Cu(II) Binding to the Peptide Ala-His-His, a Chimera of the Canonical Cu(II)-Binding Motifs Xxx-His and Xxx-Zzz-His

Modulating Copper(II) Coordination and Antimicrobial Activity: Effects of d-Amino Acid Substitution and Retro-Inverso Modification in Human Saliva MUC7 Peptide

Fragments of MUC7, a salivary protein involved in nonimmune defense, arise from proteolytic cleavage in saliva and exhibit antimicrobial properties. However, their therapeutic use is limited by low stability due to further degradation. To address this, a native MUC7 fragment was modified using d-amino acids and the retro-inverso strategy. Given the role of metal ions in enhancing antimicrobial peptides, we analyzed the bioinorganic chemistry of these systems with Cu(II) and assessed their antimicrobial activity against fungal and bacterial strains. This study is the first to explore the correlation between metal binding mode, structure, stability, and antimicrobial activity of retro-inverso peptides as well as Cu(II) coordination in such systems. A combination of experimental techniques (potentiometry, mass spectrometry, UV-vis, circular dichroism, electron paramagnetic resonance, and nuclear magnetic resonance spectroscopy) and density functional theory calculations characterized their coordination chemistry. Our results demonstrate that the "standard" enantiomeric exchange and retro-inverso modifications of the MUC7 fragment have a minimal effect on the secondary structure and biological activity of the studied peptides and their Cu(II) complexes. However, these modifications significantly influence on the thermodynamic stability of studied systems.

Design of Histidine Sequence-Associated Tripeptide Sequences for Recognition of Copper Ions and Their Application to Live Cells

Copper is central to many enzymes in living organisms, and imbalances in copper levels are linked to various diseases. Therefore, developing probes to detect copper ions is essential. Histidine, especially in the polypeptide sequences at the first three N-terminal positions (His1, His2, and His3), uniquely binds to copper ions. This study introduces three groups of tripeptide probes designed to monitor copper ion levels in living cells and organisms. The results show that tripeptides with histidine at the -2 position, specifically HDQL-2 (Asp-His-Gln-Dansyl), HMFM-2 (Met-His-Phe-Dansyl), and HDMB-2 (Asp-His-Met-Dansyl), exhibit a higher affinity for copper ions. These probes responded quickly to copper ions, demonstrating excellent fluorescence turn-off performance and stable detection within a pH range of 6.0-11.0. The detection limits for fluorescence titration, calculated using the 3σ/k equation, were 17.65 nM (HDQL-2), 18.04 nM (HMFM-2), and 15.50 nM (HDMB-2). Peptide probes are ideal for detecting copper ions in living cells via fluorescence imaging because of their low toxicity and good biocompatibility. The fluorescence intensity decreases as copper ion content changes.

Ultra-Selective and Sensitive Fluorescent Chemosensor Based on Phage Display-Derived Peptide with an N-Terminal Cu(II)-Binding Motif

Copper, along with gold, was among the first metals that humans employed. Thus, the copper pollution of the world's water resources is escalating, posing a significant threat to human health and aquatic ecosystems. It is crucial to develop detection technology that is both low-cost and feasible, as well as ultra-selective and sensitive. This study explored the use of the NH2-Xxx-His motif-derived peptide from phage display technology for ultra-selective Cu2+ detection. Various Cu-binding M13 phage clones were isolated, and their affinity and cross-reactivity for different metal ions were determined. A detailed analysis of the amino acid sequence of the unique Cu-binding peptides was employed. For the development of an optical chemosensor, a peptide with an NH2-Xxx-His motif was selected. The dansyl group was incorporated during solid-phase peptide synthesis, and fluorescence detection assays were employed. The efficacy of the Cu2+-binding peptide was verified through spectroscopic measurements. In summary, we developed a highly selective and sensitive fluorescent chemosensor for Cu2+ detection based on a peptide sequence from a phage display library that carries the N-terminal Xxx-His motif.

The effect of histidine, histamine, and imidazole on electrochemical properties of Cu(II) complexes of Aβ peptides containing His-2 and His-3 motifs

The N-truncation of amyloid beta (Aβ) peptides could lead to peptide sequences with the histidine residue at the second and third positions, creating His-2 and His-3 motifs, known as high-affinity Cu(II) binding sites. In such complexes, the Cu(II) ion is arrested in a rigid structure of a square-planar arrangement of nitrogen donors, which highly limits its susceptibility to Cu(II) reduction. Cu(II) reduction fuels the Cu(II)/Cu(I) redox cycle, which is engaged in the production of reactive oxygen species (ROS). Employing electrochemical techniques, cyclic voltammetry (CV) and differential pulse voltammetry (DPV), together with UV-vis spectroscopy, we showed that low-molecular-weight (LMW) substances, such as imidazole, histamine, and histidine, could enhance the redox activity of Cu(II) complexes of three models of N-truncated Aβ peptides, Aβ4-9, Aβ5-9, and Aβ12-16, identifying three main mechanisms. LMW compounds could effectively compete with Aβ peptides for Cu(II) ions, forming Cu(II)/LMW species, which are more prone to Cu(II) reduction. LMW substances could also shift the equilibrium between the Cu(II)/Aβ species towards the species with higher susceptibility to Cu(II) reduction. Finally, the presence of LMW molecules could promote Cu(I) reoxidation in ternary Cu(II)/Aβ/LMW systems. The obtained results raise further questions regarding the Cu(II) redox activity in Alzheimer's disease.

Unpredictable Dynamic Behaviour of Ruthenium Chelate Pyrrole Derivatives

Reaction of [Ru(H)2(CO)(PPh3)3] 1 with an equimolar amount of pyrrole-2-carboxylic acid (H2L1) leads to the homoleptic chelate derivative k2(O,O)-[RuH(CO)(HL1)(PPh3)2] 2. Prolonged acetonitrile refluxing promotes an unusual k2(O,O)- → k2(N,O)- dynamic chelate conversion, forming a neutral, stable, air- and moisture- insensitive, solvento-species k2(N,O)-[Ru(MeCN)(CO)(L1)(PPh3)2] 3. Analogously, reaction of 1 with the pyrrole-2-carboxyaldehyde (HL2) affords k2(N,O)-[RuH(CO)(HL2)(PPh3)2] 4, 5, as a couple of functional isomers. Optimized reaction conditions such as temperature and solvent polarity allow the isolation of dominant configurations. Structure 5 is a pyrrolide Ru-carbaldehyde, obtained from cyclization of the pendant CHO function, whereas species 4 can be viewed as an ethanoyl-conjugated Ru-pyrrole. Derivatives 3-5 were characterized by single crystal X-ray diffraction, ESI-Ms, IR, and NMR spectroscopy, indicating distinct features for the Ru-bonded pyrrolyl groups. DFT computational results, coplanarity, bond equalization, and electron delocalization along the fused five-membered rings support aromatic features. In accordance with the antisymbiotic trans-influence, both the isolated isomers 4 and 5 disclose CO ligands opposite to N- or O-anionic groups. The quantitative Mayer bond order evidences a stabilizing backbonding effect. Antibacterial and antifungal trials on Gram-positive (Staphylococcus aureus), Gram-negative (Escherichia coli), and Candida albicans were further carried out.

Sequence-Activity Relationship of ATCUN Peptides in the Context of Alzheimer's Disease

Amino-terminal Cu^II and Ni^II (ATCUN) binding sequences are widespread in the biological world. Here, we report on the study of eight ATCUN peptides aimed at targeting copper ions and stopping the associated formation of reactive oxygen species (ROS). This study was actually more focused on Cu(Aβ)-induced ROS production in which the Aβ peptide is the "villain" linked to Alzheimer's disease. The full characterization of Cu^II binding to the ATCUN peptides, the Cu^II extraction from Cu^II(Aβ), and the ability of the peptides to prevent and/or stop ROS formation are described in the relevant biological conditions. We highlighted in this research that all the ATCUN motifs studied formed the same thermodynamic complex but that the addition of a second histidine in position 1 or 2 allowed for an improvement in the Cu^II uptake kinetics. This kinetic rate was directly related to the ability of the peptide to stop the Cu^II(Aβ)-induced production of ROS, with the most efficient motifs being HWHG and HGHW.

Why the Ala-His-His Peptide Is an Appropriate Scaffold to Remove and Redox Silence Copper Ions from the Alzheimer's-Related Aβ Peptide

The progressive, neurodegenerative Alzheimer's disease (AD) is the most widespread dementia. Due to the ageing of the population and the current lack of molecules able to prevent or stop the disease, AD will be even more impactful for society in the future. AD is a multifactorial disease, and, among other factors, metal ions have been regarded as potential therapeutic targets. This is the case for the redox-competent Cu ions involved in the production of reactive oxygen species (ROS) when bound to the Alzheimer-related Aβ peptide, a process that contributes to the overall oxidative stress and inflammation observed in AD. Here, we made use of peptide ligands to stop the Cu(Aβ)-induced ROS production and we showed why the AHH sequence is fully appropriate, while the two parents, AH and AAH, are not. The AHH peptide keeps its beneficial ability against Cu(Aβ)-induced ROS, even in the presence of ZnII-competing ions and other biologically relevant ions. The detailed kinetic mechanism by which AHH could exert its action against Cu(Aβ)-induced ROS is also proposed.

Minimalist De Novo Design of an Artificial Enzyme

We employed a reductionist approach in designing the first heterochiral tripeptide that forms a robust heterogeneous short peptide catalyst similar to the "histidine brace" active site of lytic polysaccharide monooxygenases. The histidine brace is a conserved divalent copper ion-binding motif that comprises two histidine side chains and an amino group to create the T-shaped 3N geometry at the reaction center. The geometry parameters, including a large twist angle (73°) between the two imidazole rings of the model complex, are identical to those of native lytic polysaccharide monooxygenases (72.61°). The complex was synthesized and characterized as a structural and functional mimic of the histidine brace. UV-vis, vis-circular dichroism, Raman, and electron paramagnetic resonance spectroscopic analyses suggest a distorted square-pyramidal geometry with a 3N coordination at pH 7. Solution- and solid-state NMR results further confirm the 3N coordination in the copper center of the complex. The complex is pH-dependent and could catalyze the oxidation of benzyl alcohol in water to benzaldehyde with yields up to 82% in 3 h at pH 7 and above at 40 °C. The catalyst achieved 100% selectivity for benzaldehyde compared to conventional copper catalysis. The design of such a minimalist building block for functional soft materials with a pH switch can be a stepping stone in addressing needs for a cleaner and sustainable future catalyst.

Kinetics of Cu(II) complexation by ATCUN/NTS and related peptides: a gold mine of novel ideas for copper biology

Cu(II)-peptide complexes are intensely studied as models for biological peptides and proteins and for their direct importance in copper homeostasis and dyshomeostasis in human diseases. In particular, high-affinity ATCUN/NTS (amino-terminal copper and nickel/N-terminal site) motifs present in proteins and peptides are considered as Cu(II) transport agents for copper delivery to cells. The information on the affinities and structures of such complexes derived from steady-state methods appears to be insufficient to resolve the mechanisms of copper trafficking, while kinetic studies have recently shown promise in explaining them. Stopped-flow experiments of Cu(II) complexation to ATCUN/NTS peptides revealed the presence of reaction steps with rates much slower than the diffusion limit due to the formation of novel intermediate species. Herein, the state of the field in Cu(II)-peptide kinetics is reviewed in the context of physiological data, leading to novel ideas in copper biology, together with the discussion of current methodological issues.

Synthesis, Characterization, and Reaction Studies of Pd(II) Tripeptide Complexes

The aqueous synthesis of Pd(II) complexes with alkylated tripeptides led to the hydrolysis of the peptides at low pH values and mixtures of complexed peptides were formed. A non-aqueous synthetic route allowed the formation and isolation of single products and their characterization. Pd(II) complexes with α-Asp(OR)AlaGly(OR), β-Asp(OR)AlaGly(OR), and TrpAlaGly(OR) (R = H or alkyl) as tri and tetradentate chelates were characterized. The tridentate coordination mode was accompanied by a fourth monodentate ligand that was shown to participate in both ligand exchange reactions and a direct removal to form the tetradentate coordination mode. The tetradentate coordination revealed a rare a hemi labile carbonyl goup coordination mode to Pd(II). Reactivity with small molecules such as ethylene, acids, formate, and episulfide was investigated. Under acidic conditions and in the presence of ethylene; acetaldehyde was formed. The Pd(II) is a soft Lewis acid and thiophilic and the complexes abstract sulfur from episulfide at apparent modest catalytic rates. The complexes adopt a square planar geometry according to a spectroscopic analysis and DFT calculations that were employed to evaluate the most energetically favorable coordination geometry and compared with the observed infrared and NMR data.

A luminescent ATCUN peptide variant with enhanced properties for copper(II) sensing in biological media

The measurement of labile Cu^II in biological samples is fundamental for understanding Cu metabolism and has been emerging as a promising diagnostic marker for Cu-related pathologies such as Wilson's and Alzheimer's diseases. The use of fluorescent chelators may be useful to circumvent separation steps employed by current methods. For this purpose, we recently designed a selective and suited-affinity turn-off luminescent probe based on a peptide bearing the Cu^II-binding Xxx-Zzz-His (Amino-Terminal Cu^II- and Ni^II-binding, ATCUN) motif and a Tb^III-DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) complex. Here, we present an analogue probe bearing the ATCUN motif variant Xxx-His-His. This probe showed much faster response in biologically-relevant media and higher stability than the previous motif at low pH. These features could be beneficial to the measurement of dynamic Cu^II fluctuations and the application in slightly acidic media, such as urine.

Investigation of the structure and dynamics of gallium binding to high-affinity peptides elucidated by multi-scale simulation, quantum chemistry, NMR and ITC

Gallium (as Ga³⁺) is a Group IIIa metal and its recovery from wastewaters has become increasingly important for its reuse. The use of peptides for recycling offers a low-cost and environmentally-friendly option but the structural characteristics of peptides likely to bind Ga³⁺ are largely unknown. Multiple computational methods, coupled with experimental verification via NMR and Isothermal Calorimetry (ITC), were used to establish that Ga³⁺ binds with high affinity to peptide sequences and to elucidate the structural characteristics that contributed. It was demonstrated that peptide pre-organisation is key to Ga³⁺ binding and that a favourable binding position is necessarily governed by the size and shape of the electrostatic environment as much as individual electrostatic interactions with peptide residues themselves. Given favourable conditions, Ga³⁺ retrieved plausible binding positions involving both charged and uncharged residues that greatly increases the range of bonding possibilities with other peptide sequences and offers insights for binding other metals. The addition of pH buffer substantially improved the affinity of Ga³⁺ and a structural role for a buffer component was demonstrated.

Unraveling the implications of multiple histidine residues in the potent antimicrobial peptide Gaduscidin-1

The development of antimicrobial peptides (AMPs) as potential therapeutics requires resolving the foundational principles behind their structure-activity relationships. The role of histidine residues within AMPs remains a mystery despite the fact that several potent peptides containing this amino acid are being considered for further clinical development. Gaduscidin-1 (Gad-1) is a potent AMP from Atlantic cod fish that has a total of five His residues. Herein, the role of His residues and metal-potentiated activity of Gad-1 was studied. The five His residues contribute to the broad-spectrum activity of Gad-1. We demonstrated that Gad-1 can coordinate two Cu²⁺ ions, one at the N-terminus and one at the C-terminus, where the C-terminal binding site is a novel Cu²⁺ binding motif. High affinity Cu²⁺ binding at both sites was observed using mass spectrometry and isothermal titration calorimetry. Electron paramagnetic resonance was used to determine the coordination environment of the Cu²⁺ ions. Cu²⁺ binding was shown to be responsible for an increase in antimicrobial activity and a new mode of action. Along with the traditional AMP mode of action of pore formation, Gad-1 in the presence of Cu²⁺ (per)oxidizes lipids. Importantly, His3, His11, His17, and His21 were found to be important to lipid (per)oxidation. This insight will help further understand the inclusion and role of His residues in AMPs, the role of the novel C-terminal binding site, and can contribute to the field of designing potent AMPs that bind metal ions to potentiate activity.

Copper(ii) complex of N-truncated amyloid-β peptide bearing a His-2 motif as a potential receptor for phosphate anions

Electrochemical and spectroscopic studies demonstrated that the N-truncated amyloid β peptide Aβ_5-9 (Arg-His-Asp-Ser-Gly-NH₂) possessing histidine at position 2 (His-2) formed ternary complexes with copper(ii) and phosphate anions or phosphate groups of biomolecules. The recognition ability of Cu(ii)-Aβ_5-9 toward phosphate species provided a new perspective on designing phosphate-selective molecular receptors.

Cu(II)-Binding N-Terminal Sequences of Human Proteins

Proteins anchor copper(II) ions mainly by imidazole from histidine residues located in different positions in the primary protein structures. However, the motifs with histidine in the first three N-terminal positions (His¹ , His² , and His³ ) show unique Cu(II)-binding properties, such as availability from the surface of the protein, high flexibility, and high Cu(II) exchangeability with other ligands. It makes such sequences beneficial for the fast exchange of Cu(II) between ligands. Furthermore, sequences with His¹ and His² , thus, non-saturating the Cu(II) coordination sphere, are redox-active and may play a role in Cu(II) reduction to Cu(I). All human protein sequences deposited in UniProt Knowledgebase were browsed for those containing His¹ , His² , or His³ . Proteolytically modified sequences (with the removal of a propeptide or Met residue) were taken for the analysis. Finally, the sequences were sorted out according to the subcellular localization of the proteins to match the respective sequences with the probability of interaction with divalent copper.

Impact of N-Truncated Aβ Peptides on Cu- and Cu(Aβ)-Generated ROS: CuI Matters!

In vitro Cu(Aβ_1-x )-induced ROS production has been extensively studied. Conversely, the ability of N-truncated isoforms of Aβ to alter the Cu-induced ROS production has been overlooked, even though they are main constituents of amyloid plaques found in the human brain. N-Truncated peptides at the positions 4 and 11 (Aβ_4-x and Aβ_11-x ) contain an amino-terminal copper and nickel (ATCUN) binding motif (H₂ N-Xxx-Zzz-His) that confer them different coordination sites and higher affinities for Cu^II compared to the Aβ_1-x peptide. It has further been proposed that the role of Aβ_4-x peptide is to quench Cu^II toxicity in the brain. However, the role of Cu^I coordination has not been investigated to date. In contrast to Cu^II , Cu^I coordination is expected to be the same for N-truncated and N-intact peptides. Herein, we report in-depth characterizations and ROS production studies of Cu (Cu^I and Cu^II ) complexes of the Aβ_4-16 and Aβ_11-16 N-truncated peptides. Our findings show that the N-truncated peptides do produce ROS when Cu^I is present in the medium, albeit to a lesser extent than the unmodified counterpart. In addition, when used as competitor ligands (i.e., in the presence of Aβ_1-16 ), the N-truncated peptides are not able to fully preclude Cu(Aβ_1-16 )-induced ROS production.

Imidazole-rich copper peptides as catalysts in xenobiotic degradation

Laccases, oxidative copper-enzymes found in fungi and bacteria were used as the basis in the design of nona- and tetrapeptides. Laccases are known to be excellent catalysts for the degradation of phenolic xenobiotic waste. However, since solvent extraction of laccases is environmentally-unfriendly and yields obtained are low, they are less preferred compared to synthetic catalysts. The histidine rich peptides were designed based on the active site of laccase extracted from Trametes versicolor through RCSB Protein Data Bank, LOMETS and PyMol software. The peptides were synthesized using Fmoc-solid phase peptide synthesis (SPPS) with 30-40% yield. These peptides were purified and characterized using LC-MS (purities >75%), FTIR and NMR spectroscopy. Synthesized copper(II)-peptides were crystallized and then analyzed spectroscopically. Their structures were elucidated using 1D and 2D NMR. Standards (o,m,p-cresol, 2,4-dichlorophenol) catalysed using laccase from Trametes versicolor (0.66 U/mg) were screened under different temperatures and stirring rate conditions. After optimizing the degradation of the standards with the best reaction conditions reported herein, medications with phenolic and aromatic structures such as ibuprofen, paracetamol (acetaminophen), salbutamol, erythromycin and insulin were screened using laccase (positive control), apo-peptides and copper-peptides. Their activities evaluated using GC-MS, were compared with those of peptide and copper-peptide catalysts. The tetrapeptide was found to have the higher degradation activity towards salbutamol (96.8%) compared with laccase at 42.8%. Ibuprofen (35.1%), salbutamol (52.9%) and erythromycin (49.7%) were reported to have the highest degradation activities using Cu-tetrapeptide as catalyst when compared with the other medications. Consequently, o-cresol (84%) was oxidized by Tp-Cu while the apo-peptides failed to oxidize the cresols. Copper(II)-peptides were observed to have higher catalytic activity compared to their parent peptides and the enzyme laccase for xenobiotic degradation.

Directed coordination study of [Pd(en)(H2O)2]2+ with hetero-tripeptides containing C-terminus methyl esters employing NMR spectroscopy

Alkylation of the C-terminus acids in small peptides allows direction to amine and amide coordination, while changing the peptide composition to form tetradentate κ⁴[n,5,5], where n = 5-, 6-, 7-, or 8-membered ring coordination geometries, can be achieved. The alkylated tripeptide ligands, TrpAlaGly(OMe), β-Asp(OtBu)AlaGly(OMe), Asp(OtBu)AlaGly(OMe), and the fully methylated GSH, γ-Glu(OMe)Cys(SMe)Gly(OMe), were synthesized and their coordination properties to [Pd(en)(H₂O)₂]²⁺ were studied. pH-dependent coordination was analyzed by NMR spectroscopy and the coordination to the alkylated tripeptides at selected pH values inferred from their NMR spectra. If selective coordination of amine/amide donors results in metal complexation, allowing for flexible and adjustable ligand frameworks, then this strategy could potentially be extended to other metal ions and peptide system.

Copper Transporters? Glutathione Reactivity of Products of Cu-Aβ Digestion by Neprilysin

Aβ_4–42 is the major subspecies of Aβ peptides characterized by avid Cu(II) binding via the ATCUN/NTS motif. It is thought to be produced in vivo proteolytically by neprilysin, but in vitro experiments in the presence of Cu(II) ions indicated preferable formation of C-terminally truncated ATCUN/NTS species including Cu^IIAβ_4–16, Cu^IIAβ_4–9, and also Cu^IIAβ_12–16, all with nearly femtomolar affinities at neutral pH. Such small complexes may serve as shuttles for copper clearance from extracellular brain spaces, on condition they could survive intracellular conditions upon crossing biological barriers. In order to ascertain such possibility, we studied the reactions of Cu^IIAβ_4–16, Cu^IIAβ_4–9, Cu^IIAβ_12–16, and Cu^IIAβ_1–16 with reduced glutathione (GSH) under aerobic and anaerobic conditions using absorption spectroscopy and mass spectrometry. We found Cu^IIAβ_4–16 and Cu^IIAβ_4–9 to be strongly resistant to reduction and concomitant formation of Cu(I)–GSH complexes, with reaction times ∼10 h, while Cu^IIAβ_12–16 was reduced within minutes and Cu^IIAβ_1–16 within seconds of incubation. Upon GSH exhaustion by molecular oxygen, the Cu^IIAβ complexes were reformed with no concomitant oxidative damage to peptides. These finding reinforce the concept of Aβ_4–x peptides as physiological trafficking partners of brain copper.

Aβ₄₋₁₆, Aβ₄₋₉, and Aβ₁₂₋₁₆, oligopeptide products of β-amyloid degradation by neprilysin, bind Cu^II ions very tightly and are considered as possible Cu^II carriers in the brain. We demonstrated that Cu^II(Aβ_4−x) complexes, but not Cu^II(Aβ₁₂₋₁₆), are kinetically resistant to reduction by glutathione. No covalent Aβ peptide modifications were observed during the copper reduction and reoxidation by ambient oxygen, yielding the original complexes. These features suggest that Cu^II(Aβ_4−x) complexes might be able to cross the blood−brain barrier.

Detection of Cu2+ Ions with GGH Peptide Realized with Si-Nanoribbon ISFET

The presence of heavy metal ions such as copper in the human body at certain concentrations and specific conditions can lead to the development of different diseases. The currently available analytical detection methods remain expensive, time-consuming, and often require sample pre-treatment. The development of specific and quantitative, easy-in-operation, and cost-effective devices, capable of monitoring the level of Cu²⁺ ions in environmental and physiological media, is necessary. We use silicon nanoribbon (SiNR) ion-sensitive field effect transistor (ISFET) devices modified with a Gly-Gly-His peptide for the detection of copper ions in a large concentration range. The specific binding of copper ions causes a conformational change of the ligand, and a deprotonation of secondary amine groups. By performing differential measurements, we gain a deeper insight into the details of the ion-ligand interaction. We highlight in particular the importance of considering non-specific interactions to explain the sensors' response.

Copper complexes of synthetic peptides mimicking neurotrophin-3 enhance neurite outgrowth and CREB phosphorylation

In this work we report on the synthesis and physiochemical/biological characterization of a peptide encompassing the first thirteen residues of neurotrophin-3 (NT-3). The protein capability to promote neurite outgrowth and axonal branching by a downstream mechanism that involves the increase of the cAMP response element-binding level (CREB) was found for the NT3(1-13) peptide, thus validating its protein mimetic behaviour. Since copper ions are also involved in neurotransmission and their internalization may be an essential step in neuron differentiation and CREB phosphorylation, the peptide and its copper complexes were characterized by potentiometric and spectroscopic techniques, including UV-visible, CD and EPR. To have a detailed picture of the coordination features of the copper complexes with NT3(1-13), we also scrutinized the two peptide fragments encompassing the shorter sequences 1-5 and 5-13, respectively, showing that the amino group is the main anchoring site for Cu(ii) at physiological pH. The peptide activity increased in the presence of copper ions. The effect of copper(ii) addition is more marked for NT3(1-13) than the other two peptide fragments, in agreement with its higher affinity for metal ions. Confocal microscopy measurements carried out on fluorescently labelled NT3(1-13) indicated that copper ions increase peptide internalization.

Triggering Cu-coordination change in Cu(ii)-Ala-His-His by external ligands

A switch in copper(ii)-coordination between the two motifs Xxx-Zzz-His (ATCUN) and Xxx-His in the hybride peptide Ala-His-His is induced by adding a ligand such as imidazole.

Oligopeptides Generated by Neprilysin Degradation of β-Amyloid Have the Highest Cu(II) Affinity in the Whole Aβ Family

The catabolism of β-amyloid (Aβ) is carried out by numerous endopeptidases including neprilysin, which hydrolyzes peptide bonds preceding positions 4, 10, and 12 to yield Aβ_4-9 and a minor Aβ_{12- x} species. Alternative processing of the amyloid precursor protein by β-secretase also generates the Aβ_{11- x} species. All these peptides contain a Xxx-Yyy-His sequence, also known as an ATCUN or NTS motif, making them strong chelators of Cu(II) ions. We synthesized the corresponding peptides, Phe-Arg-His-Asp-Ser-Gly-OH (Aβ_4-9), Glu-Val-His-His-Gln-Lys-am (Aβ_11-16), Val-His-His-Gln-Lys-am (Aβ_12-16), and pGlu-Val-His-His-Gln-Lys-am (pAβ_11-16), and investigated their Cu(II) binding properties using potentiometry, and UV-vis, circular dichroism, and electron paramagnetic resonance spectroscopies. We found that the three peptides with unmodified N-termini formed square-planar Cu(II) complexes at pH 7.4 with analogous geometries but significantly varied K_d values of 6.6 fM (Aβ_4-9), 9.5 fM (Aβ_12-16), and 1.8 pM (Aβ_11-16). Cyclization of the N-terminal Glu11 residue to the pyroglutamate species pAβ_11-16 dramatically reduced the affinity (5.8 nM). The Cu(II) affinities of Aβ_4-9 and Aβ_12-16 are the highest among the Cu(II) complexes of Aβ peptides. Using fluorescence spectroscopy, we demonstrated that the Cu(II) exchange between the Phe-Arg-His and Val-His-His motifs is very slow, on the order of days. These results are discussed in terms of the relevance of Aβ_4-9, a major Cu(II) binding Aβ fragment generated by neprilysin, as a possible Cu(II) carrier in the brain.

N-Terminal Cu-Binding Motifs (Xxx-Zzz-His, Xxx-His) and Their Derivatives: Chemistry, Biology and Medicinal Applications

Peptides and proteins with N-terminal amino acid sequences NH2 -Xxx-His (XH) and NH2 -Xxx-Zzz-His (XZH) form well-established high-affinity CuII -complexes. Key examples are Asp-Ala-His (in serum albumin) and Gly-His-Lys, the wound healing factor. This opens a straightforward way to add a high-affinity CuII -binding site to almost any peptide or protein, by chemical or recombinant approaches. Thus, these motifs, NH2 -Xxx-Zzz-His in particular, have been used to equip peptides and proteins with a multitude of functions based on the redox activity of Cu, including nuclease, protease, glycosidase, or oxygen activation properties, useful in anticancer or antimicrobial drugs. More recent research suggests novel biological functions, mainly based on the redox inertness of CuII in XZH, like PET imaging (with 64 Cu), chelation therapies (for instance in Alzheimer's disease and other types of neurodegeneration), antioxidant units, Cu transporters and activation of biological functions by strong CuII binding. This Review gives an overview of the chemical properties of Cu-XH and -XZH motifs and discusses the pros and cons of the vastly different biological applications, and how they could be improved depending on the application.

A series of color-tunable light-emitting open-framework lanthanide sulfates containing extra-large 36-membered ring channels

A series of lanthanide sulfates contain 36-ring channels with the pore size (22.0 × 16.8 Å) based on Eu₁₀clusters.