Simple Peptides Research Articles

Columnar Liquid Crystals Self-Assembled by Minimalistic Peptides for Chiral Sensing and Synthesis of Ordered Mesoporous Silica

We report the spontaneous formation of lyotropic liquid crystals from the self-assembly of a series of minimalistic peptides. The highly charged peptides could self-assemble into rigid helical nanofilaments in water and spontaneously order into hexagonal liquid-crystalline phases with interfilament separations of ≤240 A. The formation of liquid crystals from the self-assembly of such simple peptides is dictated not only by the concentration and pH but also by the amino acid sequence of the peptides. Peptides containing the rigid Phe-Phe (FF) segment showed much higher propensities to form a liquid-crystalline phase with long-range order. Moreover, because of the intrinsic chirality of the peptides, the self-assembled birefringent liquid crystals could serve as sensing elements for the visual discrimination of chiral species and as templates for the biomimetic synthesis of mesoporous silica with ordered cavities. The results offer new opportunities for the design of peptide liquid crystals that are potenti...

Evaluation of Dihedral Angles of Peptides Using IR Bands of Two Successive Isotope Labeled Residues

Abstract The infrared (IR) absorption bands due to peptide bonds (amide bands) have long been used to determine the secondary structure of a peptide and to analyze intra- and intermolecular interactions between amides. In the present study, the dihedral angles of a residue in peptides have also been evaluated using the amide I IR band of two successive residues with isotope labeling. The two successive residues labeled with the 13C and 18O isotopes give the doublet amide I IR band and the intensity ratio (Rint) and the difference in peak position (Δν) of the doublet band were analyzed using GF matrix and ab initio molecular orbital calculations. We obtained the two-dimensional calculation maps of Rint and Δν against the two dihedral angles. The crossing point of the curves of Rint and Δν is the two dihedral angles of the measured residue. The evaluated dihedral angles of the simple peptides are compared with the reported values. We discuss the limitation and the application of the present method to biopolymers from the obtained results.

Concise Synthesis of Tryptanthrin Spiro Analogues with In Vitro Antitumor Activity Based on One-Pot, Three-Component 1,3-Dipolar Cycloaddition of Azomethine Ylides to Сyclopropenes

A simple, efficient and atom-economic method has been developed for the synthesis of complex alkaloid-like compounds with spiro-fused indolo[2,1-b]quinazoline and cyclopropa[a]pyrrolizine or 3-azabicyclo[3.1.0]hexane moieties. We have found that one-pot, three-component 1,3-dipolar cycloaddition reactions allow the desired products to be obtained from various cyclopropene derivatives with tryptanthrin-derived azomethine ylides generated in situ, in good to high yields and excellent diastereoselectivity. The possibility of ylide generation was exemplified by using α-amino acids (l-proline, l-4-thiazolidincarboxylic acid) and simplest peptides (dipeptide Gly-Gly, tripeptide Gly-Gly-Gly). Quantum chemical investigations indicate that the reaction proceeds through the S-shaped azomethine ylide, the interaction of which with cyclopropenes proceeds via a less sterically hindered endo-transition state. The antitumor activity of some of spiro-tryptanthrin derivatives against erythroleukemia (K562), cervical carcinoma (HeLa) and colon carcinoma (CT26) cell lines was evaluated in vitro by MTS-assay.

A Reversible and Repeatable Thiol-Ene Bioconjugation for Dynamic Patterning of Signaling Proteins in Hydrogels.

Biomolecule-functionalized hydrogels have emerged as valuable cell culture platforms to recapitulate the mechanical and biochemical properties of the extracellular niche. The typical strategy to functionalize hydrogels with biomolecules involves directly tethering them to the hydrogel backbone resulting in a static material. Thus, this approach fails to capture the dynamic changes in biomolecule composition that occur during biological processes or that may be required for regenerative medicine applications. Moreover, it also limits the scope of biomolecules to simple peptides, as signaling proteins generally have poor stability under cell culture conditions and lose their bioactivity over time. To that end, we sought to develop a bioconjugation reaction that would enable reversible and repeatable tethering of signaling proteins to hydrogels, so that spent protein could be released on-demand and replaced with fresh protein as needed. Specifically, we designed an allyl sulfide chain-transfer agent that enables a reversible, photomediated, thiol–ene bioconjugation of signaling proteins to hydrogels. Upon addition of a thiolated protein to the allyl sulfide moiety, the previously tethered protein is released, and the “ene” functionality is regenerated. Using this approach, we demonstrate that protein patterning can be achieved in hydrogels through a thiol–ene reaction, and the patterned protein can then be released through a subsequent thiol–ene reaction of a PEG thiol. Importantly, this process is repeatable through multiple iterations and proceeds at physiologically relevant signaling protein concentrations. Finally, we demonstrate that whole signaling proteins can be patterned and released in the presence of cells, and that cells respond to their presentation with spatial fidelity. Combined, these data represent the first example of a methodology that enables fully reversible and repeatable patterning and release of signaling proteins from hydrogels.

Antitubercular Marine Natural Products.

Due to the importance of nature as a source of new drug candidates, the purpose of this article is to emphasize the marine natural products, which exhibit antitubercular activity, published between January 2000 and May 2016, with 138 quotations to 250 compounds obtained from marine resources. These metabolites are organized by chemical constitution and named as simple alkyl lipids derivatives, aromatics derivatives, peptides, alkaloids, terpenoids, steroids, macrolides, and polycyclic polyketides.

Plant Viral Proteases: Beyond the Role of Peptide Cutters.

Almost half of known plant viral species rely on proteolytic cleavages as key co- and post-translational modifications throughout their infection cycle. Most of these viruses encode their own endopeptidases, proteases with high substrate specificity that internally cleave large polyprotein precursors for the release of functional sub-units. Processing of the polyprotein, however, is not an all-or-nothing process in which endopeptidases act as simple peptide cutters. On the contrary, spatial-temporal modulation of these polyprotein cleavage events is crucial for a successful viral infection. In this way, the processing of the polyprotein coordinates viral replication, assembly and movement, and has significant impact on pathogen fitness and virulence. In this mini-review, we give an overview of plant viral proteases emphasizing their importance during viral infections and the varied functionalities that result from their proteolytic activities.

Determination of Local Structure of 13C Selectively Labeled 47-mer Peptides as a Model for Gly-Rich Region of Nephila clavipes Dragline Silk Using a Combination of 13C Solid-State NMR and MD Simulation

For the first time, we elucidate the complex structure of the Gly-rich regions in Nephila clavipes dragline silk through synergistic experimental and theoretical studies. First, the 13C selectively labeled 47-mer peptides selected from the glycine (Gly)-rich region of N. clavipes dragline silk were synthesized. The 13C CP/MAS NMR spectra were analyzed to determine the fractions of the conformations of individual Gly and Ala residues through 13C conformation-dependent chemical shifts and peak deconvolution. By comparing the 13C solid-state NMR spectra of several simple model peptides, the presence of 31 helix in the 47-mer peptides was disproved, and the (Ala)6 regions were shown to form β-sheet structure in the staggered arrangement. Although the fraction of β-sheet components tended to increase and the fraction of random coil component decrease toward both chain ends, significant change in the fractions was observed depending on the amino acid position. These results were successfully rationalized throug...

Enhanced Electrochemical Ammonia Production Via Peptide-Bound Metals and Effects on the Hydrogen Evolution Reaction

Approximately half of the people on the planet are alive due to synthetically produced ammonia. However, due to the fossil fuels used in the current process, ammonia production contributes a significant amount to the world’s greenhouse gas emissions. By taking an electrochemically-based approach to this process, those emissions can be eliminated. However, current catalysts are not selective for the desired reaction. We hypothesize that 3D surface modifications can be utilized to overcome these limitations and create catalysts which mimic the selectivity of the nitrogenase enzyme, an enzyme that catalyzes the reduction of nitrogen to ammonia at mild temperatures and pressures. Therefore, in this study, peptide sequences have been designed to mimic the general function of the naturally-occurring nitrogenase enzyme. Recent results in an alkaline, solid-state electrochemical cell show promise using a synthetic peptide bound to iron(iii) oxide nanoparticles, which yield an electrochemical ammonia production rate that is 10x higher than the catalyst without bound peptide. Preliminary, repeated experiments in a liquid electrolyte cell at room temperature and atmospheric pressure illustrate similar promising results. Furthermore, gas adsorption isotherms suggest that this peptide does not significantly block nitrogen adsorption, and that the catalysts maintain the same surface area after binding to the peptides. In addition, a simple and tunable peptide system has been designed to better understand how peptides influence electrochemical reactions. Emphasis is placed on understanding the hydrogen evolution reaction (HER) on Au, Fe2O3, and Pt metal surfaces, as HER is the main competing reaction for ammonia production. One of our main tools used to understand these peptide effects is a quartz crystal microbalance with dissipation (QCM-D), which is utilized in combination with an electrochemistry module (QEM) and humidity module to give real-time electrochemical and surface information. Importantly, the QCM-D is sensitive enough to measure the mass loading of the peptide on metal surfaces. Furthermore, the humidity module allows us to study the peptides’ effects on water uptake, which will also be useful for understanding HER in a basic environment, as water is the proton source. For instance, preliminary results show that at least one tripeptide species has a significant hydrophilic effect on water uptake when used on an Au sensor. Overall, these data are anticipated to influence general guidelines when designing selective catalysts for electrochemical reactions.

Preferential Interaction of Beryllium Ion with Carboxylate‐Rich Peptides

Glycogen synthase kinase 3β (GSK3β) regulates a number of cellular pathways including glycogen metabolism, the Wnt signaling pathway, and cell proliferation. Hyperactivity of GSK3β has been hypothesized to be potentially related to Alzheimer's disease, bipolar disorder, type II diabetes, and some cancers, therefore GSK3β is of interest as a target for therapeutics. Lithium ion (Li+) is a classical inhibitor of GSK3β, but beryllium ion (Be2+) is approximately 1000‐fold more potent. We are interested in the inhibitory mechanism and thermodynamic characteristics for the metal‐protein binding of Be2+ and GSK3β. Be2+ is believed to inhibit GSK3β by competing for magnesium ion (Mg2+) binding sites that feature aspartic acid (D) or other acidic side chains. As a first step toward understanding how Be2+ could outcompete Mg2+ in metal‐protein interactions, we used model chelating compounds and simple peptides that might partially mimic a carboxylate‐rich binding pocket, and compared their binding properties towards Be2+ and Mg2+. Isothermal titration calorimetry (ITC) binding studies for nitrilotriacetic acid, 4,5‐imidazole dicarboxylic acid, and salicylic acid all showed higher binding affinity for Be2+ compared to Mg2+. ITC binding studies were used to compare Be2+ and Mg2+ interactions with three short peptides: DDDD, GGGG and KKKK. Under the conditions employed, Be2+ exhibited binding with the carboxylate‐rich DDDD peptide while Mg2+ displayed no binding. The neutral tetrapeptide GGGG and the polybasic tetrapeptide KKKK served as negative controls; no metal ion interactions were observed with these peptides in the ITC binding experiments. Further analysis with Be2+ and peptides GDDD and GGDD showed reduced binding affinity compared to DDDD. These results demonstrate that Be2+ has stronger affinity for negatively‐charged carboxylate‐rich peptide sequences compared to Mg2+. This strong affinity may lead to conformational changes that distinguish the inactive Be2+‐GSK3β complex from the active Mg2+‐GSK3β complex. Because GSK3β regulates multiple cellular pathways, the ideal therapeutic inhibitor would lead to selective downstream effects. Investigations using an unusual class of inhibitor like Be2+ might provide insight into the variety of modes potentially available for controlling GSK3β activity.Support or Funding InformationThis work was supported by the U.S. Army Research Laboratory and the U.S. Army Research Office under grant numbers W911NF‐15‐1‐0043 and W911NF‐15‐1‐0216.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Nanofibrous peptide hydrogel elicits angiogenesis and neurogenesis without drugs, proteins, or cells

The design of materials for regenerative medicine has focused on delivery of small molecule drugs, proteins, and cells to help accelerate healing. Additionally, biomaterials have been designed with covalently attached mimics of growth factors, cytokines, or key extracellular matrix components allowing the biomaterial itself to drive biological response. While the approach may vary, the goal of biomaterial design has often centered on promoting either cellular infiltration, degradation, vascularization, or innervation of the scaffold. Numerous successful studies have utilized this complex, multicomponent approach; however, we demonstrate here that a simple nanofibrous peptide hydrogel unexpectedly and innately promotes all of these regenerative responses when subcutaneously implanted into the dorsal tissue of healthy rats. Despite containing no small molecule drugs, cells, proteins or protein mimics, the innate response to this material results in rapid cellular infiltration, production of a wide range of cytokines and growth factors by the infiltrating cells, and remodeling of the synthetic material to a natural collagen-containing ECM. During the remodeling process, a strong angiogenic response and an unprecedented degree of innervation is observed. Collectively, this simple peptide-based material provides an ideal foundational system for a variety of bioregenerative approaches.

Transient effects in π-pulse sequences in MAS solid-state NMR

Dipolar recoupling techniques that use isolated rotor-synchronized π pulses are commonly used in solid-state NMR spectroscopy to gain insight into the structure of biological molecules. These sequences excel through their simplicity, stability towards radio-frequency (rf) inhomogeneity, and low rf requirements. For a theoretical understanding of such sequences, we present a Floquet treatment based on an interaction-frame transformation including the chemical-shift offset dependence. This approach is applied to the homonuclear dipolar-recoupling sequence Radio-Frequency Driven Recoupling (RFDR) and the heteronuclear recoupling sequence Rotational Echo Double Resonance (REDOR). Based on the Floquet approach, we show the influence of effective fields caused by pulse transients and discuss the advantages of pulse-transient compensation. We demonstrate experimentally that the transfer efficiency for homonuclear recoupling can be doubled in some cases in model compounds as well as in simple peptides if pulse-transient compensation is applied to the π pulses. Additionally, we discuss the influence of various phase cycles on the recoupling efficiency in order to reduce the magnitude of effective fields. Based on the findings from RFDR, we are able to explain why the REDOR sequence does not suffer in the recoupling efficiency despite the presence of effective fields.

Substarte Specificty and Immobilization Studies of Purified Solanain from the Latex of Vallaris solanacea

A novel cysteine protease has been purified to electrophoretic homogeneity from apocyanacean member Vallaris solanacea. Molecular weight of the solanain was determined as 28.5 kDa using sodium dodecyl sulphate polyacrylamide gel electrophoresis. Purified protease was named solanain and it was further characterized. An internal tryptic fragment was identified by MALDI TOF, and this peptide showed a homology (66% sequence identity) with the target sequence found as cysteine endopeptidase from Ricinus communis. The purpose of the present work is to examine the specificity towards synthetic peptide and ester substrates and also to compare with other cysteine proteases. In the present work protease solanain from V. solanacea was immobilized using various matrices such as calcium alginate, polyacrylamide, κ-carrageenan, chitosan and amberlite MB-150 employing entrapment and adsorption techniques. The purified solanain from the latex of V. solanacea exhibited broad specificity. Like other cysteine proteases it showed peptidase and amidase activity. However considerable difference was noticed in the rate of hydrolysis and also exhibited difference in specificity towards simple peptide substrates. Immobilization of solanain on amberlite MB-150 beads enhanced the enzyme stability against changes of pH and temperature. The immobilized enzyme retained 95, 89, 84 and 80% activity at the end of 2nd, 3rd, 4th and 5th cycle respectively which indicates that immobilized enzyme can be reused for 5 cycles.

Crafting of functional biomaterials by directed molecular self-assembly of triple helical peptide building blocks.

Collagen is the most abundant extracellular matrix protein in the body and has widespread use in biomedical research, as well as in clinics. In addition to difficulties in the production of recombinant collagen due to its high non-natural imino acid content, animal-derived collagen imposes several major drawbacks-variability in composition, immunogenicity, pathogenicity and difficulty in sequence modification-that may limit its use in the practical scenario. However, in recent years, scientists have shifted their attention towards developing synthetic collagen-like materials from simple collagen model triple helical peptides to eliminate the potential drawbacks. For this purpose, it is highly desirable to develop programmable self-assembling strategies that will initiate the hierarchical self-assembly of short peptides into large-scale macromolecular assemblies with recommendable bioactivity. Herein, we tried to elaborate our understanding related to the strategies that have been adopted by few research groups to trigger self-assembly in the triple helical peptide system producing fascinating supramolecular structures. We have also touched upon the major epitopes within collagen that can be incorporated into collagen mimetic peptides for promoting bioactivity.

Highly Potent Antibacterial Organometallic Peptide Conjugates.

Resistance of pathogenic bacteria against currently marketed antibiotics is again increasing. To meet the societal need for effective cures, scientists are faced with the challenge of developing more potent but equally bacteria-specific drugs. Currently, most efforts are directed toward the modification of existing antibiotics, but ideally, compounds with a new mode of action are required. In this Account, we detail our findings in the area of novel metal-based antibiotics. Our strategy is based on the modification of simple antimicrobial peptides (AMPs) with organometallic agents, resulting in organometallic AMPs (OM-AMPs). Since bacteria have most likely never encountered these synthetically prepared unnatural organometallic agents, we anticipated that such agents could well become potentiating players in the antibiotics arena. Moreover, exploiting some of the particular properties of metal complexes should also help to elucidate the mode of action of small cationic AMPs, the molecular details of which have remained elusive despite intensive efforts. Using standard Fmoc/tBu-based solid-phase peptide synthesis approaches, we have prepared various organometallic-peptide conjugates with covalently linked group 8 and 9 metallocenes (ferrocene, ruthenocene, osmocene, and cobaltocenium). As a starting point we took the (RW)3 antibacterial hexapeptide lead structure. After modifying the peptide sequence (generations 1 and 2), changing the nature and position of the organometallic group (generation 3), and optimizing the amino acid chirality (generation 5), we identified several organometallic antibacterial peptides that are currently among the most active synthetic AMPs (synAMPs) that have ever been prepared. Through these rational and systematic optimizations, we were able to increase the antibacterial activity of a short non-organometallic synAMP 18-fold to submicromolar activity, rivaling the activity of vancomycin (often the drug of last resort) against methicillin-resistant Staphylococcus aureus (MRSA). Moreover, by making use of the unique physicochemical properties of ruthenocene, we were able to determine the mode of action of these short AMPs in unprecedented detail. We propose that the OM-AMP integrates into the bacterial membrane and changes its biophysical properties, which ultimately results in detachment of vital enzymes for respiration and cell-wall biosynthesis such as specifically cytochrome c and MurG from their locations in the membrane. Further explorations of these small OM-AMP derivatives that are summarized in this Account include lipid substitution, multivalent display of metalated di- or tripeptides on a trivalent scaffold with different linkers, and increasing the metal-to-peptide ratio such that every tryptophan in the (RW)3 scaffold is eventually replaced by a metalated lysine. While initial experiments with our OM-AMPs for systemic applications were largely disappointing, these OM-AMPs turned out to be potent antibiotics for topical applications. In this sense, two applications are described as examples in this Account, namely, bacterial decontamination of wastewater by reverse osmosis membranes (coated with our OM-AMPs by Cu-catalyzed azide-alkyne cycloaddition reaction) and synergistic activities of one of our synAMPs with colistin and tobramycin for the treatment of Pseudomonas aeruginosa infections that are associated with cystic fibrosis.

Improvement of a fermentation process for the production of two PfAMA1-DiCo-based malaria vaccine candidates in Pichia pastoris

Pichia pastoris is a simple and powerful expression platform that has the ability to produce a wide variety of recombinant proteins, ranging from simple peptides to complex membrane proteins. A well-established fermentation strategy is available comprising three main phases: a batch phase, followed by a glycerol fed-batch phase that increases cell density, and finally an induction phase for product expression using methanol as the inducer. We previously used this three-phase strategy at the 15-L scale to express three different AMA1-DiCo-based malaria vaccine candidates to develop a vaccine cocktail. For two candidates, we switched to a two-phase strategy lacking the intermediate glycerol fed-batch phase. The new strategy not only provided a more convenient process flow but also achieved 1.5-fold and 2.5-fold higher space-time yields for the two candidates, respectively, and simultaneously reduced the final cell mass by a factor of 1.3, thus simplifying solid–liquid separation. This strategy also reduced the quantity of host cell proteins that remained to be separated from the two vaccine candidates (by 34% and 13%, respectively), thus reducing the effort required in the subsequent purification steps. Taken together, our new fermentation strategy increased the overall fermentation performance for the production of two different AMA1-DiCo-based vaccine candidates.

Array-Based Rational Design of Short Peptide Probe-Derived from an Anti-TNT Monoclonal Antibody.

Complementarity-determining regions (CDRs) are sites on the variable chains of antibodies responsible for binding to specific antigens. In this study, a short peptide probe for recognition of 2,4,6-trinitrotoluene (TNT), was identified by testing sequences derived from the CDRs of an anti-TNT monoclonal antibody. The major TNT-binding site in this antibody was identified in the heavy chain CDR3 by antigen docking simulation and confirmed by an immunoassay using a spot-synthesis based peptide array comprising amino acid sequences of six CDRs in the variable region. A peptide derived from heavy chain CDR3 (RGYSSFIYWF) bound to TNT with a dissociation constant of 1.3 μM measured by surface plasmon resonance. Substitution of selected amino acids with basic residues increased TNT binding while substitution with acidic amino acids decreased affinity, an isoleucine to arginine change showed the greatest improvement of 1.8-fold. The ability to create simple peptide binders of volatile organic compounds from sequence information provided by the immune system in the creation of an immune response will be beneficial for sensor developments in the future.

Current Status and Future Prospects of Marine Natural Products (MNPs) as Antimicrobials.

The marine environment is a rich source of chemically diverse, biologically active natural products, and serves as an invaluable resource in the ongoing search for novel antimicrobial compounds. Recent advances in extraction and isolation techniques, and in state-of-the-art technologies involved in organic synthesis and chemical structure elucidation, have accelerated the numbers of antimicrobial molecules originating from the ocean moving into clinical trials. The chemical diversity associated with these marine-derived molecules is immense, varying from simple linear peptides and fatty acids to complex alkaloids, terpenes and polyketides, etc. Such an array of structurally distinct molecules performs functionally diverse biological activities against many pathogenic bacteria and fungi, making marine-derived natural products valuable commodities, particularly in the current age of antimicrobial resistance. In this review, we have highlighted several marine-derived natural products (and their synthetic derivatives), which have gained recognition as effective antimicrobial agents over the past five years (2012–2017). These natural products have been categorized based on their chemical structures and the structure-activity mediated relationships of some of these bioactive molecules have been discussed. Finally, we have provided an insight into how genome mining efforts are likely to expedite the discovery of novel antimicrobial compounds.

Development and comprehensive comparison of two on-line capillary electrophoretic methods for β-secretase inhibitor screening

Alzheimer’s disease is the most common cause of dementia, afflicting over 34 million patients worldwide. Since β-secretase is a rate-limiting enzyme of the production of neurotoxic β-amyloid peptide oligomers abnormally accumulated in the affected brain tissue, its specific inhibition appears to be a promising approach to slowing down or even stopping the progression of the disease. Hence two on-line capillary electrophoretic methods for studies of β-secretase activity based on the principles of transverse diffusion of laminar flow profiles and electrophoretically mediated microanalysis were developed, both using a simple unlabeled peptide substrate and UV detection. The optimized procedures were thoroughly validated and applied for determining the enzyme’s kinetic parameters and the inhibition characteristics of two potent probe inhibitors. The resulting values were found to be comparable to literature data obtained with other analytical techniques. The suitability of the employed methodologies for different experimental designs is discussed on the basis of a statistical evaluation of the experimental data. The presented methods constitute a miniaturized and fully automated tool, which should be suitable for kinetic and inhibition studies of β-secretase as a target for Alzheimer’s disease drug discovery in the early stages of the development of a new drug.

First observation of metal ion-induced structural fluctuations of α-helical peptides by using diffracted X-ray tracking

In order to analyze protein structural dynamics, we designed simple model peptides whose structures changed from random-coil to helix-bundle structures by forming stable hydrophobic core in the presence of metal ions. The strategy involved destabilizing a de novo designed three helix-bundle protein by substituting the residues present in its hydrophobic core with histidine and small amino acids. The conformational changes of peptides induced upon binding of Zn2+ to histidine were analyzed using circular dichroism spectroscopy, which revealed peptides, HA and HG, to be good candidates for further analyses. The diffracted X-ray tracking experiments showed that the structural fluctuations of both HA and HG were suppressed upon binding of Zn2+. We succeeded in observing the differences in fluctuations of HA and HG in solution between random-coil like and helix-bundle structures. The metal-binding energies determined using the angular diffusion coefficients were in good agreement with those determined using isothermal titration calorimetry.

Increasing the histidine ‘density’ in tripodal peptides by gradual N -functionalization of tris (2-aminoethyl)amine (tren) with l -histidyl units: The effect on zinc(II) complexes

Abstract Tripodal peptidomimetics have received increasing interest among others as efficient metal ion chelators. Most of these studies have focused on symmetrical, tri-substituted ligands. Our aim was to establish how the increasing donor group ‘density’, i.e. the gradual N-histidyl substitution, alters the coordination chemical properties of the tripodal platform. To this end we synthesized mono- , bis- and tris ( l -histidyl)-functionalized tren derivatives ( L1 , L2 and L3 , respectively), and studied their zinc(II) complexes by pH potentiometry, 1 H NMR and MS spectroscopy. The three ligands provide a variety of donor sites, and consequently different stability and structure for their zinc(II) complexes depending on the pH and metal-to-ligand ratios. In the neutral pH range histamine-like coordination is operating in all cases. Due to the formation of macrochelate between the two/three {N im ,NH 2 } binding sites, L2 and L3 have considerably higher zinc(II) binding ability than histamine, or any other simple peptide with N-terminal His unit. The situation is fundamentally different at higher pH. The tren-like subunit in L1 acts as an anchoring site for amide deprotonation, and the {3NH 2 ,N − ,N tert } type coordination, a rare example where zinc(II)-amide N − coordination takes place, results in outstanding stability. Although L1 provides tight binding above pH 7, it forms only mononuclear species. However, the increasing level of functionalization in L2 and L3 allows the formation of oligonuclear complexes, and at threefold zinc(II) excess the three ligands share nearly the same amount of zinc(II). Moreover, the high histidine ‘density’ in L2 and L3 also provides the formation of imidazolato-bridged structures, which has never been observed before in zinc(II) complexes of simple linear peptides.