Effect of the Peptidic Scaffold in Copper(II) Coordination and the Redox Properties of Short Histidine‐Containing Peptides

A linear decapeptide containing three His and one Asp residues and a β‐turn‐inducing dProPro unit was synthesised. A detailed potentiometric, mass spectrometric and spectroscopic study showed that at a 1:1 ratio of C_Cu/C_peptide this peptide formed a major [CuH(O_dPro?Asp)]²⁺ species (pH range 5.5–7.0), in which the Cu²⁺ ion was bound to the His and Asp residues in square‐planar or square‐pyramidal geometries. The stability constant corrected for protonated species (log K*=9.33) is almost equal to the value obtained for the parent [CuH(O?Asp)]²⁺ species (log K*_CuH(O‐Asp)=9.28), but lower than that obtained for the cyclic [CuH(C?Asp)]²⁺ complex (log K*_CuH(C‐Asp)=10.79) previously published. Thus, the replacement of the ProGly unit by the stronger β‐turn‐inducing dProPro unit did not generate a more stable copper(II) species, although the O_dPro?Asp peptide was structured in solution, as shown by circular dichroism (CD) spectroscopy. Interestingly, the calculated value of K_eff showed that this peptide behaved similarly to the O?Asp or C?Asp counterparts, depending on the pH value. The cyclic voltammetry data indicated that the most easily reducible species were [CuH(O?Asp)]²⁺ (E′⁰=262 mV versus a normal hydrogen electrode (NHE)) and [CuH(O_dPro?Asp)]²⁺ (E′⁰=294 mV versus NHE) complexes, the peptidic scaffolds of which are open. A lower value was obtained for [CuH(C?Asp)]²⁺ (E′⁰=24 mV versus NHE). A different degree of non‐reversibility was observed for the three copper(II) complexes; this could reflect a different degree of flexibility in their respective peptidic scaffolds.     

Neutral Penta‐ and Hexacoordinate Silicon(IV) Complexes Containing Two Bidentate Ligands Derived from the α‐Amino Acids (S)‐Alanine, (S)‐Phenylalanine,and (S)‐tert‐Leucine

The neutral hexacoordinate silicon(IV) complex 6 (SiO₂N₄ skeleton) and the neutral pentacoordinate silicon(IV) complexes 7 – 11 (SiO₂N₂C skeletons) were synthesized from Si(NCO)₄ and RSi(NCO)₃ (R=Me, Ph), respectively. The compounds were structurally characterized by solid‐state NMR spectroscopy ( 6 – 11 ), solution NMR spectroscopy ( 6 and 10 ), and single‐crystal X‐ray diffraction ( 8 and 11 were studied as the solvates 8? CH₃CN and 11? C₅H₁₂ ? 0.5 CH₃CN, respectively). The silicon(IV) complexes 6 (octahedral Si‐coordination polyhedron) and 7 – 11 (trigonal‐bipyramidal Si‐coordination polyhedra) each contain two bidentate ligands derived from an α‐amino acid: (S)‐alanine, (S)‐phenylalanine, or (S)‐tert‐leucine. The deprotonated amino acids act as monoanionic ( 6 ) or as mono‐ and dianionic ligands ( 7 – 11 ). The experimental investigations were complemented by computational studies of the stereoisomers of 6 and 7 .     

Copper(II) coordination chemistry of westiellamide and its imidazole, oxazole, and thiazole analogues

The copper(II) coordination chemistry of westiellamide (H(3)L(wa)), as well as of three synthetic analogues with an [18]azacrown-6 macrocyclic structure but with three imidazole (H(3)L(1)), oxazole (H(3)L(2)), and thiazole (H(3)L(3)) rings instead of oxazoline, is reported. As in the larger patellamide rings, the N(heterocycle)-N(peptide)-N(heterocycle) binding site is highly preorganized for copper(II) coordination. In contrast to earlier reports, the macrocyclic peptides have been found to form stable mono- and dinuclear copper(II) complexes. The coordination of copper(II) has been monitored by high-resolution electrospray mass spectrometry (ESI-MS), spectrophotometric and polarimetric titrations, and EPR and IR spectroscopies, and the structural assignments have been supported by time-dependent studies (UV/Vis/NIR, ESI-MS, and EPR) of the complexation reaction of copper(II) with H(3)L(1). Density functional theory (DFT) calculations have been used to model the structures of the copper(II) complexes on the basis of their spectroscopic data. The copper(II) ion has a distorted square-pyramidal geometry with one or two coordinated solvent molecules (CH(3)OH) in the mononuclear copper(II) cyclic peptide complexes, but the coordination sphere in [Cu(H(2)L(wa))(OHCH(3))](+) differs from those in the synthetic analogues, [Cu(H(2)L)(OHCH(3))(2)](+) (L = L(1), L(2), L(3)). Dinuclear copper(II) complexes ([Cu(II) (2)(HL)(mu-X)](+); X = OCH(3), OH; L = L(1), L(2), L(3), L(wa)) are observed in the mass spectra. While a dipole-dipole coupled EPR spectrum is observed for the dinuclear copper(II) complex of H(3)L(3), the corresponding complexes with H(3)L (L = L(1), L(2), L(wa)) are EPR-silent. This may be explained in terms of strong antiferromagnetic coupling (H(3)L(1)) and/or a low concentration of the dicopper(II) complexes (H(3)L(wa), H(3)L(2)), in agreement with the mass spectrometric observations.     

Synthesis and Conformation of Fluorinated β‐Peptidic Compounds

Experimental and theoretical data indicate that, for α‐fluoroamides, the F? C? C(O)? N(H) moiety adopts an antiperiplanar conformation. In addition, a gauche conformation is favoured between the vicinal C? F and C? N(CO) bonds in N‐β‐fluoroethylamides. This study details the synthesis of a series of fluorinated β‐peptides ( 1 – 8 ) designed to use these stereoelectronic effects to control the conformation of β‐peptide bonds. X‐ray crystal structures of these compounds revealed the expected conformations: with fluorine β to a nitrogen adopting a gauche conformation, and fluorine α to a C?O group adopting an antiperiplanar conformation. Thus, the strategic placement of fluorine can control the conformation of a β‐peptide bond, with the possibility of directing the secondary structures of β‐peptides.     

A Crystalline Copper(II) Coordination Polymer for the Efficient Visible‐Light‐Driven Generation of Hydrogen

A crystalline coordination polymer (CP) photocatalyst (Cu‐RSH) which combines redox‐active copper centers with photoactive rhodamine‐derived ligands remains stable in acid and basic solutions from pH 2 to 14, and efficiently catalyzes dihydrogen evolution at a maximum rate of 7.88 mmol g^?1 h^?1 in the absence of a mediator and a co‐catalyst. Cyclic voltammetry, control experiments, and DFT calculations established that copper nodes with open coordination sites and favorable redox potentials, aided by spatially ordered stacking of rhodamine‐based linkers, account for the high catalytic performance of Cu‐RSH. Emission quenching, time‐resolved fluorescence decay, and transient photocurrent experiments disclosed the charge separation and transfer process in the catalytic system. The present study demonstrates the potential of crystalline copper CPs for the practical utilization of light.     

One Dimensional Coordination Polymer Constructed by1,2—Bis（4—pyridinecarboxamido）ethane and Copper（Ⅱ）

The ligand [1,2‐bis(4‐pyridinecarboxamido)ethane] (L) and the coordination polymer |[Cu(L)₂(H₂O)]‐(NO₃)₂·6H₂O|·(1) haw been synthesized and characterized by ER and ¹H NMR spectra. Their molecular structures and the packing of 1 have been determined by single‐crystal X‐ray diffraction analysis. The Cu(n) in 1 is bridged by two ligands forming an infinite one‐dimensional chain like structure and L in 1 adopts a different conformation from its free state. 1 belongs to monoclinic, space group P2₁/n, a = 1.2896(3) nm, b = 1.2552(8) nm, c = 2.2903(19) nm, β = 93.04(5)°, Z = 4, V = 3.702(4) nm³. The TG and DTG experiments showed that the uncoordinated H₂O can be removed at low temperature by heating, and it does not decompose until 250 °C.     

Metal‐Catalyzed Derivatization of Cα‐Tetrasubstituted Amino Acids and Their Use in the Synthesis of Cyclic Peptides

Making circles with N and O : Cyclic tripeptides containing an unnatural C^α‐tetrasubstituted THF amino acid are prepared by copper(I) and palladium(0)‐catalyzed N‐ and O‐arylation reactions. The reactions give access to side chain‐modified derivatives of the unnatural amino acid and macrocyclic peptidomimetics.