Solid-state conformation of a hybrid tripeptide between beta-amino acid; 8-aminocyclooct-4-enecarboxylic acid and 2-aminoisobutyric acid

Exploring a β-Amino Acid with a Seven-Membered Ring Constraint as a Foldamer Building Block for Nontraditional Helices

We explored trans- and cis-2-aminocycloheptanecarboxylic acid (ACHpC) as potential building blocks for helical foldamers. trans-ACHpC does not show sufficient folding propensity in unnatural peptides. cis-ACHpC promotes nontraditional helices of two unnatural peptide backbones: the 11/9-helix for 1:1 α/β-peptides and the 12/10-helix for β-peptides with interconvertible handedness. The two opposite-handed 12/10-helices rapidly interconvert in solution by pseudorotation of the two twist chair forms of the cycloheptane moiety in each cis-ACHpC residue.

Chirality Control of Screw-Sense in Aib-Polymers: Synthesis and Helicity of Amino Acid Functionalized Polymers

2-Aminoisobutyric acid (Aib) is an essential amino acid, leading to the formation of peptAibols as microbiologically active peptides and proteins. We here report on the ring-opening polymerization (ROP) of Aib-NCA (N-carboxy-anhydrides), enabling to prepare distinct Aib-polymers up to molecular weights of 1400 g/mol with precise end-group control. ROP of Aib-NCA was accomplished via various amine initiators in frozen and liquid solvent systems, in all cases revealing the desired structures as detected by MALDI-TOF-MS and ¹H NMR. We can prove living polymerization behavior until the limit of solubility via inline-IR spectroscopy in both solution and solid state polymerization, characterizing the final polymers via MALDI-TOF-analysis. The attachment of chiral (d or l)-amino acids onto the polymer' headgroup allows to systematically investigate the helical screw-sense of poly(Aib)_n, resulting in chiral induction to form either left (M)- or right (P)-handed screw-senses, depending on the chirality of the attached amino acid. The approach is extended toward a switchable, chiral azo-headgroup, able to change chirality of the attached poly(Aib)_n via a light-induced trigger, revealing a thermally stable cis-isomer when polymer units are attached. In contrast to many other amino acids easily polymerized into poly(amino-acids) via ring-opening polymerization (ROP) of their respective N-carboxyanhydrides (NCAs), the ROP of Aib-NCA (N-carboxy-anhydrides) here is novel and opens a general approach toward chiral, end group-modified helices with a defined screw-sense.

Side chain-specific 11/9-helix propensity of α/β-peptides with alternating residue types

The 11/9-helix propensity of α/β-peptides is dependent on a specific side chain group of α- or β³-residue.

Expanding the polypeptide backbone: hydrogen-bonded conformations in hybrid polypeptides containing the higher homologues of alpha-amino acids

Half a century has passed since the hydrogen-bonded secondary structures of polypeptides and proteins were first recognized. An extraordinary wealth of conformational information is now available on peptides and proteins, which are formed of alpha-amino acid residues. More recently, the discovery of well-folded structures in oligopeptides containing beta-amino acids has focused a great deal of current interest on the conformational properties of peptides constructed from higher homologues (omega) of alpha-amino acids. This review examines the nature of intramolecularly hydrogen-bonded conformations of hybrid peptides formed by amino acid residues, with a varying number of backbone atoms. The beta-turn, a ubiquitous structural feature formed by two residue (alphaalpha) segments in proteins and peptides, is stabilized by a 10-atom (C10) intramolecular 4-->1 hydrogen bond. Hybrid turns may be classified by comparison with their alphaalpha counterparts. The available crystallographic information on hydrogen-bonded hybrid turns is surveyed in this review. Several recent examples demonstrate that individual omega-amino acid residues and hybrid dipeptide segments may be incorporated into the regular structures of alpha-peptides. Examples of both peptide helices and hairpins are presented. The present review explores the relationships between folded conformations in hybrid sequences and their counterparts in all alpha-residue sequences. The use of stereochemically constrained omega-residues promises to expand the range of peptide design strategies to include omega-amino acids. This approach is exemplified by well-folded structures like the C12 (alphagamma) and C14 (gammagamma) helices formed in short peptides containing multiply substituted gamma-residues. The achiral gamma-residue gabapentin is a readily accessible building block in the design of peptides containing gamma-amino acids. The construction of globular polypeptide structures using diverse hybrid sequences appears to be a realistic possibility.

Hybrid Peptides: Expanding the β Turn in Peptide Hairpins by the Insertion of β-, γ-, and δ-Residues

The beta turn segment in designed peptide hairpins has been expanded by the insertion of beta-, gamma- and delta-amino acids at the i+2 position. The model octapeptides Boc-Leu-Phe-Val-DPro-Ac6c-Leu-Phe-Val-OMe (1), Boc-Leu-Phe-Val-DPro-beta3-Ac6c-Leu-Phe-Val-OMe (2), and Boc-Leu-Phe-Val-DPro-Gpn-Leu-Phe-Val-OMe (3) have been shown to adopt beta hairpin conformations in methanol by the observation of key diagnostic nuclear Overhauser effects. Boc-Leu-Val-Val-DPro-delta-Ava-Leu-Val-Val-OMe (4) adopts a beta hairpin conformation in crystals; this is stabilized by three cross-strand hydrogen bonds as demonstrated by X-ray diffraction. The canonical C10 turn in an alpha-alpha segment is expanded to C11, C12, and C13 turns in alpha-beta, alpha-gamma, and alpha-delta segments, respectively. The crystal structures of Piv-LPro-beta3-Ac6c-NHMe (5) and Boc-Ac6c-Gpn-Ac6c-OMe (6) reveal intramolecularly hydrogen-bonded C11 and C12 conformations, respectively. Computer modeling of octapeptide sequences that contain centrally positioned hybrid-turn segments, by using turn parameters derived from the structures of peptides 5 and 6, establishes the stereochemical acceptability of the beta hairpins in the cases of peptides 2 and 3. Accommodation of omega-amino acids into the turn segments is achieved by the adoption of gauche conformations around the backbone C--C bonds.

Structural studies of model peptides containing β-, γ- and δ-amino acids

The crystal structures of five model peptides Piv-Pro-Gly-NHMe (1), Piv-Pro-betaGly-NHMe (2), Piv-Pro-betaGly-OMe (3), Piv-Pro-deltaAva-OMe (4) and Boc-Pro-gammaAbu-OH (5) are described (Piv: pivaloyl; NHMe: N-methylamide; betaGly: beta-glycine; OMe: O-methyl ester; deltaAva: delta-aminovaleric acid; gammaAbu: gamma-aminobutyric acid). A comparison of the structures of peptides 1 and 2 illustrates the dramatic consequences upon backbone homologation in short sequences. 1 adopts a type II beta-turn conformation in the solid state, while in 2, the molecule adopts an open conformation with the beta-residue being fully extended. Piv-Pro-betaGly-OMe (3), which differs from 2 by replacement of the C-terminal NH group by an O-atom, adopts an almost identical molecular conformation and packing arrangement in the solid state. In peptide 4, the observed conformation resembles that determined for 2 and 3, with the deltaAva residue being fully extended. In peptide 5, the molecule undergoes a chain reversal, revealing a beta-turn mimetic structure stabilized by a C-H...O hydrogen bond.