We have synthesized by solution methods and fully characterized the Nα-blocked heptapeptide methylamide mBrBz-[L-Iva-L-(αMe)Val]2-L-(αMe)Phe-L-(αMe)Val-L-Iva-NHMe, fully based on conformationally constrained C α-methylated α-amino acids. An X-ray diffraction investigation of the N α-benzyloxycarbonylated analogue showed that in the crystal state both independent molecules (A and B) in the asymmetric unit of the peptide adopt a fully developed, regular, right-handed 310-helical structure, although molecule A would be slightly distorted at the C-terminal residue. Solution conformational analysis on the mBrBz-blocked peptide was carried out in CDCl3 by means of NMR spectroscopy. For structure determination we performed restrained molecular dynamics simulations in CDCl3 based on a search of the conformational space derived from a simulated annealing strategy. For this peptide the NMR observables can be described by a single backbone conformation, more specifically a rigid 310-helix spanning the amino acid sequence from residue 1 to residue 6. The C-terminal methylamido NH group seems to be involved simultaneously in two H-bonds (with the preceding i − 3 and i − 4 carbonyl groups). Although in this peptide model there are no distinct NOE distances for discriminating 310- versus α-helix conformation, the sum of all NMR-derived restraints clearly results in a 310-helical structure. Convergence from different starting structures (including an α-helix) into a 310-helix was observed.

No abstract

The cyclic octapeptide cyclo(-Pro-Pro-Tyr-Val-Pro-Leu-Ile-Ile-) (l), isolated from the Hymeniucidon sponge, was synthesized and examined conformationally using NMR and molecular-dynamics simulations. Most structural parameters of synthetic 1 are in accord with those reported for the isolated material. Our study indicates some small but significant differences in the assignment of the 'H-and I3C-NMR resonances from those of the natural material. The conformation was determined in both CHCl, and DMSO using 'H-NMR and molecular-dynamics simulations. Both NOES and coupling constants were used as experimental restraints during the simulations which utilized explicitly the same solvent as in the NMR study. The differences in the interaction of the solvent with 1 were examined, providing insight into the observed differences in conformation. The dominant conformation contains a,WIa turn about lle8-Tyr3 including a Pro1-Pro2 cis-peptide bond and a@I orb11 turn about VaI4-Ile7 in CHCI, and DMSO, respectively.

Recently, a cyclic heptapeptide with cytostatic activity was isolated from marine sources by three different groups independently. The sequence of the isolated peptide was ambiguous, since two different isomers have been proposed: cyclo(-Asn'-Pro2-Phe3-Val4-Val5-Pro6-Val7-) (1) also called axinastatin 1 resp. pseudoaxinellin and cyclo(-Asn1"-Pro2"-Pro3"-Phe4*-Val5"-Val6*-Val7"-) (2) called rnalaysiatin. We synthesized both peptides 1 and 2 and compared their optical rotation, FAB-MS, 'H-and 13C-NMR data with those of the native compounds. Our results prove that peptide 1 has been assigned correctly, whereas the data of 2 differ significantly from those of the natural peptide. Peptide 1 adopts two conformations (90: 10 ratio) in DMSO, interconverting slowly on the NMR time scale. According to MD simulations, using NOES and J couplings as experimental restraints, a PVIa-turn with a cis peptide bond between Val5-Pro6 and a PI-turn in the Asn'-Val4 region are the characteristic secondary structural elements of the major conformer. Its backbone conformation is very similar to the X-ray structure of a related peptideDuring the past years many cyclic peptides like hymenistatin[',*], ev~lidine [~I, hymenamide~[~] and pseudostell~rins~~] were isolated from different biological sources. Recently, three cyclic heptapeptides were found independently in marine organisms: axinastatin I in Axinella spongueL6], pseudoxainellin in Pseudoaxinella ma~sa [~], and malaysiatin in Pseudoaxinyssa spongud81. Although similar biological activities and almost identical NMR data were found, two significantly different structures have been proposed: cyclo-(-Asn'-Pro2-Phe3-Val4-Vals-Pro6-Val7-) (1) and cyclo-(-Asn'#-Pro2#-Pro3#-Phe4#-Val5*-VaI6#-Val7#) (2). Both peptides have the same amino acid composition but differ in their sequence.Pettit et al. were the first to describe the isolation of 1 from Axinella spongue. This compound displayed a strong activity in a P388 test of murine lymphoblastic leukemia [6]. The same substance was isolated from Pseudoaxinella massa by Kong et al.L7]. They confirmed the structure and activity of 1. Fernandez et al. also isolated a highly cytotoxic substance (P388 test) from Pseudoaxinyssa spongue [8]. They proposed a different sequence 2 for this compound, although the NMR data, especially the characteristic I3C resonances, were very similar to those of the previously isolated substances. To reveal the correct sequence and structure of this cytotoxic material, both cyclic peptides 1 and 2 were synthesized and their physical data compared with those of the native compounds. Synthesis and Characterizationusing the Fmoc strategy['o,ll] and TBTU { 0- (1H-benzotriazol-l-yl)-N,N,N',N ["] were applied to the synthesis of the linear peptides. These precursors were also cyclized to 1 and 2 by the use of TBTU/HOBt[141. Purification was performed by using RP HPLC and a standard acetonitrile/water gradient system. The products were characterized by FAB-MS, optical rotation analysis and NMR spectroscopy. FAB-MSThe ami...