A Fiberlike Peptide Material Stabilized by Single Intermolecular Hydrogen Bonds

A Proline Mimetic for the Design of New Stable Secondary Structures: Solvent-Dependent Amide Bond Isomerization of (S)-Indoline-2-carboxylic Acid Derivatives

A thorough experimental and computational study on the conformational properties of (S)-indoline-2-carboxylic acid derivatives has been conducted. Methyl (S)-1-acetylindoline-2-carboxylate, both a mimetic of proline and phenylalanine, shows a remarkable tendency toward the cis amide isomer when dissolved in polar solvents. This behavior is opposite to the general preference of proline for the trans isomer, making indoline-2-carboxylic acid a good candidate for the design of different secondary structures and new materials.

Designing a Transparent and Fluorine Containing Hydrogel

Physical hydrogels are supramolecular materials obtained by self-assembly of small molecules called gelators. Aromatic amino acids and small peptides containing aromatic rings are good candidates as gelators due to their ability to form weak bonds as π-π interactions and hydrogen bonds between NH and CO of the peptide chain. In this paper we show our results in the preparation of a transparent hydrogel that was obtained by self-assembly of a fluorine-containing dipeptide that relies on the additional formation of halogen bonds due to the fluorine atoms contained in the dipeptide. We used Boc-D-F2Phe-L-Oxd-OH (F2Phe = 3,4-difluorophenylalainine; Oxd = 4-methyl-5-carboxy-oxazolidin-2-one) that formed a strong and transparent hydrogel in 0.5% w/w concentration at pH = 4.2. The formation of a hydrogel made of unnatural fluorinated amino acids may be of great interest in the evaluation of patients with parkinsonian syndromes and may be used for controlled release.

Tuning Mechanical Properties of Pseudopeptide Supramolecular Hydrogels by Graphene Doping

Supramolecular hydrogels, obtained from small organic molecules, may be advantageous over polymeric ones for several applications, because these materials have some peculiar properties that differentiate them from the traditional polymeric hydrogels, such as elasticity, thixotropy, self-healing propensity, and biocompatibility. We report here the preparation of strong supramolecular pseudopeptide-based hydrogels that owe their strength to the introduction of graphene in the gelling mixture. These materials proved to be strong, stable, thermoreversible and elastic. The concentration of the gelator, the degree of graphene doping, and the nature of the trigger are crucial to get hydrogels with the desired properties, where a high storage modulus coexists with a good thixotropic behavior. Finally, NIH-3T3 cells were used to evaluate the cell response to the presence of the most promising hydrogels. The hydrogels biocompatibility remains good, if a small degree of graphene doping is introduced.

Synthesis and structure analysis of ferrocene-containing pseudopeptides

Ferrocene with its aromaticity and facile redox properties is an attractive moiety to be incorporated into functional moieties. Medicinal applications of ferrocene are well known and ferrocene itself shows cytotoxic and antianemic properties. In this article, we will describe the synthesis and the structure analysis of two pseudopeptides containing a ferrocene moiety as N-terminal group. After purification, Fc-l-Phe-d-Oxd-OBn [l-Phel-phenylalanine; d-Oxd(4R,5S)-4-Methyl-5-carboxy-oxazolidin-2-one] appears as bright brown solid that spontaneously forms brown needles. The X-ray diffraction of the crystals shows the presence of strong π interactions between the ferrocenyl moiety and the phenyl rings, while no NH•••OC hydrogen bonds are formed. This result is confirmed by FT-IR and 1 H NMR analysis. In contrast, both FT-IR and 1 H NMR analysis suggest that Fc-(l-Phe-d-Oxd)2 -OBn forms a turn conformation stabilized by intramolecular NH•••OC hydrogen bonds in solution. Chiroptical spectroscopies (ECD and VCD) substantially confirmed the absence of a well-defined folded structure. The presence of the Fc moiety is responsible for specific ECD signals, one of which displayed pronounced temperature dependence and is directly related with the helicity assumed by the Fc core. Solid-state ECD spectra were recorded and rationalized on the basis of the X-ray geometry and quantum-mechanical calculations.

Thixotropic Peptide-Based Physical Hydrogels Applied to Three-Dimensional Cell Culture

Pseudopeptides containing the d-Oxd or the d-pGlu [Oxd = (4R,5S)-4-methyl-5-carboxyl-oxazolidin-2-one, pGlu = pyroglutamic acid] moiety and selected amino acids were used as low-molecular-weight gelators to prepare strong and thixotropic hydrogels at physiological pH. The addition of calcium chloride to the gelator solutions induces the formation of insoluble salts that get organized in fibers at a pH close to the physiological one. Physical characterization of hydrogels was carried out by morphologic evaluation and rheological measurements and demonstrated that the analyzed hydrogels are thixotropic, as they have the capability to recover their gel-like behavior. As these hydrogels are easily injectable and may be used for regenerative medicine, they were biologically assessed by cell seeding and viability tests. Human gingival fibroblasts were embedded in 2% hydrogels; all of the hydrogels allow the growth of encapsulated cells with a very good viability. The gelator toxicity may be correlated with their tendency to self-assemble and is totally absent when the hydrogel is formed.

Self-healing hydrogels triggered by amino acids

Nine amino acids with different chemical properties have been chosen to promote the formation of hydrogels based on the bolamphiphilic gelator A: three basic amino acids (arginine, histidine and lysine), one acidic amino acid (aspartic acid), two neutral aliphatic amino acids (alanine and serine) and three neutral aromatic amino acids (phenylalanine, tyrosine and tryptophan).

Exploring structural features of folded peptide architectures in the construction of nanomaterials

Supramolecular assemblies of conformationally biased α,γ-hybrid peptides composed of alternating α-Phe and 4,4-dimethyl substituted γ-amino acids are investigated.

Hydrogelation Induced by Fmoc-Protected Peptidomimetics

Four new low molecular weight hydrogelators (LMWGs) have been prepared in multigram scale and their attitude to form hydrogels has been tested. The gelation trigger is pH variation. The resulting gels have been characterized with several techniques: measurement of the melting points (T(gel)), transparency, gelation time, and viscoelastic properties, together with ECD analysis. Among them, Fmoc-L-Tyr-D-Oxd-OH 1 is an excellent gelator that leads to the preparation of strong, transparent, and viscoelastic gels, by pH variation. UV-visible analyses have demonstrated that the gels obtained with the LMWG 1 possess high transparency, with a transmittance up to 25.6% at a wavelength of 600 nm. Results of the amplitude sweep experiments showed that the elastic response component (G') was approximately an order of magnitude larger than the viscous component, indicating an elastic rather than viscous attitude of the gels, confirmed by the frequency independence of G' and G″ values, in the range from 0.1 to 100 rad·s(-1). The thermal behavior of gel obtained from Fmoc-L-Tyr-D-Oxd-OH 1 was characterized performing an "ad hoc" rheological temperature sweep experiment, that indicated that G' remained almost constant from 23 °C up to about 65 °C while G″ increased in the same temperature range. At higher temperatures, both G' and G″ values started to slightly decrease without displaying a crossover point.

Solvent assisted structural diversity: supramolecular sheet and double helix of a short aromatic γ-peptide

Solvent interaction has a significant effect on the molecular motions, folding and self-assembly of short aromatic γ-peptides that leads to a change in initial helical conformation.

A peptidic hydrogel that may behave as a "Trojan Horse"

A physical hydrogel prepared with the low-molecular-weight hydrogelator (LMWHG) CH₂(C₃H₆CO-L-Phe-D-Oxd-OH)₂ and water/ethanol mixture was applied as a potential “Trojan Horse” carrier into cells. By SEM and XRD analysis we could demonstrate that a fibrous structure is present in the xerogel, making a complex network. The gelator is derived from α-amino acids (Thr, Phe) and a fatty acid (azelaic acid) and is biocompatible: it was dosed to IGROV-1 cells, which internalized it, without significantly affecting the cell proliferation. To check the internalization process by confocal microscopy, fluorescent hydrogels were prepared, introducing the fluorescent dansyl moiety into the mixture.

Triplet excited state behavior of naphthalene-based pseudopeptides in the presence of energy donors

In this work, the triplet state behavior of naphthalene-based pseudopeptides with amide-based macrocyclic or lateral chain substructures has been investigated in the presence of benzophenone and/or biphenyl, as suitable energy-donating chromophores. Their behavior has been compared with that of 1,4-dimethylnaphthalene as model compound. In all the cases, the triplet-triplet absorption of naphthalene is detected by transient absorption spectroscopy, upon selective excitation of benzophenone at 355 nm. The kinetics of formation and decay of this species is markedly slower in the pseudopeptides, due to retardation of triplet-triplet energy transfer and exciplex formation. Finally, the delayed fluorescence detected in the model naphthalene is absent in the pseudopeptides. The concept can, in principle, be exploited for the study of excited-state interactions in supramolecular systems.

Conformational studies of Phe-rich foldamers by VCD spectroscopy and ab initio calculations

Employing VCD spectroscopy, we demonstrate that the structural behavior of the oligomers Boc-(L-Phe-L-Oxd)(n)-OBn is similar from n = 2 to n = 6; ab initio calculations for the n = 1 case provide physical insight into the conformational properties. Further information is gained by IR, (1)H NMR, and ECD spectroscopies. ECD spectra suggest the presence of different conformations between n = 1 on one side and longer chain foldamers on the other side. VCD and absorption IR spectra in methanol solutions can be interpreted as indicative of a PPII structure. In the case of Boc-L-Phe-L-Oxd-OBn, VCD spectra in CCl(4) and detailed DFT computational analysis allow one to demonstrate that the most populated conformers exhibit backbone dihedral angles similar to those of a PPII geometry. This is a remarkable outcome, as we had previously demonstrated that the Boc-(L-Ala-D-Oxd)(n)-OBn series folds in a β-band ribbon spiral that is a subtype of the 3(10) helix.

Self-assembly of chiral trans-cyclobutane-containing β-dipeptides into ordered aggregates

Two chiral synthetic β-dipeptides have been constructed, one with two trans-cyclobutane residues and the other with one trans and one cis fragment, 1 and 2, respectively, and investigated to get insight into the non-covalent interactions responsible for their self-assembly to form ordered aggregates, as well into parameters such as their morphology and size. Experimental evidence of the formation of these assemblies was provided by spectroscopy, microscopy and X-ray diffraction experiments that suggest the formation of nanoscale helical aggregates. This process involves a conformational change in the molecules of each dipeptide with respect to the preferred conformation of the isolated molecules in solution. A high-resolution NMR spectroscopy study allowed the determination of the dynamics of the gelation process in [D(8)]toluene and the sol-gel transition temperature, which was around 270 K in this solvent at a concentration of 15 mM. NMR spectroscopy experiments also provided some information about conformational changes involved in the sol-gel transition and also suggested a different gel packing for each dipeptide. These observations have been nicely explained by computational studies. The self-assembly of the molecules has been modelled and suggested a head-to-head molecular arrangement for 1 and a head-to-tail arrangement for 2 to give helical structures corresponding to hydrogen-bonded single chains. These chains interact with one another in an antiparallel way to afford bundles, the significant geometry parameters of which fit well to the main peaks observed in wide-angle X-ray diffraction spectra of the aggregates in the solid state.

Nanofibers from oxazolidi-2-one containing hybrid foldamers: what is the right molecular size?

A series of oligomers of the type Boc-(L-Phe-D-Oxd)(n)-OBn (Boc = tert-butoxycarbonyl; Oxd = 4-methyl-5-carboxy oxazolidin-2-one; Bn = benzyl) were prepared for n = 2-5. The shortest oligomer, Boc-(L-Phe-D-Oxd)(2)-OBn, aggregates and forms a fiber-like material with an anti-parallel beta-sheet structure in which the oligopeptide units are connected to each other by only one intermolecular hydrogen bond. The longer oligomers exhibit structural heterogeneity. They start to organize into secondary structures by the formation of intramolecular hydrogen bonds at the pentamer level. Microscopy and diffraction of the oligomers indicated a crystalline character for only the shorter ones.