Formation Constants of Copper(II) Complexes with Tripeptides Containing Glu, Gly, and His: Potentiometric Measurements and Modeling by Generalized Multiplicative Analysis of Variance

Quantification of underivatized amino acids in solid beverages using high-performance liquid chromatography and a potentiometric detector

The simultaneous quantification of amino acids (AAs) in solid beverages without prior derivatization was explored by high-performance liquid chromatography (HPLC) coupled to a potentiometric detector. Included were threonine, leucine, methionine, phenylalanine, and histidine. The potentiometric detector was made consisting of a copper(II)-selective electrode based on a polyvinyl chloride (PVC) membrane, and the potential changes in the detector were determined according to the coordination interactions between cupric copper ions released from the inner filling solution of the electrode and AAs. Conditions were optimized for effective separation and sensitive detection. Fundamental characteristics such as linearity, limits of detection, limits of quantitation, accuracy, precision, and robustness were validated experimentally. The calibration curves showed a linear relationship between peak heights and the injection concentrations of the AAs. The detection limits down to the sub-micromolar range were achieved under isocratic conditions, outperforming ultraviolet detection. The copper(II)-selective electrode had a minimum lifetime of one month. Some real samples were examined to further demonstrate the feasibility of the proposed approach. The measurement results obtained by the present method were in good agreement with those obtained by the HPLC-mass spectrometry (MS), indicating that the combined HPLC-potentiometric method is a potential option for quantifying AAs.

Impact of the Histidine-Triazole and Tryptophan-Pyrene Exchange in the WHW Peptide: Cu(II) Binding, DNA/RNA Interactions and Bioactivity

In three novel peptidoids based on the tryptophan—histidine—tryptophan (WHW) peptide, the central histidine was replaced by Ala-(triazole), and two derivatives also had one tryptophan replaced with pyrene-alkyls of different lengths and flexibility. Pyrene analogues show strong fluorescence at 480–500 nm, attributed to intramolecular exciplex formation with tryptophan. All three peptidoids bind Cu2+ cation in water with strong affinity, with Trp- Ala-(triazole)-Trp binding comparably to the parent WHW, and the pyrene analogues even stronger, demonstrating that replacement of histidine with triazole in peptides does not hamper Cu2+ coordination. The studied peptidoids strongly bind to ds-DNA and ds-RNA, whereby their complexes with Cu2+ exhibit distinctively different interactions in comparison to metal-free analogues, particularly in the stabilization of ds-DNA against thermal denaturation. The pyrene peptidoids efficiently enter living cells with no apparent cytotoxic effect, whereby their red-shifted emission compared to the parent pyrene allows intracellular confocal microscopy imaging, showing accumulation in cytoplasmic organelles. However, irradiation with 350 nm light resulted in evident antiproliferative effect on cells treated with micromolar concentrations of the pyrene analogues, presumably attributed to pyrene-induced production of singlet oxygen and consecutive cellular damage.

Fluorescent Analogues of FRH Peptide: Cu(II) Binding and Interactions with ds-DNA/RNA

Four novel peptidoids, derived from the Phe-Arg-His (FRH) peptide motif, were prepared by replacing the histidine heterocycle with triazole and consequent triazole-fluorophore (coumarin) extension and also replacing arginine with less voluminous lysine. So the constructed Phe-Lys-Ala(triazole) (FKA(triazole)) peptidoids bind Cu2+ cations in water with a strong, nanomolar affinity comparable to the parent FRH and its known analogs, demonstrating that triazole can coordinate copper similarly as histidine. Moreover, even short KA(triazole)coumarin showed submicromolar affinity to Cu2+. Only FKA(triazole)coumarin with free amino groups and its shorter analog KA(triazole)coumarin showed strong induced CD spectra upon Cu2+ cation binding. Thus, KA(triazole)coumarin can be considered as the shortest peptidoid sequence with highly sensitive fluorescent and chiral CD response for Cu2+ cation, encouraging further studies with other metal cations. The FKA(triazole) coumarin peptidoids show biorelevant, 10 µM affinity to ds-DNA and ds-RNA, binding within DNA/RNA grooves. Intriguingly, only peptidoid complexes with Cu2+ strongly stabilize ds-DNA and ds-RNA against thermal denaturation, suggesting significant interactions of Cu2+ cation within the DNA/RNA binding site.

Coordination of Redox Ions within a Membrane-Binding Peptide: A Tale of Aromatic Rings

The amino-terminal-copper-and-nickel-binding (ATCUN) motif, a tripeptide sequence ending with a histidine, confers important functions to proteins and peptides. Few high-resolution studies have been performed on the ATCUN motifs of membrane-associated proteins and peptides, limiting our understanding of how they stabilize Cu²⁺/Ni²⁺ in membranes. Here, we leverage solid-state NMR to investigate metal-binding to piscidin-1 (P1), a host-defense peptide featuring F1F2H3 as its ATCUN motif. Bound to redox ions, P1 chemically and physically damages pathogenic cell membranes. We design ¹³C/¹⁵N correlation experiments to detect and assign the deprotonated nitrogens produced and/or shifted by Ni²⁺-binding. Occupying multiple chemical states in P1-apo, H3 and the neighboring H4 respond to metalation by populating only the τ-tautomer. H3, as a proximal histidine, directly coordinates the metal, compared to the distal H4. Density functional theory calculations reflect this noncanonical arrangement and point toward cation-π interactions between the F1/F2/H4 aromatic rings and metal. These structural findings, which are relevant to other ATCUN-containing membrane peptides, could help design new therapeutics and materials for use in the areas of drug-resistant bacteria, neurological disorders, and biomedical imaging.

The copper(II)-binding tripeptide GHK, a valuable crystallization and phasing tag for macromolecular crystallography

The growth of diffraction-quality crystals and experimental phasing remain two of the main bottlenecks in protein crystallography. Here, the high-affinity copper(II)-binding tripeptide GHK was fused to the N-terminus of a GFP variant and an MBP-FG peptide fusion. The GHK tag promoted crystallization, with various residues (His, Asp, His/Pro) from symmetry molecules completing the copper(II) square-pyramidal coordination sphere. Rapid structure determination by copper SAD phasing could be achieved, even at a very low Bijvoet ratio or after significant radiation damage. When collecting highly redundant data at a wavelength close to the copper absorption edge, residual S-atom positions could also be located in log-likelihood-gradient maps and used to improve the phases. The GHK copper SAD method provides a convenient way of both crystallizing and phasing macromolecular structures, and will complement the current trend towards native sulfur SAD and MR-SAD phasing.

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.

Gas-phase metal ion chelation investigated with IRMPD spectroscopy: A brief review of Robert Dunbar's contributions

With the passing of Prof. Robert C. Dunbar on 31 October 2017, the field of ion chemistry lost one of its modern heroes. Throughout his career in mass spectrometry, two of his main research interests involved the interaction of trapped ions with electromagnetic radiation and the chelation motifs of metal ions with organic ligands. The focus of his early career was on the fundamental processes that take place in molecules upon ultraviolet and infrared excitation. From 2003 to 2017, his scientific interests shifted to more structural questions, notably to resolving the structures and binding motifs of metal ion chelation complexes by application of infrared photodissociation spectroscopy. These experiments were carried out during numerous visits to the (Free Electron Laser for Infrared eXperiments) (FELIX) facility in the Netherlands and were complemented by extensive theoretical investigations by Rob. As a tribute to our friend, we present in this contribution a brief review of this work.

Copper ion interaction with the RNase catalytic site fragment of the angiogenin protein: an experimental and theoretical investigation

The angiogenin protein (Ang) is a member of the vertebrate-specific secreted ribonucleases and one of the most potent angiogenic factors known. Ang is a normal constituent of human plasma and its concentration increases under some physiological and pathological conditions to promote neovascularization. Ang was originally identified as an angiogenic tumour factor, but its biological activity has been found to extend from inducing angiogenesis to promoting cell survival in different neurodegenerative diseases. Ang exhibits weak ribonucleolytic activity, which is critical for its biological functions. The RNase catalytic sites are two histidine residues, His-13 and His-114, and the lysine Lys-40. Copper is also an essential cofactor in angiogenesis and influences angiogenin's biological properties. The main Cu(ii) anchoring site of Ang is His-114, where metal binding inhibits RNase activity of the protein. To reveal the Cu(ii) coordination environment in the C-terminal domain of the Ang protein, we report on the characterization, by means of potentiometric, voltammetric, and spectroscopic (CD, UV-Vis and EPR) methods and DFT calculations, of Cu(ii) complexes formed with a peptide fragment including the Ang sequence 112-117 (PVHLDQ). Potentiometric titrations indicated that [CuLH_-2] is the predominant species at physiological pH. EPR, voltammetric data and DFT calculations are consistent with a CuN₃O₂ coordination mode in which a distorted square pyramidal arrangement of the peptide was observed with the equatorial positions occupied by the nitrogen atoms of the deprotonated amides of the Asp and Leu residues, the δ-N atom of histidine and the oxygen atom of the aspartic carboxylic group. Moreover, two analogous peptides encompassing the PVHLNQ and LVHLDQ sequences were also characterized by using thermodynamic, spectroscopic and DFT studies to reveal the role they play in Cu(ii) complex formation by the carboxylate side chain of the Asp and Pro residues, a known breaking-point in metal coordination.

Binding of Divalent Metal Ions with Deprotonated Peptides: Do Gas-Phase Anions Parallel the Condensed Phase?

Chelation complexes of the histidine-containing tripeptides HisAlaAla, AlaHisAla, and AlaAlaHis with Ni(II) and Cu(II) having a −1 net charge are characterized in the gas phase by infrared multiple-photon dissociation (IRMPD) spectroscopy and density functional theory calculations. We address the question of whether the gas-phase complexes carry over characteristics from the corresponding condensed-phase species. We focus particularly on three aspects of their structure: (i) square-planar chelation by the deprotonated amide nitrogens around the metal ion (low-spin for the Ni case), (ii) metal-ion coordination of the imidazole side chain nitrogen, and (iii) the exceptional preference for metal-ion chelation by peptides with His in the third position from the N-terminus, as in the amino terminal Cu and Ni (ATCUN) motif. We find that square-planar binding around the metal ion, involving bonds to both deprotonated backbone nitrogens, one of the carboxylate oxygens and the N-terminal nitrogen, is the dominant binding motif for all three isomers. In contrast to the condensed-phase behavior, the dominant mode of binding for all three isomers does not involve the imidazole side chain, which is instead placed outside the coordination zone. Only for the AlaAlaHis isomer, the imidazole-bound structure is also detected as a minority population, as identified from a distinctive short-wavelength IR absorption. The observation that this conformation exists only for AlaAlaHis correlates with condensed-phase behavior at neutral-to-basic pH, in the sense that the isomer with His in the third position is exceptionally disposed to metal ion chelation by four nitrogen atoms (4N) when compared with the other isomers. These results also emphasize the divergence between the conformational stabilities in the gas phase and in solution or crystalline environments: in the gas phase, direct metal binding of the imidazole is overall less favorable than the alternative of a remote imidazole that can act as an intramolecular H-bond donor enhancing the gas-phase stability.