Gamma4-aminoxy peptides as new peptidomimetic foldamers

The conformational properties of peptides 1-3 of gamma-aminoxy acids have been investigated by using FT-IR, NMR spectroscopy, and X-ray crystallography. Diamide 1 consisting of unsubstituted gamma-aminoxy acid cannot form intramolecular hydrogen bond. A novel gamma N-O turn involving a 10-membered-ring intramolecular hydrogen bond between NHi+2 and COi is formed in gamma4-aminoxy peptides 2 and 3. Triamides 3 prefers a new helical structure featuring two consecutive gamma N-O turns. Therefore, gamma4-aminoxy peptides represent new peptidomimetic foldamers.     

Functionalized foldamers: synthesis and characterization of a glycosylated beta-peptide 314-helix conveying the TN-antigen

Herein we describe the design, synthesis, and solution structure of a novel type of conjugate composed of a naturally occurring bio-active ligand bound to an artificial peptidomimetic backbone; in this first report on such functionalized foldamers we utilized a beta-peptide as backbone and a GalNAc carbohydrate residue as ligand.     

Oligonuclear ferrocene amides: mixed-valent peptides and potential redox-switchable foldamers

Trinuclear ferrocene tris-amides were synthesized from an Fmoc- or Boc-protected ferrocene amino acid, and hydrogen-bonded zigzag conformations were determined by NMR spectroscopy, molecular modelling, and X-ray diffraction. In these ordered secondary structures orientation of the individual amide dipole moments approximately in the same direction results in a macrodipole moment similar to that of α-helices composed of α-amino acids. Unlike ordinary α-amino acids, the building blocks in these ferrocene amides with defined secondary structure can be sequentially oxidized to mono-, di-, and trications. Singly and doubly charged mixed-valent cations were probed experimentally by Vis/NIR, paramagnetic 1H NMR and M?ssbauer spectroscopy and investigated theoretically by DFT calculations. According to the appearance of intervalence charge transfer (IVCT) bands in solution, the ferrocene/ferrocenium amides are described as Robin-Day class II mixed-valent systems. M?ssbauer spectroscopy indicates trapped valences in the solid state. The secondary structure of trinuclear ferrocene tris-amides remains intact (coiled form) upon oxidation to mono- and dications according to DFT calculations, while oxidation to the trication should break the intramolecular hydrogen bonding and unfold the ferrocene peptide (uncoiled form).     

Expedient synthesis of pseudo-Pro-containing peptides: towards constrained peptidomimetics and foldamers

The reaction of sulfonyl peptides containing L- or D-configured Ser or Thr with bis(succinimidyl) carbonate in the presence of a catalytic amount of a base affords, in solution or in the solid phase, the corresponding peptides with one or two, consecutive or alternate oxazolidin-2-ones (Oxd). The Oxd ring can be regarded to as a pseudo-Pro with an exclusively trans conformation of the preceding peptide bond; homochiral Oxd-containing peptides adopt extended conformations, while the presence of a D-configured Oxd favours folded conformations.     

Catalytic hydrogenation of vinylogous peptides: a route towards γ-peptide foldamers

Catalytic hydrogenation over Pd/C of vinylogous aminoacids and aminoamides has been studied. The configuration of the ethylenic bond has an important effect on the diastereoselectivity. The higher selectivity is observed with the E-vinylogous aminoamides. The conformational preferences of the α,γ-disubstituted γ-peptides have been determined. The 2S,4S-γ-peptide moiety induces a β-like folded structure stabilized by an intramolecular hydrogen bond, whereas the 2S,4R-diastereomer assumes an open structure.     

Peptides of aminoxy acids as foldamers

This Feature Article summarizes our efforts in developing a new family of foldamers from alpha-, beta- and gamma-aminoxy acids. From a series of conformational studies, we demonstrate that peptides consisting of aminoxy acids adopt several well-defined secondary structures, such as alpha N-O turns (which feature an eight-membered-ring hydrogen bond), beta N-O turns (a nine-membered-ring hydrogen bond), gamma N-O turns (a ten-membered-ring hydrogen bond), 1.8(8) helices (consecutive homochiral alpha N-O turns), 7/8 helices (alternating alpha N-O turns and gamma-turns), 1.7(9) helices (consecutive beta N-O turns), reverse turns (consecutive heterochiral alpha N-O turns) and sheet-like structures.     

Secondary structural preferences of 2,2-disubstituted pyrrolidine-4-carboxylic acid oligomers: beta-peptide foldamers that cannot form internal hydrogen bonds

We examine a new class of beta-peptides, 2,2-disubstituted pyrrolidine-4-carboxylic acid oligomers, and show that they manifest discrete conformational preferences despite the impossibility of internal hydrogen bonding. Numerous beta-peptide families have been described that display specific secondary structural preferences, but all of the conformations characterized in detail so far have contained internal hydrogen bonds. Internal hydrogen bonding is observed within the most common secondary structures of conventional peptides as well. Identifying foldamers in which shape control is independent of hydrogen bonding is significant in two ways. At a fundamental level, foldamers in this small but growing class are interesting because their shapes are controlled by distinctive networks of noncovalent forces. At a practical level, non-hydrogen bonded foldamers may be useful in biomedical applications because the low intrinsic polarity of their backbones may promote bioavailability.     

Pseudopeptide foldamers: the homo-oligomers of pyroglutamic acid

As a part of a program evaluating substituted gamma-lactams as conformationally constrained building blocks of pseudopeptide foldamers, we synthesized the homo-oligomers of L-pyroglutamic acid up to the tetramer level by solution methods. The preferred conformation of this pseudopeptide series in structure-supporting solvents was assessed by FT-IR absorption, 1H NMR and CD techniques. In addition, the crystal structure of the N alpha-protected dimer was established by X-ray diffraction. A high-level DFT computational modeling was performed based on the crystallographic parameters. In this analysis, we demonstrated that an alpha C-H...O=C intramolecular hydrogen bond is responsible for the stabilization of the s-trans L-pGlu-L-pGlu conformation by 1.4 kcal mol-1. This effect can be easily detected by 1H NMR spectroscopy, owing to the anomalous chemical shifts of the alpha CH protons present in all of the oligomers. In summary, we have developed a new polyimide-based, foldameric structure that, if appropriately functionalized, has promise as a rigid scaffold for novel functions and applications.     

The photochemistry of N-p-toluenesulfonyl peptides: the peptide bond as an electron donor

The scope of photobiological processes that involve absorbers within a protein matrix may be limited by the vulnerability of the peptide group to attack by highly reactive redox centers consequent upon electronic excitation. We have explored the nature of this vulnerability by undertaking comprehensive product analyses of aqueous photolysates of 12 N-p-toluenesulfonyl peptides with systematically selected structures. The results indicate that degradation includes a major pathway that is initiated by intramolecular electron transfer in which the peptide bond serves as electron donor, and the data support the likelihood of a relay process in dipeptide derivatives.     

Translocation of a beta-peptide across cell membranes.

Short cationic peptides derived from DNA-binding proteins, of which HIV Tat is a prototype, can cross the membranes of living cells, and they can bring covalently attached moieties (proteins, drugs) along with them. We show that a beta-amino acid analogue of Tat 47-57 enters HeLa cells with comparable efficiency to Tat 47-57 itself (YGRKKRRQRRR). The beta-peptide is comprised of residues that bear the appropriate side chain at the beta-carbon. Both the alpha- and the beta-peptide were conjugated to fluorescein at the N terminus, and cell penetration was monitored by confocal fluorescence microscopy. Deletion of the three C-terminal arginine residues from the alpha-peptide abolished translocation activity, consistent with prior reports, and deletion of the three C-terminal beta3-homoarginine residues from the beta-peptide had a similarly adverse effect. Thus, alpha- and beta-peptide translocation processes show similar length/charge dependence. The beta-peptide appeared to be largely unfolded in water, which is consistent with the behavior of short Tat-derived alpha-peptides, but in methanol the beta-peptide adopted a helical conformation, in contrast to short Tat-derived alpha-peptides. Our results show that neither altering the oligomeric backbone (amide group spacing) nor increasing the intrinsic propensity to adopt a specific secondary structure affects translocation activity.     

A novel polypseudorotaxane composed of cyclic beta-peptide as bead component

A polypseudorotaxane composed of cyclic hexamers of beta-glucosamino acid and poly(ethylene glycol) (PEG) was constructed, where the beads were stacked unidirectionally together via hydrogen bonds to form a peptide nanotube.     

Application of Hydrazino Dinitrophenyl-Amino Acids as Chiral Derivatizing Reagents for Liquid Chromatographic Enantioresolution of Carbonyl Compounds

A new series of chiral derivatizing reagents (CDRs) consisting of five hydrazino dinitrophenyl (HDNP)-amino acids (CDR 1?C5) was prepared by a two-step synthesis procedure starting from 1,5-difluoro-2,4-dinitrobenzene (DFDNB). In the first step, five fluoro-dinitrophenyl (FDNP)-reagents, namely FDNP-l-Leu, FDNP-l-Val, FDNP-l-Phe, FDNP-l-Ala and FDNP-d-Phg were synthesized by substituting one of the fluorine atoms in DFDNB moiety with amino acids l-Leu, l-Val, l-Phe, l-Ala and d-Phg, respectively. In the following step, the remaining fluorine atom of the FDNP reagents was substituted with hydrazine hydrate to obtain five HDNP reagents (i.e. CDR 1?C5; HDNP-l-Leu, HDNP-l-Val, HDNP-l-Phe, HDNP-l-Ala and HDNP-d-Phg). These five CDRs were used for synthesis of diastereomers of six racemic carbonyl compounds which were resolved by high-performance liquid chromatography using C18 column and gradient eluting mixture of acetonitrile or methanol with triethylammonium phosphate buffer with UV detection at 348 nm. Microwave irradiation was used for synthesis of both the CDRs and the diastereomers. The newly synthesized CDRs were observed to be superior in comparison to their counterparts having amino acid amides as chiral auxiliaries in terms of cost effectiveness and providing better resolution of diastereomers. The method was validated for limit of detection, linearity, accuracy and precision.     

Adsorption of amyloid beta-peptide at polymer surfaces: a neutron reflectivity study.

The adsorption of amyloid beta-peptide at hydrophilic and hydrophobic modified silicon-liquid interfaces was characterized by neutron reflectometry. Distinct polymeric films were used to obtain noncharged (Formvar), negatively (sodium poly(styrene sulfonate)) and positively charged (poly(allylamine hydrochloride)) hydrophilic as well as hydrophobic surfaces (polystyrene and a polysiloxane-dodecanoic acid complex). Amyloid beta-peptide was found to adsorb at positively charged hydrophilic and hydrophobic surfaces, whereas no adsorbed layer was detected on hydrophilic noncharged and negatively charged films. The peptide adsorbed at the positively charged film as patches, which were dispersed on the surface, whereas a uniform layer was observed at hydrophobic surfaces. The thickness of the adsorbed peptide layer was estimated to be approximately 20 A. The peptide formed a tightly packed layer, which did not contain water. These studies provide information about the affinity of the amyloid beta-peptide to different substrates in aqueous solution and suggest that the amyloid fibril formation may be driven by interactions with surfaces.     

Liposome fusion with orthogonal coiled coil peptides as fusogens: the efficacy of roleplaying peptides

Biological membrane fusion is a highly specific and coordinated process as a multitude of vesicular fusion events proceed simultaneously in a complex environment with minimal off-target delivery. In this study, we develop a liposomal fusion model system with specific recognition using lipidated derivatives of a set of four de novo designed heterodimeric coiled coil (CC) peptide pairs. Content mixing was only obtained between liposomes functionalized with complementary peptides, demonstrating both fusogenic activity of CC peptides and the specificity of this model system. The diverse peptide fusogens revealed important relationships between the fusogenic efficacy and the peptide characteristics. The fusion efficiency increased from 20% to 70% as affinity between complementary peptides decreased, (from K_F ≈ 10⁸ to 10⁴ M⁻¹), and fusion efficiency also increased due to more pronounced asymmetric role-playing of membrane interacting ‘K’ peptides and homodimer-forming ‘E’ peptides. Furthermore, a new and highly fusogenic CC pair (E₃/P1_K) was discovered, providing an orthogonal peptide triad with the fusogenic CC pairs P2_E/P2_K and P3_E/P3_K. This E₃/P1_k pair was revealed, via molecular dynamics simulations, to have a shifted heptad repeat that can accommodate mismatched asparagine residues. These results will have broad implications not only for the fundamental understanding of CC design and how asparagine residues can be accommodated within the hydrophobic core, but also for drug delivery systems by revealing the necessary interplay of efficient peptide fusogens and enabling the targeted delivery of different carrier vesicles at various peptide-functionalized locations.

We developed a liposomal fusion model system with specific recognition using a set of heterodimeric coiled coil peptide pairs. This study unravels important structure–fusogenic efficacy relationships of peptide fusogens.