Two novel hexadepsipeptides with several modified amino acid residues isolated from the fungus isaria

Two new cyclohexadepsipeptides have been isolated from the fungus Isaria. Fungal growth in solid media yielded hyphal strands from which peptide fractions were readily isolable by organic-solvent extraction. Two novel cyclodepsipeptides, isaridin A and isaridin B, have been isolated by reverse-phase HPLC, and characterized by ESI-MS and 1H-NMR. Single crystals of both peptides have been obtained, and their 3D structures were elucidated by X-ray diffraction. The isaridins contain several unusual amino acid residues. The sequences are cyclo(β-Gly-HyLeu-Pro-Phe-NMeVal-NMePhe) and cyclo(β-Gly-HyLeu-β-MePro-Phe-NMeVal-NMePhe), where NMeVal is N-methylvaline, NMePhe N-methylphenylalanine, and HyLeu hydroxyleucine (=2-hydroxy-4-methylpentanoic acid). The two peptides differ from one another at residue 3, isaridin A having an (S)-proline at this position, while β-methyl-(S)-proline (=(2S,3S)-2,3,4,5-tetrahydro-3-methyl-1H-pyrrole-2-carboxylic acid) is found in isaridin B. The solid-state conformations of both cyclic depsipeptides are characterized by the presence of two cis peptide bonds at HyLeu(2)-Pro(3)/HyLeu(2)-β-MePro(3) and NMeVal(5)-NMePhe(6), respectively. In isaridin A, a strong intramolecular H-bond is observed between Phe(4)CO...HNβ-Gly(1), and a similar, but weaker, interaction is observed between β-Gly(1)COHNPhe(4). In contrast, in isaridin B, only a single intramolecular H-bond is observed between β-Gly(1)CO...HNPhe(4)

NMR Analysis of Aromatic Interactions in Designed Peptide \beta -Hairpins

Designed octapeptide \beta -hairpins containing a central $^DPro-Gly$ segment have been used as a scaffold to place the aromatic residues Tyr and Trp at various positions on the antiparallel \beta -strands. Using a set of five peptide hairpins, aromatic interactions have been probed across antiparallel \beta-sheets, in the non-hydrogen bonding position $(Ac-L-Y-V-^DP-G-L-Y/W-V-OMe: peptides 1 and 2)$, diagonally across the strands $(Boc-Y/W-L-V-^DP-G-W-L-V-OMe: peptides 3 and 6)$, and along the strands at positions iand i+ 2 $(Boc-L-L-V-^DP-G-Y-L-W-OMe: peptide 4)$. Two peptides served as controls $(Boc-L-L-V-^DP-G-Y-W-V-OMe: peptide 5; Boc-L-Y-V-^DP-G-L-L-V-OMe: peptide 7)$ for aromatic interactions. All studies have been carried out using solution NMR methods in $CDCI_3 + 10$ and have been additionally examined in $CD_3OH$ for peptides 1 and 2. Inter-ring proton-proton nuclear Overhauser effects (NOEs) and upfield shifted aromatic proton resonances have provided firm evidence for specific aromatic interactions. Calculated NMR structures for peptides 1 and 2, containing aromatic pairs at facing non-hydrogen bonded positions, revealed that T-shaped arrangements of the interacting pairs of rings are favored, with ring current effects leading to extremely upfield chemical shifts and temperature dependences for specific aromatic protons. Anomalous far-UV CD spectra appeared to be a characteristic feature in peptides where the two aromatic residues are spatially proximal. The observation of the close approach of aromatic rings in organic solvents suggests that interactions of an electrostatic nature may be favored. This situation may be compared to the case of aqueous solutions, where clustering of aromatic residues is driven by solvophobic (hydrophobic) forces

Beta-Hairpin nucleation by Pro-Gly beta-turns. Comparison of D-Pro-Gly and L-Pro-Gly sequences in an apolar octapeptide

The solution conformation of the synthetic octapeptide Boc-Leu-Val-Val-D-Pro-Gly-Leu-Val-Val-OMe 1 and Boc-Leu-Val-Val-Pro-Gly-Leu-Val-Val-OMe 2 have been investigated in organic solvents by NMR spectroscopy. Peptide 1 adopts well-defined beta-hairpin conformations in CDCl3, C6D6 and (CD3)2SO, nucleated by a D-Pro-Gly Type II9 beta-turn, as demonstrated by the observation of characteristic nuclear Overhauser effects (NOEs) between backbone protons and solvent shielding of NH groups involved in cross-strand hydrogen bonding. Chemical shifts and coupling constants provide further support for the beta-hairpin conformation, which is consistent with the observation of a single negative circular dichroism band at 216 nm in methanol. In peptide 2, there is no characteristic interstrand NOE observed in (CD3)2SO, while in CDCl3 pronounced aggregation results in line broadening. The observation of a low temperature coefficient for the Leu(6)NH proton favours a population of Pro-Gly Type II beta-turn conformations. These results suggest that in short peptide sequences, the precise nature of the beta-turn is critical for hairpin formation, with Type II9 beta-turns being particularly effective

Conformational variability in short acyclic peptides. Stabilization of multiple beta-turn structures in organic solvents

The conformational characteristics of three hexapeptides Boc-Leu-Xxx-Val-Leu-Aib-Val-OMe(Xxx = Ala 1, D-Ala2, Gly 3; Aib =alpha-aminoisobutyryl) have been probed in CDCl3 solution by NMR methods using solvent perturbation of chemical shifts and radical broadening of NH resonances to delineate intramolecularly hydrogen bonded NH groups. Nuclear Overhauser effects (NOEs) provide additional information on preferred backbone conformations. The substituent at position 2 acts as a major conformational determinant. While a continuous 310 helical conformation is favoured for the peptide with Xxx = Ala, a multiple beta-turns conformation is supported by both NMR and CD data for the peptide with Xxx =D-Ala. In the peptide with Xxx = Gly CD and NMR data suggest that both 310 helical and multiple turns conformations are simultaneously populated. The results suggest that incorporation of D-amino acids and Aib residues into all L-sequences may prove useful in generating sequences containing multiple turns

Ambidextrous Molecules: Cylindrical Peptide Structures Formed by Fusing Left- and Right-Handed Helices

The handedness or chirality of molecules, organisms, and elementary particles has been widely appreciated, vindicating Pasteur’s perceptive generalization, Luni-vers est dissymktrique. Ambidexterity is rare not only in biological organisms but also in molecules. One of the most striking chiral features in biopolymers is the twist or handedness of the helices that are frequently formed. Cylindrical helical structures formed by polypeptide chains are an important constituent of peptide and protein structure, The most widespread helical structures are the alpha-helix and 310-helix, both of which have a right-handed screw sense in proteins, determined by the L configuration of the component amino acids. Peptide sequences with alternating L and D residues occur in the membrane channel-forming I 5-residue peptide gramicidin A, resulting in a large diameter cylindrical structure formed by wrapping beta strands about the helix axis.’ Alternating L,D sequences in cyclic peptides have yielded novel tubular stack. Helical polymers with alternating left- and right-handed blocks have been studies in polyisocyanates, providing new insights into the effect of chiral biases on polymer properties. In order to develop novel synthetic structures using chiral blocks, we have investigated peptides with strong helix-forming tendencies generated so as to fuse left- and right-handed cylindrical screws in the same molecule. We describe below the structural characterization of a 14-residue hydrophobic peptide containing two heptapeptide blocks of opposite chirality

Beta-Hairpins Generated from Hybrid Peptide Sequences Containing both Alpha- and Beta-Amino Acids

The incorporation of the -amino acid residues into specific positions in the strands and -turn segments of peptide hairpins is being systematically explored. The presence of an additional torsion variable about the C()C() bond () enhances the conformational repertoire in -residues. The conformational analysis of three designed peptide hairpins composed of /-hybrid segments is described: Boc-Leu-Val-Val-DPro-Phe- Leu-Val-Val-OMe (1), Boc-Leu-Val-Val-DPro-Gly-Leu-Val-Val-OMe (2), and Boc-Leu-Val-Phe-Val-DPro- Gly-Leu-Phe-Val-Val-OMe (3). 500-MHz 1H-NMR Analysis supports a preponderance of -hairpin conformation in solution for all three peptides, with critical cross-strand NOEs providing evidence for the proposed structures. The crystal structure of peptide 2 reveals a -hairpin conformation with two -residues occupying facing, non-H-bonded positions in antiparallel -strands. Notably, Val(3 ) adopts a gauche conformation about the C()C() bond (65) without disturbing cross-strand H-bonding. The crystal structure of 2, together with previously published crystal structures of peptides 3 and Boc-Phe-Phe-DPro-Gly- Phe-Phe-OMe, provide an opportunity to visualize the packing of peptide sheets with local −polar segments× formed as a consequence of reversal peptide-bond orientation. The available structural evidence for hairpins suggests that -residues can be accommodated into nucleating turn segments and into both the H-bonding and non-H-bonding positions on the strands

Two Novel Hexadepsipeptides with Several Modified Amino Acid Residues Isolated from the Fungus

Two new cyclohexadepsipeptides have been isolated from the fungus Isaria. Fungal growth in solid media yielded hyphal strands from which peptide fractions were readily isolable by organic-solvent extraction. Two novel cyclodepsipeptides, isaridin A and isaridin B, have been isolated by reverse-phase HPLC, and characterized by ESI-MS and 1H-NMR. Single crystals of both peptides have been obtained, and their 3D structures were elucidated by X-ray diffraction. The isaridins contain several unusual amino acid residues. The sequences are cyclo(β-Gly-HyLeu-Pro-Phe-NMeVal-NMePhe) and cyclo(β-Gly-HyLeu-β-MePro-Phe-NMeVal-NMePhe), where NMeVal is N-methylvaline, NMePhe N-methylphenylalanine, and HyLeu hydroxyleucine (=2-hydroxy-4-methylpentanoic acid). The two peptides differ from one another at residue 3, isaridin A having an (S)-proline at this position, while β-methyl-(S)-proline (=(2S,3S)-2,3,4,5-tetrahydro-3-methyl-1H-pyrrole-2-carboxylic acid) is found in isaridin B. The solid-state conformations of both cyclic depsipeptides are characterized by the presence of two cis peptide bonds at HyLeu(2)-Pro(3)/HyLeu(2)-β-MePro(3) and NMeVal(5)-NMePhe(6), respectively. In isaridin A, a strong intramolecular H-bond is observed between Phe(4)CO⋅⋅⋅HNβ-Gly(1), and a similar, but weaker, interaction is observed between β-Gly(1)CO⋅⋅⋅HNPhe(4). In contrast, in isaridin B, only a single intramolecular H-bond is observed between β-Gly(1)CO⋅⋅⋅HNPhe(