Photoinduced Color Change and Photomechanical Effect of Naphthalene Diimides Bearing Alkylamine Moieties in the Solid State

Photoinduced color change of naphthalene diimides (NDIs) bearing alkylamine moieties has been observed in the solid state. The color change is attributed to the generation of a NDI radical‐anion species, which may be formed through a photoinduced electron‐transfer process from the alkylamine moiety to the NDI. The photosensitivity of NDIs is highly dependent on the structures of the alkylamine moieties. Crystallographic analysis, kinetic analysis, UV/Vis/NIR spectroscopic measurements, and analysis of the photoproduct suggested that a radical anion was formed through an irreversible process initiated by proton abstraction between an amine radical cation and the neutral amine moiety. The radical anions formed stacks including mixed‐valence stacks and radical‐anion stacks, as shown by the broad absorption bands in near‐IR spectra. These photosensitive NDIs also showed crystal bending upon photoirradiation, which may be associated with a change in the intermolecular distance of the NDI stacks by the formation of monomeric radical anions, mixed‐valence stacks, and radical‐anion stacks.     

α‐Aminoxy Oligopeptides: Synthesis,Secondary Structure,and Cytotoxicity of a New Class of Anticancer Foldamers

α‐Aminoxy peptides are peptidomimetic foldamers with high proteolytic and conformational stability. To gain an improved synthetic access to α‐aminoxy oligopeptides we used a straightforward combination of solution‐ and solid‐phase‐supported methods and obtained oligomers that showed a remarkable anticancer activity against a panel of cancer cell lines. We solved the first X‐ray crystal structure of an α‐aminoxy peptide with multiple turns around the helical axis. The crystal structure revealed a right‐handed 2₈‐helical conformation with precisely two residues per turn and a helical pitch of 5.8 Å. By 2D ROESY experiments, molecular dynamics simulations, and CD spectroscopy we were able to identify the 2₈‐helix as the predominant conformation in organic solvents. In aqueous solution, the α‐aminoxy peptides exist in the 2₈‐helical conformation at acidic pH, but exhibit remarkable changes in the secondary structure with increasing pH. The most cytotoxic α‐aminoxy peptides have an increased propensity to take up a 2₈‐helical conformation in the presence of a model membrane. This indicates a correlation between the 2₈‐helical conformation and the membranolytic activity observed in mode of action studies, thereby providing novel insights in the folding properties and the biological activity of α‐aminoxy peptides.     

Helix Nucleation by the Smallest Known α‐Helix in Water

Cyclic pentapeptides (e.g. Ac‐(cyclo‐1,5)‐[KAXAD]‐NH₂; X=Ala, 1 ; Arg, 2 ) in water adopt one α‐helical turn defined by three hydrogen bonds. NMR structure analysis reveals a slight distortion from α‐helicity at the C‐terminal aspartate caused by torsional restraints imposed by the K(i)–D(i+4) lactam bridge. To investigate this effect on helix nucleation, the more water‐soluble 2 was appended to N‐, C‐, or both termini of a palindromic peptide ARAARAARA (≤5 % helicity), resulting in 67, 92, or 100 % relative α‐helicity, as calculated from CD spectra. From the C‐terminus of peptides, 2 can nucleate at least six α‐helical turns. From the N‐terminus, imperfect alignment of the Asp5 backbone amide in 2 reduces helix nucleation, but is corrected by a second unit of 2 separated by 0–9 residues from the first. These cyclic peptides are extremely versatile helix nucleators that can be placed anywhere in 5–25 residue peptides, which correspond to most helix lengths in protein–protein interactions.     

New Helical Folds in α‐Peptides with Alternating Chirality

In α‐peptides, the 8/10 helix is theoretically predicted to be energetically unstable and has not been experimentally observed so far. Based on our earlier studies on ‘helical induction’ and ‘hybrid helices’, we have adopted the ‘end‐capping’ strategy to induce the 8/10 helix in α‐peptides by using short α/β‐peptides. Thus, α‐peptides containing a regular string of α‐amino acids with alternating chirality were end capped by α/β‐peptides with 11/9‐helical motifs at the termini. Extensive NMR spectroscopy studies of these peptides revealed the presence of a hitherto unknown 8/10‐helical pattern; the H‐bonds in the shorter pseudorings were rather weak. The approach of using short helical motifs to induce new mixed helices in α‐peptides could provide avenues for more versatile design strategies.     

Asymmetric Anion–π Catalysis on Perylenediimides

Anion–π catalysis, that is the stabilization of anionic transition states on π‐acidic aromatic surfaces, has so far been developed with naphthalenediimides (NDIs). This report introduces perylenediimides (PDIs) to anion–π catalysis. The quadrupole moment of PDIs (+23.2 B) is found to exceed that of NDIs and reach new records with acceptors in the core (+70.9 B), and their larger surface provides space to better accommodate chemical transformations. Unlike NDIs, the activity of PDI catalysts for enolate and enamine addition is determined by the twist of their π surface rather than their reducibility. These results, further strengthened by nitrate inhibition and circular dichroism spectroscopy, support an understanding of anion–π interactions centered around quadrupole moments, i.e., electrostatic contributions, rather than redox potentials and charge transfer. The large PDI surfaces provide access to the highest enantioselectivities observed so far in anion–π catalysis (96 % ee).     

Photoswitchable,DNA‐Binding Helical Peptides Assembled with Two Independently Designed Sequences for Photoregulation and DNA Recognition

Diarylethene‐bridged peptides were developed to photoregulate biomolecular interactions. The peptides are made up of diarylethene‐bridged and DNA‐binding regions at their N‐ and C termini, respectively. The two regions could be independently designed and combined as desired. The α‐helicities of the peptides were photoregulated in on/off or off/on manners, and the manner depended on the positions of two ornithine (Orn) residues for cross‐linking reaction at the diarylethene‐bridged region. In the case of the on/off manner, when the diarylethene structure adopted the open form on the peptides, the peptides folded into stable α‐helices. Upon UV irradiation, the diarylethene moiety isomerized to its closed form to destabilize the helical structures. Quartz crystal microbalance (QCM) analysis showed that the open isomer strongly associated with a target DNA, as compared with the closed one. When the closed‐form peptide existing in the DNA complex was irradiated with a fluorescent lamp in the middle of the QCM monitoring, the frequency change (ΔF) was enhanced by the diarylethene photoisomerization.     

α‐Peptide–Oligourea Chimeras: Stabilization of Short α‐Helices by Non‐Peptide Helical Foldamers

Short α‐peptides with less than 10 residues generally display a low propensity to nucleate stable helical conformations. While various strategies to stabilize peptide helices have been previously reported, the ability of non‐peptide helical foldamers to stabilize α‐helices when fused to short α‐peptide segments has not been investigated. Towards this end, structural investigations into a series of chimeric oligomers obtained by joining aliphatic oligoureas to the C‐ or N‐termini of α‐peptides are described. All chimeras were found to be fully helical, with as few as 2 (or 3) urea units sufficient to propagate an α‐helical conformation in the fused peptide segment. The remarkable compatibility of α‐peptides with oligoureas described here, along with the simplicity of the approach, highlights the potential of interfacing natural and non‐peptide backbones as a means to further control the behavior of α‐peptides.     

Conformational Studies on Peptides of α‐Aminoxy Acids with Functionalized Side‐Chains

Peptides of homochiral α‐aminoxy acids of nonpolar side chains can form a 1.8₈‐helix. In this paper, we report the conformational studies of α‐aminoxy peptides 1 , 2 , 3 , which have functionalized side chains, in both nonpolar and polar solvents. ¹H NMR, XRD, and FTIR absorption studies confirm the presence of the eight‐membered‐ring intramolecular hydrogen bonds (the N‐O turns) in nonpolar solvents as well as in methanol. CD studies of peptides 1 , 2 , 3 in different solvents indicate that a substantial degree of helical content is retained in methanol and acidic aqueous buffers. The introduction of functionalized side chains in α‐aminoxy peptides provides opportunities for designing biologically active peptides.     

Remote Control of Anion–π Catalysis on Fullerene‐Centered Catalytic Triads

The design, synthesis and evaluation of catalytic triads composed of a central C₆₀ fullerene with an amine base on one side and polarizability enhancers on the other side are reported. According to an enolate addition benchmark reaction, fullerene–fullerene–amine triads display the highest selectivity in anion–π catalysis observed so far, whereas NDI–fullerene–amine triads are not much better than fullerene–amine controls (NDI=naphthalenediimide). These large differences in activity are in conflict with the small differences in intrinsic π acidity, that is, LUMO energy levels and π holes on the central fullerene. However, they are in agreement with the high polarizability of fullerene–fullerene–amine triads. Activation and deactivation of the fullerene‐centered triads by intercalators and computational data on anion binding further indicate that for functional relevance, intrinsic π acidity is less important than induced π acidity, that is, the size of the oriented macrodipole of polarizable π systems that emerges only in response to the interaction with anions and anionic transition states. The resulting transformation is thus self‐induced, the anionic intermediates and transition states create their own anion–π catalyst.     

Selection of Secondary Structures of Heterotypic Supramolecular Peptide Assemblies by an Enzymatic Reaction

In a model study to investigate the consequence of reactions of intrinsically disordered regions (IDRs) of proteins in the context of the formation of highly ordered structures, we found that enzymatic reactions control the secondary structures of peptides during assembly. Specifically, phosphorylation of an α‐helix‐dominant peptide results in mostly disordered conformations, which become β‐strand‐dominant after enzymatic dephosphorylation to regenerate the peptide. In the presence of another peptide largely with a β‐strand conformation, direct coassembly of the peptides results in amorphous aggregates consisting of α‐helix and β‐strand peptides, but the enzymatically generated peptide coassemblies (from the phosphopeptide) mainly adopt a β‐strand conformation and form ordered structures (e.g., nanofibers). These results indicate that enzymatic dephosphorylation instructs conformationally flexible peptides to adopt thermodynamically favorable conformations in homotypic or heterotypic supramolecular assemblies.     

Micelle‐Triggered β‐Hairpin to α‐Helix Transition in a 14‐Residue Peptide from a Choline‐Binding Repeat of the Pneumococcal Autolysin LytA

Choline‐binding modules (CBMs) have a ββ‐solenoid structure composed of choline‐binding repeats (CBR), which consist of a β‐hairpin followed by a short linker. To find minimal peptides that are able to maintain the CBR native structure and to evaluate their remaining choline‐binding ability, we have analysed the third β‐hairpin of the CBM from the pneumococcal LytA autolysin. Circular dichroism and NMR data reveal that this peptide forms a highly stable native‐like β‐hairpin both in aqueous solution and in the presence of trifluoroethanol, but, strikingly, the peptide structure is a stable amphipathic α‐helix in both zwitterionic (dodecylphosphocholine) and anionic (sodium dodecylsulfate) detergent micelles, as well as in small unilamellar vesicles. This β‐hairpin to α‐helix conversion is reversible. Given that the β‐hairpin and α‐helix differ greatly in the distribution of hydrophobic and hydrophilic side chains, we propose that the amphipathicity is a requirement for a peptide structure to interact and to be stable in micelles or lipid vesicles. To our knowledge, this “chameleonic” behaviour is the only described case of a micelle‐induced structural transition between two ordered peptide structures.     

A One‐Pot Chemically Cleavable Bis‐Linker Tether Strategy for the Synthesis of Heterodimeric Peptides

Heterodimeric peptides linked by disulfide bonds are attractive drug targets. However, their chemical assembly can be tedious, time‐consuming, and low yielding. Inspired by the cellular synthesis of pro‐insulin in which the two constituent peptide chains are expressed as a single‐chain precursor separated by a connecting C‐peptide, we have developed a novel chemically cleavable bis‐linker tether which allows the convenient assembly of two peptide chains as a single “pro”‐peptide on the same solid support. Following the peptide cleavage and post‐synthetic modifications, this bis‐linker tether can be removed in one‐step by chemical means. This method was used to synthesize a drug delivery‐cargo conjugate, TAT‐PKCi peptide, and a two‐disulfide bridged heterodimeric peptide, thionin (7‐19)‐(24‐32R), a thionin analogue. To our knowledge, this is the first report of a one‐pot chemically cleavable bis‐linker strategy for the facile synthesis of cross‐bridged two‐chain peptides.     

Conformational Dynamics of Monomer‐ versus Dimer‐like Features in a Naphthalenediimide‐Based Conjugated Cyclophane

The design and synthesis of an enantiomeric pair of 1,8‐diethynylanthracene‐bridged naphthalenediimide (NDI)‐based cyclophanes ( Cyclo‐NDI s) are reported. Each enantiomer of Cyclo‐NDI exhibits a circularly polarized luminescence signal with a relatively large luminescence dissymmetry factor (g_lum=±8×10^?3). We have further investigated the modulation of through‐space electronic communication between co‐facially oriented NDIs in a discrete Cyclo‐NDI with changes in the temperature. Tuning of the electronic communication results from the conformational transformation of monomer‐ versus dimer‐like features of Cyclo‐NDI , as confirmed by UV/Vis, fluorescence, circular dichroic, and NMR spectroscopic analysis. The temperature‐dependent optical response in the Cyclo‐NDI through the conformational transformation could be utilized as a highly sensitive and reversible optical thermometer in a wide temperature range (100 to ?80 °C).     

Anion–π Catalysis of Diels–Alder Reactions

Among concerted cycloadditions, the Diels–Alder reaction is the grand old classic, which is usually achieved with acid catalysis. In this report, hydroxypyrones, oxa‐, and thiazolones are explored because they provide access to anionic dienes. Their [4+2] cycloaddition with cyclic and acyclic dienophiles, such as maleimides and fumarates, affords bicyclic products with four new stereogenic centers. Bifunctional anion–π catalysts composed of amine bases next to the π surface of naphthalenediimides (NDIs) are shown to selectively stabilize the “open”, fully accessible anionic exo transition state on the π‐acidic aromatic surface. Our results also include reactivities that are hard to access with conventional organocatalysts, such as the exo ‐specific and highly enantioselective Diels–Alder reaction of thiazolones and maleimides with complete suppression of the otherwise dominant Michael addition. With increasing π acidity of the anion–π catalysts, the rates, chemo‐, diastereo‐, and enantioselectivities increase consistently.     

Application of Z‐sinapinic matrix in peptide MALDI‐MS analysis

Since introduction of sinapinic acid (SA) and α‐cyano‐4‐hydroxycinnamic acid as matrices, successful application of matrix‐assisted laser desorption/ionization mass spectrometry started for protein/polypeptides. Both show some limitations in short peptide analysis because matrix clusters are quite abundant. Cinnamics currently used are E‐cinnamics. Here, Z‐SA as matrix for peptides is studied and compared with E‐SA and α‐cyano‐4‐hydroxycinnamic acid. Minor number of clusters is always observed in the low m/z region allowing the detection of short peptides. The results here described show that this novel matrix is a tool of choice for direct, rapid and sensitive detection of hydrophilic and hydrophobic peptides. Copyright © 2017 John Wiley & Sons, Ltd.     

12/14/14‐Helix Formation in 2:1 α/β‐Hybrid Peptides Containing Bicyclo[2.2.2]octane Ring Constraints

The highly constrained β‐amino acid ABOC induces different types of helices in β urea and 1:1 α/β amide oligomers. The latter can adopt 11/9‐ and 18/16‐helical folds depending on the chain length in solution. Short peptides alternating proteinogenic α‐amino acids and ABOC in a 2:1 α/β repeat pattern adopted an unprecedented and stable 12/14/14‐helix. The structure was established through extensive NMR, molecular dynamics, and IR studies. While the 1:1 α‐AA/ABOC helices diverged from the canonical α‐helix, the helix formed by the 9‐mer 2:1 α/β‐peptide allowed the projection of the α‐amino acid side chains in a spatial arrangement according to the α‐helix. Such a finding constitutes an important step toward the conception of functional tools that use the ABOC residue as a potent helix inducer for biological applications.     

Novel naphthalene diimides as activatable precursors of bisalkylating agents, by reduction and base catalysis

Mild activation of water-soluble naphthalene diimides (NDIs) as bisalkylating agents has been achieved by base catalysis and by chemical and electrochemical reductions. NDI activation by a single electron reduction represents a novelty in the field of activatable electrophiles. Under mild reduction, induced by S2O4(2-) in aqueous solution, the resulting NDI radical anion (NDI*-) undergoes a monomolecular fragmentation to yield a new transient species, where the NDI radical anion is tethered to a quinone methide moiety. The latter still retains electrophilic properties, reacting with amines, thiols, and ethyl vinyl ether. Owing to the NDI recognition properties, these results represent the first step toward selective and bioactivatable cross-linking agents.     

Metal‐Helix Frameworks from Short Hybrid Peptide Foldamers

The potential of structured peptides has not been explored much in the design of metal‐organic frameworks (MOFs). This is partly due to the difficulties in obtaining stable secondary structures from the short α‐peptide sequences. Here we report the design, crystal conformations, coordination site dependent different silver coordinated frameworks of short α,γ‐hybrid peptide 12‐helices consisting of terminal pyridyl moieties and the utility of metal‐helix frameworks in the adsorption of CO₂. Upon silver ion coordination the 12‐helix terminated by the 3‐pyridyl derivatives adopted a 2:2 macrocyclic structure, while the 12‐helix terminated by the 4‐pyridyl derivatives displayed remarkable porous metal‐helix frameworks. Both head‐to‐tail intermolecular H‐bonds of the 12‐helix and metal ion coordination have played an important role in stabilizing the ordered metal‐helix frameworks. The studies described here open the door to design a new class of metal‐organic‐frameworks from peptide foldamers.     

Amyloid formation from an α-helix peptide bundle is seeded by 3(10)-helix aggregates

Transformation of proteins and peptides to fibrillar aggregates rich in β sheets underlies many diseases, but mechanistic details of these structural transitions are poorly understood. To simulate aggregation, four equivalents of a water‐soluble, α‐helical (65 %) amphipathic peptide (AEQLLQEAEQLLQEL) were assembled in parallel on an oxazole‐containing macrocyclic scaffold. The resulting 4α‐helix bundle is monomeric and even more α helical (85 %), but it is also unstable at pH 4 and undergoes concentration‐dependent conversion to β‐sheet aggregates and amyloid fibrils. Fibrils twist and grow with time, remaining flexible like rope (>1 μm long, 5–50 nm wide) with multiple strings (2 nm), before ageing to matted fibers. At pH 7 the fibrils revert back to soluble monomeric 4α‐helix bundles. During α→β folding we were able to detect soluble 3₁₀ helices in solution by using 2D‐NMR, CD and FTIR spectroscopy. This intermediate satisfies the need for peptide elongation, from the compressed α helix to the fully extended β strand/sheet, and is driven here by 3₁₀‐helix aggregation triggered in this case by template‐promoted helical bundling and by hydrogen‐bonding glutamic acid side chains. A mechanism involving α?α₄?(3₁₀)₄?(3₁₀)_n?(β)_n?m(β)_n equilibria is plausible for this peptide and also for peptides lacking hydrogen‐bonding side chains, with unfavourable equilibria slowing the α→β conversion.     

Structure–Activity Studies of Cysteine‐Rich α‐Conotoxins that Inhibit High‐Voltage‐Activated Calcium Channels via GABAB Receptor Activation Reveal a Minimal Functional Motif

α‐Conotoxins are disulfide‐rich peptides that target nicotinic acetylcholine receptors. Recently we identified several α‐conotoxins that also modulate voltage‐gated calcium channels by acting as G protein‐coupled GABA_B receptor (GABA_BR) agonists. These α‐conotoxins are promising drug leads for the treatment of chronic pain. To elucidate the diversity of α‐conotoxins that act through this mechanism, we synthesized and characterized a set of peptides with homology to α‐conotoxins known to inhibit high voltage‐activated calcium channels via GABA_BR activation. Remarkably, all disulfide isomers of the active α‐conotoxins Pu1.2 and Pn1.2, and the previously studied Vc1.1 showed similar levels of biological activity. Structure determination by NMR spectroscopy helped us identify a simplified biologically active eight residue peptide motif containing a single disulfide bond that is an excellent lead molecule for developing a new generation of analgesic peptide drugs.