesis of two amatoxin-type isomers, for which we propose the term ansamers. The natural structure of the P-ansamer can be ansa-selectively synthesized using an optimized synthetic strategy. We believe that the newly described terminology does also have implications for many other peptide structures, e.g. norbornapeptides, lasso peptides, tryptorubins and others, and helps to unambiguously describe conformational isomery of cyclic peptides.

The influence of N-methylation on the ansamers of an amatoxin: Gly5Sar-amanullin

“…For amatoxins, this ring is the peptide ring (macrolactam), where the direction is defined by the sequence N → C α → carbonyl-C. Because of this directionality, the main ring has two distinct sides, and the position of the second cycle with respect to the main ring defines two distinct isomers. In our previous study, 29 we suggested the term ansamers for this type of isomerism, with P -ansamer denoting one option and M -ansamer the other. To assign the stereochemical descriptor (M or P configuration), the CIP (Cahn-Ingold-Prelog) rules can be followed.…”

“…So far, only the P -ansamers of amatoxins have been isolated from mushrooms, 16 but in the laboratory we could also synthesise M -ansamers. 24,29 We synthesised Gly5Sar-amanullin (Figure 1.b) using our previously developed strategy: 24,29 the iodine-mediated tryptathionine bridge formation and the bicycle ring-closure between Hyp2 and Ile3. We verified the product by HPLC-HRMS, 1 H-NMR spectroscopy and UV-VIS spectroscopy.…”

“…[26][27][28] However, recently we were able to synthesise, isolate and characterise both isomers of the amatoxin amanullin. 29 We also proposed the term 'ansamers' for this particular isomerism, where P ansa is the natural isomer with the bridge above the macrolactam ring, and M ansa is the newly synthesized non-natural isomer with the bridge below the macrolactam ring. Formally, the two ansamers can be interconverted by rotating the tryptathionine bridge around the (imaginary) axis between the two bridgeheads, but this rotation is hindered by the macrolactam ring.…”

“…Spectroscopic data indicated that amanullin only forms a single dominant backbone conformation in each of the ansamers of amanullin (with two conformations of the Ile3-ethyl group in the crystal structure of the M -ansamer). 29 The conformation of the P -ansamer is stabilized by a strong hydrogen bond from the amide of Gly5 to the carbonyl oxygen of Asn1. This hydrogen bond is known to contribute to the overall rigidity of the amatoxin scaffold.…”

“…This hydrogen bond is known to contribute to the overall rigidity of the amatoxin scaffold. 29,40,41 Here, we investigate the amatoxin Gly5Sar-amanullin, in which the amide of Gly5 has been methylated (sarcosine), thereby prohibiting the stabilizing hydrogen bond. N-methylation is a well-established strategy to modify the structure and dynamics of monocyclic peptides.…”