Why Are Copper(II) Amino Acid Complexes Not Planar in Their Crystal Structures? An ab Initio and Molecular Mechanics Study

Bis(amino acidato)copper(II) compounds in blood plasma: a review of computed structural properties and amino acid affinities for Cu2+ informing further pharmacological research

Neutral bis(amino acidato)copper(II) [Cu(aa)2] coordination compounds are the physiological species of copper(II) amino acid compounds in blood plasma taking the form of bis(l-histidinato)copper(II) and mixed ternary copper(II)-l-histidine complexes, preferably with l-glutamine, l-threonine, l-asparagine, and l-cysteine. These amino acids have three functional groups that can bind metal ions: the common α-amino and carboxylate groups and a side-chain polar group. In Cu(aa)2, two coordinating groups per amino acid bind to copper(II) in-plane, while the third group can bind apically, which yields many possibilities for axial and planar bonds, that is, for bidentate and tridentate binding. So far, the experimental studies of physiological Cu(aa)2 compounds in solutions have not specified their complete geometries. This paper provides a brief review of my group's research on structural properties of physiological Cu(aa)2 calculated using the density functional theory (DFT) to locate low-energy conformers that can coexist in aqueous solutions. These DFT investigations have revealed high conformational flexibility of ternary Cu(aa)2 compounds for tridentate or bidentate chelation, which may explain copper(II) exchange reactions in the plasma and inform the development of small multifunctional copper(II)-binding drugs with several possible copper(II)-binding groups. Furthermore, our prediction of metal ion affinities for Cu2+ binding with amino-acid ligands in low-energy conformers with different coordination modes of five physiological Cu(aa)2 in aqueous solution supports the findings of their abundance in human plasma obtained with chemical speciation modelling.

Structure prediction of physiological bis(amino acidato)copper(II) species in aqueous solution: The copper(II) compounds with l-glutamine and l-histidine

Neutral (l-histidinato)(l-glutaminato)copper(II) [Cu(His)(Gln)] has been established as the most abundant ternary copper(II) amino acid compound of the exchangeable copper(II) pool in blood plasma. The experimental studies of Cu(His)(Gln) and bis(glutaminato)copper(II) [Cu(Gln)2] in solutions did not specify their complete geometries. To determine the geometries, this paper investigates the conformers, energy landscapes, and a structure-magnetic parameters relation of Cu(Gln)2 and Cu(His)(Gln) by the density functional theory (DFT) calculations. We assume a glycine-like coordination of Gln (other coordination patterns are dismissed because of steric reasons), and three His in-plane copper(II) binding modes. The conformational analyses are performed in the gas phase and implicitly modeled aqueous solution. The reliability of the DFT relative electronic and Gibbs free energies of the Cu(His)(Gln) conformers is confirmed by benchmarking against the corresponding energies obtained by the domain-based local pair natural orbital coupled-cluster method with singles, doubles, and perturbative triples [DLPNO-CCSD(T)]. Several cis- and trans-Cu(His)(Gln) conformers with His in the histaminate-like and glycine-like modes have low Gibbs free energies, and the greatest estimated metal-binding affinities. The DFT-calculated magnetic parameters of the low-energy conformers reproduce best the experimental electron paramagnetic resonance parameters measured in aqueous solutions for trans- and cis-Cu(Gln)2 conformers having two oxygen atoms (either from Gln or water molecules) at the apical positions, and Cu(His)(Gln) conformers having His in the histaminate-like mode with an apically placed carboxylato oxygen atom. The predicted conformational flexibility of His‑copper(II)-amino acid compounds may be connected with their physiological abundance, and the role in copper(II) exchange reactions in blood plasma.

Conformational Analyses of Physiological Binary and Ternary Copper(II) Complexes with l-Asparagine and l-Histidine; Study of Tridentate Binding of Copper(II) in Aqueous Solution

This study explores the structural properties and energy landscapes of the physiologically important bis(l-asparaginato)copper(II) [Cu(l-Asn)2] and (l-histidinato)(l-asparaginato)copper(II) [Cu(l-His)(l-Asn)]. The conformational analyses in the gas phase and implicitly modeled water medium, and magnetic parameters of electron paramagnetic resonance spectra were attained using density functional theory calculations. The apical CuII coordination and hydrogen bonding were analyzed. Predicted lower-energy structures enabled the confirmation and, for apical bonding, also the refinement of structural proposals from literature. Available experimental results were indecisive regarding the amido-group binding in the CuII equatorial plane in solutions, but the examination of the relative stability of Cu(l-Asn)2 conformers in 30 binding modes confirms the glycine-like mode as the most stable one. Previously reported experimental results for Cu(l-His)(l-Asn) were interpreted for l-His to have a tridentate histamine-like mode. However, the aqueous conformers with l-His in the glycinato mode are also predicted to have low energies, which does not contradict the tridentate l-His binding. The predicted magnetic parameters of conformers with an apical oxygen atom (intramolecular or from a water molecule) can reproduce the experimental data. An extent of conformational flexibility and abundance of l-His-containing ternary copper(II) amino acid complexes under physiological conditions may be related.

A new and recyclable system based on tropin ionic liquids for resolution of several racemic amino acids

A new, recyclable solid-liquid resolution system was developed based on tropin ionic liquids [C_nDTr][L-Pro]₂ for the enantiomeric resolution of racemic phenylalanine and other α-substituted carboxylic acids including tryptophan, tyrosine, benzene glycine and mandelic acid. With racemic phenylalanine as resolution model, effect factors were investigated for better resolution conditions. On the conditions, some efficient resolution were achieved, for instance the e.e. (98%) and product yield (76%) in solid phase for phenylalanine, and the e.e. 99.71% in solid phase for tryptophan. Chiral product was verified with fourier transform infrared spectroscopy (FT-IR), Raman spectrum, thermal gravity analysis (TG), elemental analysis (EA) and chiral HPLC. Further, the resolution mechanism was studied with computer molecular dynamic simulations and UV-vis. The resolution was closely related to the formation of complexes (L-Phe-Cu(Ⅱ)-L-pro^-) and the spatial configuration of D/L-Phe. The system is characteristic of high resolution, no organic solvent, easy isolation of solid-liquid and recycle of all chemical materials as much as possible.

High resolution of racemic phenylalanine with dication imidazolium-based chiral ionic liquids in a solid-liquid two-phase system

A novel solid-liquid two-phase system was developed for the chiral separation of racemic phenylalanine with new dication imidazolium-based chiral ionic liquids. Preliminary experiments showed distinct enantioselectivity in amino acid extraction with the novel solid-liquid two-phase system, more L-enantiomer of amino acid cooperatively interacted with ionic liquids and copper ions to be the solid phase. Various factors, including the alkyl chain length of cations of ionic liquids, the amount of copper acetate, the ratio of n_(ILs)/n_(Cu²⁺), the amount of water and racemic phenylalanine, the resolution time together with the resolution temperature, were systematically investigated for their influence on resolution efficiency. The results showed that, under a certain condition, the enantiomeric excess value and the yield of phenylalanine in liquid phase (mainly containing D-enantiomer) were 67.8% and 96.5%, the enantiomeric excess value and the yield of phenylalanine in solid phase (mainly containing L-enantiomer) were 99.2% and 85.2%. Finally, 2D NMR technology, infrared spectroscopy and molecular simulation method were used to study the interaction mechanism. The results indicated that L-enantiomer of phenylalanine interacts more strongly with chiral ILs and Cu²⁺. The novel system has characteristics of free-organic solvent, simple operation, fast separation process and very high resolution efficiency for racemic phenylalanine. This work could provide a new and alternative resolution approach for other chiral separations.

Fast differentiation of meats from fifteen animal species by liquid chromatography with electrochemical detection using copper nanoparticle plated electrodes

A simple, rapid and reliable method based on high-performance liquid chromatography with electrochemical detection was developed to routinely differentiate among meat products from fifteen food animal species. Samples from cattle, pigs, goats, deer, horses, chickens, ducks, ostriches, salmon, cod, shrimp, crabs, scallops, bullfrogs and alligators each exhibited unique electrochemical profiles. Species-specific markers exhibited reproducible peak retention times with coefficients of variation less then 6% across different runs, body regions and subjects. The method requires no derivatization or extraction steps and may be applicable to fresh or cooked meats. Incubation of fresh beef, pork or chicken at room temperature for 24h or repeated freezing and thawing changed the intensity but not the pattern of species-specific peaks. In conclusion, this method appears suitable for rapid differentiation of meats from various food animal species and demonstrates the utility of electrochemical detection to supplement existing immunochemical and molecular biological methods. The possibility of using this method to detect adulteration and degradative changes of meat proteins is discussed.

Chemical Exchange Reaction of Glycinatocopper(II) Complex in Water: A Theoretical Study

The axial water exchange on glycinatocopper(II) complexes was theoretically investigated by using density functional theory (DFT) calculations. Glycinatocopper(II) complexes are well-known by the diffusion controlled exchange of axial ligands. Calculations using explicitly coordinating water molecules and solvent models showed that bis-glycinatocopper(II) complexes have a four-coordinate planar structure, in which waters are excluded from the axial positions of Cu(II) due to the Jahn-Teller effect. This may be because coordinating axial waters induce the discrepancy in the most stable ligand field splittings of inner 3d and outer 4d orbitals of the Cu(II) cation. To estimate the reactivity of the axial water exchange, we calculated the rate constant by calculating Gibbs free energies for the activation. As a result, we obtained the rate constant as k = 3.61 x 10(10) s(-1) in aqueous solution at T = 298.15 K. This rate constant is slightly larger than that of the diffusion controlled exchange of axial waters, which is experimentally observed in the order of 10(9) s(-1). Finally, we determined the structures of tris-glycinatocopper(II) complexes. It was consequently found that the third glycine is coordinated to Cu with the amino groups as experimentally observed.

Theoretical model of the aqua-copper [Cu(H2O)5]+cation interactions with guanine

Pentaaqua complexes of Cu(I) with guanine were optimized at the DFT B3PW91/6-31G(d) level. For the most stable structures, vibration frequencies and NBO charges were computed followed by energy analyses. The order of individual conformers was very sensitive to the method and basis sets used for the calculation. Several conformers are practically degenerated in energy. The inclusion of an entropy term changes the order of the conformers' stability. Water molecules associated at the N9 position of guanine are favored by the inclusion of the entropy correction. Bonding energies of Cu-O(aqua) interactions were estimated to be about 60 kcal mol(-1) and for Cu-N7 bonding in the range of 75-83 kcal mol(-1). The broad range in Cu-N interaction energies demonstrates the role of induction effects caused by water molecules associated at the various sites of guanine. The charge distribution of the guanine molecule is changed remarkably by the coordination of a Cu(I) cation, which can also change the base-pairing pattern of the guanine.

Is the Enthalpy of Fusion of Tris(acetylacetonato)metal(III) Complexes Affected by Their Potential Energy in the Crystal State?

In this Article we present enthalpies of fusion and melting points obtained from new thermochemical measurements of tris(acetylacetonato)metal(III), M(acac)(3), complexes (M = Fe, Al, Cr, Mn, Co) using differential scanning calorimetry (DSC) and evaluate them in relation to their different values found in the literature. An enthalpy of fusion of 27.67 kJ mol(-)(1) was derived for Mn(acac)(3) from a symmetrical DSC thermogram captured for the first time. The enthalpy value was indirectly confirmed with the solubility measurements of Mn(acac)(3) in acetylacetone. A hypothesis has been stated that the enthalpy of fusion and the potential energy of M(acac)(3) in the crystal state may be related. To calculate molecular in-crystal potential energy, in this Article we proposed a molecular mechanics model for the M(acac)(3) class of compounds. Nine X-ray crystal structures of M(acac)(3) complexes (M = Fe, Al, V, Mn, Co, Cr, Sc) were included in the modeling. The conformational potential energy was minimized for a molecule surrounded by other molecules in the crystal lattice. The partial charges from two schemes, the electrostatic potential (ESP) fit and the natural population analysis (NPA), were used to construct two types of force fields to examine which force field type would yield a better fit with the experimental thermal properties. The final force fields were named FF-ESP and FF-NPA. Both force field sets reproduced well the experimental crystal data of nine M(acac)(3) complexes as well as of tris(3-methyl-2,4-pentanedionato-O,O')cobalt(III). Only in-crystal potential energies derived by FF-NPA yielded a significant correlation (correlation coefficient R = -0.71) with the measured enthalpies of fusion. The enthalpy of fusion for Co(acac)(3) could not be determined experimentally because of simultaneous decomposition and fusion, and it is predicted to be 33.2 kJ mol(-)(1) from the correlation regression line.

Metal complexation by the peptide-bound maillard reaction products N(epsilon)-fructoselysine and N(epsilon)-carboxymethyllysine

Although the Maillard reaction between proteins and carbohydrates is of central importance for food processing and in vivo processes, only little is known about changes of the metal-binding properties induced by protein glycation. The purpose of this study was to examine the complex formation of the quantitatively important peptide-bound Maillard reaction products (MRPs) N(epsilon)-fructoselysine and N(epsilon)-carboxymethyllysine with the biologically relevant metal ions copper(II) and zinc(II). The MRPs were synthesized as the N(alpha)-hippuryllysine derivatives in order to block the coordination function of the alpha-amino group. Stability constant measurements were performed in aqueous solution using pH potentiometry. N(alpha)-Hippuryl-N(epsilon)-fructoselysine forms moderate Cu(II) complexes (Log(10) K(1) = 5.8; Log(10) K(2) = 4.0) but fails to form any complexes with Zn(II). N(alpha)-Hippuryl-N(epsilon)-carboxymethyllysine gives slightly stronger complexes with Cu(II) (Log(10) K(1) = 7.3; Log(10) K(2) = 6.3), but again no complexation with Zn(II) was observed. These results show that post-translational modification of proteins by carbohydrates leads to the formation of new coordination centers for metal ions within a protein chain. Further studies are necessary to clarify the consequences of this phenomenon in terms of protein quality and physiological processes.

Crystal environments probed by EPR spectroscopy. Variations in the EPR spectra of Co(II)(octaethylporphyrin) doped in crystalline diamagnetic hosts and a reassessment of the electronic structure of four-coordinate cobalt(II)

The powder and single-crystal EPR spectra of Co(II)(OEP) (OEP is the dianion of octaethylporphyrin) doped into a range of diamagnetic crystals including simple four-coordinate hosts, H(2)(OEP), the triclinic B form of Ni(II)(OEP), the tetragonal form of Ni(II)(OEP) and Zn(II)(OEP); five-coordinate hosts, micro-dioxane)[Zn(II)(OEP)](2) and (py)Zn(II)(OEP); six-coordinate hosts, (py)(2)Zn(II)(OEP) and (py)(2)Mg(II)(OEP); and hosts containing fullerenes, C(60).2Zn(II)(OEP).CHCl(3), C(70).Ni(II)(OEP).C(6)H(6).CHCl(3), and C(60).Ni(II)(OEP).2C(6)H(6) have been obtained and analyzed. Spectra were simulated using a program that employed the exact diagonalization of the 16 x 16 complex spin Hamiltonian matrix. The EPR spectra of these doped samples are very sensitive to the environment within each crystal with the crystallographic site symmetry determining whether axial or rhombic resonance patterns are observed. For Co(II)(OEP) doped into tetragonal Ni(II)(OEP) (which displays a very large g( perpendicular ) of 3.405 and a very small g( parallel ) of 1.544) and several other crystals containing four-coordinate metal sites, the g components could not be fit using existing theory with the assumption of the usual z(2) ground state. However, reasonable agreement of the observed EPR parameters could be obtained by assuming that the unpaired electron resides in an xy orbital in the four-coordinate complexes.

Copper(II) diamino acid complexes: quantum chemical computations regarding diastereomeric effects on the energy of complexation

[reaction: see text] Quantum chemical calculations were used to rationalize the observed enantiodifferentiation in the complexation of alpha-amino acids to chiral Cu(II) complexes. Apart from Cu(II)[bond]pi interactions and steric repulsions between the anchoring cholesteryl-Glu moiety and an aromatic amino acid R group, hydrogen bonding also plays a role. In fact, in the case of tryptophan, C[double bond]O...H[bond]N hydrogen bonding between the glutamate moiety and the tryptophan N[bond]H group compensates for the loss of intramolecular hydrogen-bonding and diminished Cu(II)[bond]pi interactions.

Modeling anhydrous and aqua copper(II) amino acid complexes: a new molecular mechanics force field parametrization based on quantum chemical studies and experimental crystal data

This paper presents the vacuum structures of aquacopper(II) bis(amino acid) complexes with glycine, sarcosine, N,N-dimethylglycine, and N-tert-butyl-N-methylglycine estimated using the B3LYP method. The differences between the B3LYP vacuum structures and experimental crystal structures suggested considerable influence of crystal lattice packing effects on the changes in the complexes' geometries. A previously developed molecular mechanics force field for modeling anhydrous copper(II) amino acidates was reoptimized to simulate these changes and predict the properties of both trans and cis anhydrous and aqua copper(II) amino acid complexes. The modeling included experimental molecular and crystal structures of 13 anhydrous and 10 aqua copper(II) amino acidates with the same atom types (Cu(II), C, H, N, and O) but various copper(II) coordination polyhedron geometries, crystal symmetries, and intermolecular interactions. The empirical parameters of the selected potential energy functions were optimized on the B3LYP vacuum copper(II) coordination geometries of three anhydrous copper(II) amino acidates and on experimental crystalline internal coordinates and unit cell dimensions of six anhydrous and six aqua copper(II) amino acid complexes. The respective equilibrium structures were calculated in vacuo and in simulated crystalline environment. The efficacy of the final force field, FFW, was examined. The total root-mean-square deviations between the experimental and theoretical crystal values were 0.018 A in the bond lengths, 2.2 degrees in the valence angles, 5.5 degrees in the torsion angles, and 0.395 A in the unit cell lengths. FFW reproduced the unit cell volumes in the range from -8.1 to 9.6%. The means of Cu to axial water oxygen distances were 2.4 +/- 0.1 A (experiment) and 2.6 +/- 0.1 A (FFW). This paper describes the ability of the molecular mechanics model and FFW force field to simulate the flexibility of the metal coordination polyhedron. The new force field proved effective in predicting the most stable molecular conformation of copper(II) amino acidato systems in vacuo.