Hexane-soluble octadecadepsipeptide forming hydrophobic alpha-helical rod end-capped by the C-terminal gamma-turn

Solid-state conformations of linear depsipeptide amides with an alternating sequence of α,α-disubstituted α-amino acid and α-hydroxy acid

Depsipeptides and cyclodepsipeptides are analogues of the corresponding peptides in which one or more amide groups are replaced by ester functions. Reports of crystal structures of linear depsipeptides are rare. The crystal structures and conformational analyses of four depsipeptides with an alternating sequence of an α,α-disubstituted α-amino acid and an α-hydroxy acid are reported. The molecules in the linear hexadepsipeptide amide in (S)-Pms-Acp-(S)-Pms-Acp-(S)-Pms-Acp-NMe₂ acetonitrile solvate, C₄₇H₅₈N₄O₉·C₂H₃N, (3b), as well as in the related linear tetradepsipeptide amide (S)-Pms-Aib-(S)-Pms-Aib-NMe₂, C₂₈H₃₇N₃O₆, (5a), the diastereoisomeric mixture (S,R)-Pms-Acp-(R,S)-Pms-Acp-NMe₂/(R,S)-Pms-Acp-(R,S)-Pms-Acp-NMe₂ (1:1), C₃₂H₄₁N₃O₆, (5b), and (R,S)-Mns-Acp-(S,R)-Mns-Acp-NMe₂, C₃₀H₃₇N₃O₆, (5c) (Pms is phenyllactic acid, Acp is 1-aminocyclopentanecarboxylic acid and Mns is mandelic acid), generally adopt a β-turn conformation in the solid state, which is stabilized by intramolecular N-H...O hydrogen bonds. Whereas β-turns of type I (or I') are formed in the cases of (3b), (5a) and (5b), which contain phenyllactic acid, the torsion angles for (5c), which incorporates mandelic acid, indicate a β-turn in between type I and type III. Intermolecular N-H...O and O-H...O hydrogen bonds link the molecules of (3a) and (5b) into extended chains, and those of (5a) and (5c) into two-dimensional networks.

Conformational change from antiparallel beta-sheet to alpha-helix in a series of depsipeptide, -(Leu-Leu-Lac)(n)-: syntheses, spectroscopic studies, and crystal structures of Boc-Leu-Lac-OEt and Boc-(Leu-Leu-Lac)(n)-OEt (n = 1, 2)

The depsipeptides Boc-Leu-Lac-OEt (1) and Boc-(Leu-Leu-Lac)(n)-OEt (n = 1, 2) (2 and 3, respectively) (Boc = tert-butyloxycarbonyl, Lac = L-lactic acid residue) has been synthesized and studied by crystallographic, CD spectroscopic, and ESI-MS analyses. In the packing cells, those three compounds adopt beta-strand conformations. Each molecule is linked into a dimer (1) or an infinite assembly (2 and 3) by tight hydrogen bonds of the type NH...O==C. Interestingly, the hexamer, 3 shows the first example of antiparallel pleated beta-sheet crystal structure for a depsipeptide molecule. In the packing cells, especially for 3, the ester groups O--C==O are perpendicularly oriented to the amide groups NH--C==O and beta-sheet planes to avoid the interaction between --O--(ester) and O==C. Therefore, when the chain length become longer, the O...O==C repulsion interaction works as a beta-sheet breaker and hence promotes an alpha-helical structure as observed for Boc-(Leu-Leu-Lac)(3)-Leu-Leu-OEt (4) (Oku et al. Biopolymers 2004, 75, 242-254) and Boc-(Leu-Leu-Lac)(n)-OEt (n = 4-6) (5-7) (Katakai et al., Biopolymers 1996, 38, 285-290), in which the O...O==C repulsion does not cause significant structural changes in alpha-helical main chains. Therefore from the structural and spectroscopic analyses, we have found governing factors for the specificity in the beta-sheet and alpha-helix decision in this series of depsipeptides, -(Leu-Leu-Lac)(n)-.

Addition of a peptide fragment on an ?-helical depsipeptide induces ?/310-conjugated helix: Synthesis, crystal structure, and CD spectra of Boc-Leu-Leu-Ala-(Leu-Leu-Lac)3-Leu-Leu-OEt

The depsipeptide Boc(1)-Leu(2)-Leu(3)-Ala(4)-Leu(5)-Leu(6)-Lac(7)-Leu(8)-Leu(9)-Lac(10)-Leu(11)-Leu(12)-Lac(13)-Leu(14)-Leu(15)-OEt(16) (1) (Boc = tert-butyloxycarbonyl, Lac = L-lactic acid residue) has been synthesized from the peptide Boc-Leu-Leu-Ala-OEt (2) and a depsipeptide, Boc-(Leu-Leu-Lac)(3)-Leu-Leu-OEt (3). Single crystals of 1 were successfully obtained and the structure has been solved by direct methods (such as Sir2002 and Shake-and-Bake). Interestingly, 1 adopts an alpha/3(10)-conjugated helix containing a kink at the junction of peptide and depsipeptide segments, Leu3-Lac7. This is significantly different from the conformation of 3, which has a straight alpha-helical structure with standard phi and psi angles. Microcrystalline CD spectra were also studied to compare structural properties of 1 and 3. The differences between alpha/3(10)- and alpha-helices appear in these CD spectra.

Synthesis of sequential polydepsipeptides utilizing a new approach for the synthesis of depsipeptides

Sequential polydepsipeptides were synthesized by the depsipeptide active ester method using a new approach for the direct synthesis of N-protected depsipeptide free acids from hydroxy acids. The method uses synthesis of Boc-didepsipeptides by reaction of free hydroxy acids with Boc-amino acid N-hydroxysuccinimide esters catalyzed by 4-dimethylaminopyridine and chain elongation of the free depsipeptides by the reaction with Boc-amino acid N-hydroxysuccinimide esters in an organic solvent system of acetonitrile-tetrahydrofuran. The Boc-depsipeptide free acids were activated as their N-hydroxysuccinimide esters, which were polymerized after removal of the Boc-protecting group.

Conformation of the transmembrane domains in peripheral myelin protein 22. Part 1. Solution-phase synthesis and circular dichroism study of protected 17-residue partial peptides in the first putative transmembrane domain

Charcot-Marie-Tooth disease (CMT) is the most commonly inherited peripheral neuropathy. DNA duplication and point mutation of the gene encoding peripheral myelin protein 22 (PMP22) have been found in CMT type 1A dominants. To investigate the influence of the point mutation of PMP22 on the secondary structure, protected partial peptides in the putative first transmembrane domain, wild type Boc-IVLH(Bom)VAVLVLLFVSTIV-OMe (1) and its Pro16 mutant Boc-IVLH(Bom)VAVPVLLFVSTIV-OMe (2) were synthesized. Circular dichorism (CD)-spectral analysis suggested that peptide 1 adopts a stable alpha-helical conformation in membrane-mimetic solvent,1-BuOH/1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) system. On the contrary, the mutant 2 favors beta-sheet conformation in the same solvent system. Interestingly, alpha-helix to beta-sheet transition of 2 was observed at higher contents of 1-BuOH than 70%.

Crystal structure of a depsipeptide, Boc-(Leu-Leu-Lac)3-Leu-Leu-OEt

A sequential polydepsipeptide, Boc-(Leu-Leu-Lac)3-Leu-Leu-OEt (1) (Lac = L-lactic acid residue) has been synthesized by the segment condensation method. The sequential unit of 1, -Leu-Leu-Lac-, is consisted of two amino acid residues and one hydroxy acid residue. X-ray diffraction measurement with an imaging plate detector and a direct-methods procedure of Shake-and-Bake successfully revealed the crystal structure of 1. In the solid state, the 11-mer depsipeptide, 1, have clear alpha-helical conformation even with the three ester linkages.