Exploring the Esterase Catalytic Activity of Minimalist Heptapeptide Amyloid Fibers.

This paper investigates the esterase activity of minimalist amyloid fibers composed of short seven-residue peptides, IHIHIHI (IH7) and IHIHIQI (IH7Q), with a particular focus on the role of the sixth residue position within the peptide sequence. Through computational simulations and analyses, we explore the molecular mechanisms underlying catalysis in these amyloid-based enzymes. Contrary to initial hypotheses, our study reveals that the twist angle of the fiber, and thus the catalytic site's environment, is not notably affected by the sixth residue. Instead, the sixth residue interacts with the p-nitrophenylacetate (pNPA) substrate, particularly through its -NO2 group, potentially enhancing catalysis. Quantum mechanics/molecular mechanics (QM/MM) simulations of the reaction mechanism suggest that the polarizing effect of glutamine enhances catalytic activity by forming a stabilizing network of hydrogen bonds with pNPA, leading to lower energy barriers and a more exergonic reaction. Our findings provide valuable insights into the intricate interplay between peptide sequence, structural arrangement, and catalytic function in amyloid-based enzymes, offering potentially valuable information for the design and optimization of biomimetic catalysts.

Enhanced Metallophilicity in Metal-Carbene Systems: Stronger Character of Aurophilic Interactions in Solution.

Metallophilicity is an essential concept that builds upon the attraction between closed shell metal ions. We report on the [M2 (bisNHC)2 ]2+ (M=AuI , AgI ; NHC=N-heterocyclic carbene) systems, which display almost identical features in the solid state. However, in solution the Au2 cation exhibits a significantly higher degree of rigidity owed to the stronger character of the aurophilic interactions. Both Au2 and Ag2 cationic constructs are able to accommodate Ag+ ions via M-M interactions, despite their inherent Coulombic repulsion. When electrostatic repulsion between host and guest is partially diminished, M-M distances are substantially shortened. Quantum chemical calculations estimate intermetallic bond orders up to 0.2. Although at the limit of (or beyond) the van der Waals radii, metallophilic interactions are responsible for their behavior in solution.

Drastic Effect of the Peptide Sequence on the Copper-Binding Properties of Tripeptides and the Electrochemical Behaviour of Their Copper(II) Complexes.

The binding and electrochemical properties of the complexes CuII -HAH, CuII -HWH, CuII -Ac-HWH, CuII -HHW, and CuII -WHH have been studied by using NMR and UV/Vis spectroscopies, CV, and density functional calculations. The results obtained highlight the importance of the peptidic sequence on the coordination properties and, consequently, on the redox properties of their CuII complexes. For CuII -HAH and CuII -HWH, no cathodic processes are observed up to -1.2 V; that is, the complexes exhibit very high stability towards copper reduction. This behaviour is associated with the formation of very stable square-planar (5,5,6)-membered chelate rings (ATCUN motif), which enclose two deprotonated amides. In contrast, for non-ATCUN CuII -Ac-HWH, CuII -HHW complexes, simulations seem to indicate that only one deprotonated amide is enclosed in the coordination sphere. In these cases, the main electrochemical feature is a reductive irreversible one electron-transfer process from CuII to CuI , accompanied with structural changes of the metal coordination sphere and reprotonation of the amide. Finally, for CuII -WHH, two major species have been detected: one at low pH (<5), with no deprotonated amides, and another one at high pH (>10) with an ATCUN motif, both species coexisting at intermediate pH. The present study shows that the use of CV, using glassy carbon as a working electrode, is an ideal and rapid tool for the determination of the redox properties of CuII metallopeptides.

Intramolecular Photocycloaddition of 2(5 H)-Furanones to Temporarily Tethered Terminal Alkenes as a Stereoselective Source of Enantiomerically Pure Polyfunctionalyzed Cyclobutanes.

Allyloxymethyloxymethyl and 4-pentenoyloxymethyl substituents have been used as tethering groups to study the intramolecular [2 + 2] photocycloaddition of chiral 5-substituted 2(5 H)-furanones. The photoreactions proceed in good yield and provide the expected regio- and diastereoselective tricyclic compounds with complementary regioselectivity, which depends on whether the vinyl chain is attached to the furanone by an acetal or an ester linkage. Computational simulations agree with experimental observations and indicate that the origin of the different observed regioselectivity in the intramolecular photochemical reaction of lactones 5 and 6 arises from the relative stability of the initial conformers. The synthetic potential of the enantiomerically pure photoadducts is illustrated by preparing an all- cis 1,2,3-trisubstituted cyclobutane bearing fully orthogonally protected hydroxyl groups.

Multiple Condensation Reactions Involving Pt(II) /Pd(II) -OH2 , Pt-NH3 , and Cytosine-NH2 Groups: New Twists in Cisplatin-Nucleobase Chemistry.

The coordination chemistry of the antitumor agent cisplatin and related complexes with DNA and its constituents, that is, the nucleobases, appears to be dominated by 1:1 and 1:2 adducts of the types cis-[Pta2 (nucleobase)X] and cis-[Pta2 (nucleobase)2 ] (a=NH3 or amine; a2 =diamine or diimine; X=Cl, OH or OH2 ). Here, we have studied the interactions of the putative 1:1 adducts cis-[Pta2 (1-MeC-N3)(OH2 )](2+) (with a=NH3 , a2 =2,2'-bpy (2,2'-bipyridine), 1-MeC=model nucleobase 1-methylcytosine) with additional cis-[Pt(NH3 )2 (OH2 )2 ](2+) or its kinetically superior analogues [Pd(en)(OH2 )2 ](2+) (en=ethylenediamine) and [Pd(2,2'-bpy)(OH2 )2 ](2+) . Depending upon the conditions applied different compounds of different nuclearity are formed. Without exception they represent condensation products of the components, containing µ-1-MeC-H , µ-OH(-) , as well as µ-NH2 (-) bridges. In the presence of Ag(+) ions, the isolated products in several cases display additionally Pt→Ag dative bonds. On the basis of the cytosine-containing structures established by X-ray crystallography, it is proposed that any of the feasible initial 1:1 nucleobase adducts of cisplatin could form dinuclear Pt complexes upon reaction with additional hydrolyzed cisplatin, thereby generating nucleobase adducts other than the presently established ones. Two findings appear to be of particular significance: First, hydrolyzed cisplatin can have a moderately accelerating effect on the formation of a secondary nucleobase product. Second, NH3 ligands of the cisplatin moiety can be converted into bridging amido ligands following condensation with the diaqua species of cisplatin.

Coordination properties of a metal chelator clioquinol to Zn(2+) studied by static DFT and ab initio molecular dynamics.

Several lines of evidence supporting the role of metal ions in amyloid aggregation, one of the hallmarks of Alzheimer's disease (AD), have turned metal ion chelation into a promising therapeutic treatment. The design of efficient chelating ligands requires proper knowledge of the electronic and molecular structure of the complexes formed, including their hydration properties. Among various potential chelators, clioquinol (5-chloro-7-iodo-8-hydroxyquinoline, CQH) has been evaluated with relative success in in vitro experiments and even in phase 2 clinical trials. Clioquinol interacts with Zn(ii) to lead to a binary metal/ligand 1 : 2 stoichiometric complex in which the phenolic group of CQH is deprotonated, resulting in Zn(CQ)2 neutral complexes, to which additional water molecules may coordinate. In the present work, the coordinative properties of clioquinol in aqueous solution have been analyzed by means of static, minimal cluster based DFT calculations and explicit solvent ab initio molecular dynamics simulations. Results from static calculations accounting for solvent effects by means of polarized continuum models suggest that the preferred metal coordination environment is tetrahedral Zn(CQ)2, whereas ab initio molecular dynamics simulations point to quasi degenerate penta Zn(CQ)2(H2O) and hexa Zn(CQ)2(H2O)2 coordinated complexes. The possible reasons for these discrepant results are discussed.

Dioxygen activation in the Cu-amyloid ß complex.

We investigate, by means of density-functional theory, the binding of dioxygen to Cu(I)-amyloid ß (Aß), one of the first steps in the oxidation of ascorbate by dioxygen. Cu, Aß, ascorbate and dioxygen are all present in the synapse during neurodegeneration, when the above species can trigger an irreversible oxidative stress inducing the eventual death of neurons. The binding of dioxygen to Cu(I) is possible and its role in dioxygen activation of Cu ligands and of residues in the first coordination sphere is described in atomic detail. Dioxygen is activated when a micro-environment suitable for a square-planar Cu(2+) coordination is present and a negatively charged group like Asp 1 carboxylate takes part in the Cu coordination anti to O2.

Mixed adenine/guanine quartets with three trans-a2 Pt(II) (a=NH(3) or MeNH(2)) cross-links: linkage and rotational isomerism, base pairing, and loss of NH(3).

Of the numerous ways in which two adenine and two guanines (N9 positions blocked in each) can be cross-linked by three linear metal moieties such as trans-a2 Pt(II) (with a=NH3 or MeNH2 ) to produce open metalated purine quartets with exclusive metal coordination through N1 and N7 sites, one linkage isomer was studied in detail. The isomer trans,trans,trans-[{Pt(NH3 )2 (N7-9-EtA-N1)2 }{Pt(MeNH2 )2 (N7-9-MeGH)}2 ][(ClO4 )6 ]⋅3H2 O (1) (with 9-EtA=9-ethyladenine and 9-MeGH=9-methylguanine) was crystallized from water and found to adopt a flat Z-shape in the solid state as far as the trinuclear cation is concerned. In the presence of excess 9-MeGH, a meander-like construct, trans,trans,trans-[{Pt(NH3 )2 (N7-9-EtA-N1)2 }{Pt(MeNH2 )2 (N7-9-MeGH)2 }][(ClO4 )6 ]⋅[(9-MeGH)2 ]⋅7 H2 O (2) is formed, in which the two extra 9-MeGH nucleobases are hydrogen bonded to the two terminal platinated guanine ligands of 1. Compound 1, and likewise the analogous complex 1 a (with NH3 ligands only), undergo loss of an ammonia ligand and formation of NH4 (+) when dissolved in [D6 ]DMSO. From the analogy between the behavior of 1 and 1 a it is concluded that a NH3 ligand from the central Pt atom is lost. Addition of 1-methylcytosine (1-MeC) to such a DMSO solution reveals coordination of 1-MeC to the central Pt. In an analogous manner, 9-MeGH can coordinate to the central Pt in [D6 ]DMSO. It is proposed that the proton responsible for formation of NH4 (+) is from one of the exocyclic amino groups of the two adenine bases, and furthermore, that this process is accompanied by a conformational change of the cation from Z-form to U-form. DFT calculations confirm the proposed mechanism and shed light on possible pathways of this process. Calculations show that rotational isomerism is not kinetically hindered and that it would preferably occur previous to the displacement of NH3 by DMSO. This displacement is the most energetically costly step, but it is compensated by the proton transfer to NH3 and formation of U(-H(+) ) species, which exhibits an intramolecular hydrogen bond between the deprotonated N6H(-) of one adenine and the N6H2 group of the other adenine. Finally the question is examined, how metal cross-linking patterns in closed metallacyclic quartets containing two adenine and two guanine nucleobases influence the overall shape (square, rectangle, trapezoid) and the planarity of a metalated purine quartet.

DFT study on the recovery of Hoveyda-grubbs-type catalyst precursors in enyne and diene ring-closing metathesis.

DFT (B3LYP-D) calculations have been used to better understand the origin of the recovered Hoveyda-Grubbs derivative catalysts after ring-closing diene or enyne metathesis reactions. For that, we have considered the activation process of five different Hoveyda-Grubbs precursors in the reaction with models of usual diene and enyne reactants as well as the potential precursor regeneration through the release/return mechanism. The results show that, regardless of the nature of the initial precursor, the activation process needs to overcome relatively high energy barriers, which is in agreement with a relatively slow process. The precursor regeneration process is in all cases exergonic and it presents low energy barriers, particularly when compared to those of the activation process. This indicates that the precursor regeneration should always be feasible, unlike the moderate recoveries sometimes observed experimentally, which suggests that other competitive processes that hinder recovery should take place. Indeed, calculations presented in this work show that the reactions between the more abundant olefinic products and the active carbenes usually require lower energy barriers than those that regenerate the initial precatalyst, which could prevent precursor regeneration. On the other hand, varying the precursor concentration with time obtained from the computed energy barriers shows that, under the reaction conditions, the precursor activation is incomplete, thereby suggesting that the origin of the recovered catalyst probably arises from incomplete precursor activation.

Enhancing magneto-ionic effects in cobalt oxide films by electrolyte engineering.

Electric-field-driven ion motion to tailor magnetic properties of materials (magneto-ionics) offers much promise in the pursuit of voltage-controlled magnetism for highly energy-efficient spintronic devices. Electrolyte gating is a relevant means to create intense electric fields at the interface between magneto-ionic materials and electrolytes through the so-called electric double layer (EDL). Here, improved magneto-ionic performance is achieved in electrolyte-gated cobalt oxide thin films with the addition of inorganic salts (potassium iodide, potassium chloride, and calcium tetrafluoroborate) to anhydrous propylene carbonate (PC) electrolyte. Ab initio molecular dynamics simulations of the EDL structure show that K+ is preferentially located on the cobalt oxide surface and KI (when compared to KCl) favors the accumulation of positive charge close to the surface. It is demonstrated that room temperature magneto-ionics in cobalt oxide thin films is dramatically enhanced in KI-containing PC electrolyte at an optimum concentration, leading to 11-fold increase of generated magnetization and 35-fold increase of magneto-ionic rate compared to bare PC.

Three dimensional models of Cu(2+)-Aß(1-16) complexes from computational approaches.

Elucidation of the coordination of metal ions to Aß is essential to understand their role in its aggregation and to rationally design new chelators with potential therapeutic applications in Alzheimer disease. Because of that, in the last 10 years several studies have focused their attention in determining the coordination properties of Cu(2+) interacting with Aß. However, more important than characterizing the first coordination sphere of the metal is the determination of the whole Cu(2+)-Aß structure. In this study, we combine homology modeling (HM) techniques with quantum mechanics based approaches (QM) to determine plausible three-dimensional models for Cu(2+)-Aß(1-16) with three histidines in their coordination sphere. We considered both ε and δ coordination of histidines 6, 13, and 14 as well as the coordination of different possible candidates containing oxygen as fourth ligand (Asp1, Glu3, Asp7, Glu11, and CO(Ala2)). Among the 32 models that enclose COO(-), the lowest energy structures correspond to [O(E3),N(δ)(H6),N(ε)(H13),N(ε)(H14)] (1), [O(E3),N(δ)(H6),N(δ)(H13),N(δ)(H14)] (2), and [O(D7),N(ε)(H6),N(δ)(H13),N(δ)(H14)] (3). The most stable model containing CO(Ala2) as fourth ligand in the Cu(2+) coordination sphere is [O(c)(A2),N(ε)(H6),N(δ)(H13),N(ε)(H14)] (4). An estimation of the relative stability between Glu3 (1) and CO(Ala2) (4) coordinated complexes seems to indicate that the preference for the latter coordination may be due to solvent effects. The present results also show the relationship between the peptidic and metallic moieties in defining the overall geometry of the complex and illustrate that the final stability of the complexes results from a balance between the metal coordination site and amyloid folding upon complexation.

Mechanistic insights into ring-closing enyne metathesis with the second-generation Grubbs-Hoveyda catalyst: a DFT study.

The full catalytic process (precatalyst activation, propagating cycle and active-species interconversion) of the ring-closing enyne metathesis (RCEYM) reaction of 1-allyloxy-2-propyne with the Grubbs-Hoveyda complex as catalyst was studied by B3LYP density functional theory. Both the ene-then-yne and yne-then-ene pathways are considered and, for the productive catalytic cycle, the feasibility of the endo-yne-then-ene route is also explored. Calculations predict that the ene-then-yne and yne-then-ene pathways proceed through equivalent steps, the only major difference being the order in which they take place. In this way, all alkene metathesis processes studied here involve four steps: olefin coordination, cycloaddition, cycloreversion and olefin decoordination. Among them, the two more energetically demanding ones are the olefin coordination and decoordination steps. The reaction of the alkyne fragment consists of two steps: alkyne coordination and alkyne skeletal reorganization, the latter of which has the highest Gibbs energy barrier. Comparison between the ene-then-yne and yne-then-ene pathways shows that there is no clear energetic preference for either of the two processes, and thus both should be operative when unsubstituted enynes are involved. In addition, although the endo orientation is computed to be slightly disfavored, it is not ruled out for 1-allyloxy-2-propyne, and thus calculations seem to indicate that the exo versus endo selectivity is strongly influenced by the presence of substituents in the reagent.

Pt(II) coordination to N1 of 9-methylguanine: why it facilitates binding of additional metal ions to the purine ring.

The preparation and X-ray crystal structure analysis of {trans-[Pt(MeNH(2))(2)(9-MeG-N1)(2)]}⋅{3 K(2)[Pt(CN)(4)]}⋅6 H(2)O (3 a) (with 9-MeG being the anion of 9-methylguanine, 9-MeGH) are reported. The title compound was obtained by treating [Pt(dien)(9-MeGH-N7)](2+) (1; dien=diethylenetriamine) with trans-[Pt(MeNH(2))(2)(H(2)O)(2)](2+) at pH 9.6, 60 °C, and subsequent removal of the [(dien)Pt(II)] entities by treatment with an excess amount of KCN, which converts the latter to [Pt(CN)(4)](2-). Cocrystallization of K(2)[Pt(CN)(4)] with trans-[Pt(MeNH(2))(2)(9-MeG-N1)(2)] is a consequence of the increase in basicity of the guanine ligand following its deprotonation and Pt coordination at N1. This increase in basicity is reflected in the pK(a) values of trans-[Pt(MeNH(2))(2)(9-MeGH-N1)(2)](2+) (4.4±0.1 and 3.3±0.4). The crystal structure of 3 a reveals rare (N7,O6 chelate) and unconventional (N2,C2,N3) binding patterns of K(+) to the guaninato ligands. DFT calculations confirm that K(+) binding to the sugar edge of guanine for a N1-platinated guanine anion is a realistic option, thus ruling against a simple packing effect in the solid-state structure of 3 a. The linkage isomer of 3 a, trans-[Pt(MeNH(2))(2)(9-MeG-N7)(2)] (6 a) has likewise been isolated, and its acid-base properties determined. Compound 6 a is more basic than 3 a by more than 4 log units. Binding of metal entities to the N7 positions of 9-MeG in 3 a has been studied in detail for [(NH(3))(3)Pt(II)], trans-[(NH(3))(2)Pt(II)], and [(en)Pd(II)] (en=ethylenediamine) by using (1)H NMR spectroscopy. Without exception, binding of the second metal takes place at N7, but formation of a molecular guanine square with trans-[(Me(2)NH(2))Pt(II)] cross-linking N1 positions and trans-[(NH(3))(2)Pt(II)] cross-linking N7 positions could not be confirmed unambiguously, despite the fact that calculations are fully consistent with its existence.

Computational calculations of pKa values of imidazole in Cu(II) complexes of biological relevance.

The imidazole ring is part of the lateral chain of histidine. One of the main features of this amino acid is the ability to coordinate copper, especially Cu(2+), because of the intermediate base nature of its imidazole ring, which has a great biological relevance. Proteins such as cytochrome c oxidase, a crucial enzyme in the respiratory chain, and ß-amyloid peptide, implicated in the pathology of Alzheimer's disease, are examples of proteins containing histidines in their coordination sphere. Several studies indicate that the presence of this metal ion produces a decrease in the pK(a) of the imidazole ring of histidine. However, there are no reports of systematic studies of pK(a) variation in these types of metal cation complexes. In this work we use density functional theory to study the dependence of imidazole pK(a) with the number of imidazole rings in Cu(2+) coordination environments. The pK(a) of isolated imidazole (ImH), and the pK(a) of imidazole in Cu(2+)(ImH)(m)(H(2)O)(4-m) (m=1-3) complexes have been studied using two different functionals, B3LYP and MPWB1K, which have different percentage of exact exchange, and the highly-correlated CCSD(T) method. Results show that imidazole pK(a) decreases between 2 and 7 units depending on the method employed and the number of imidazole rings coordinating the metal cation. Taking into account that the pK(a) of imidazole is 14, this decrease could be relevant in biological processes.

Structures and stabilities of Fe2+/3+ complexes relevant to Alzheimer's disease: an ab initio study.

Iron is one of the most abundant metals found in senile plaques of post mortem patients with Alzheimer's disease. However, the interaction mode between iron ions and ß-amyloid peptide as well as their precise affinity is unknown. In this study we apply ab initio computational methodology to calculate binding energies of Fe(2+/3+) with the His13-His14 sequence of Aß, as well as other important ligands such as His6 and Tyr10. Calculations were carried out at the "MP2/6-311+G(2df,2p)"//B3LYP/6-31+G(d) level of theory and solvent effects included by the IEFPCM procedure. Several reaction paths for the binding of imidazole, phenol, and the His13-His14 fragment (modeled by N-(2-(1H-imidazol-4-yl)ethyl)-3-(1H-imidazol-4-yl)propanamide) were sequentially explored. The results show that the most stable complexes containing His13-His14 and phenolate of Tyr10 are the pentacoordinated [Fe(2+)(O-HisHis)(PhO(-))(H(2)O)](+) and [Fe(3+)(N-HisHis)(PhO(-))(H(2)O)](+) compounds and that simultaneous coordination of tyrosine and His13-His14 to Fe(2+/3+) is thermodynamically favorable in water at physiological pH. Computed Raman spectra confirm the conclusion obtained by Miura et al. ( Biochemistry 2000 , 39 , 7024 ) that tyrosine is coordinated to Fe(3+) but do not exclude coordination of imidazoles. Finally, calculations of standard reduction potentials indicate that phenol coordination reduces the redox activity of the iron/Aß complexes.

Surface morphology controls water dissociation on hydrated IrO2 nanoparticles.

Iridium oxide is a highly efficient catalyst for the oxygen evolution reaction, whose large-scale application requires decreasing the metal content. This is achieved using small nanoparticles. The knowledge of the water-IrO2 nanoparticle interface is of high importance to understand the IrO2 behavior as electrocatalyst in aqueous solutions. In this contribution, DFT (PBE-D2) calculations and AIMD simulations on IrO2 nanoparticle models of different sizes ((IrO2)33 and (IrO2)115) are performed. Results show that two key factors determine the H2O adsorption energy and the preferred adsorption structure (molecular or dissociated water): metal coordination and hydrogen bonding with oxygen bridge atoms of the IrO2 surface. Regarding metal coordination, and since the tetragonal distortion existing in IrO2 is retained on the nanoparticle models, the adsorption at iridium axial vacant sites implies stronger Ir-H2O interactions, which favors water dissociation. In contrast, Ir-H2O interaction at equatorial vacant sites is weaker and thus the relative stability of molecular and dissociated forms becomes similar. Hydrogen bonding increases adsorption energy and favors water dissociation. Thus, tip and corner sites of the nanoparticle, with no oxygen bridge atoms nearby, exhibit the smallest adsorption energies and a preference for the molecular form. Overall, the presence of rather isolated tip and corner sites in the nanoparticle leads to lower adsorption energies and a smaller degree of water dissociation when compared with extended surfaces.

The role of exact exchange in the description of Cu(2+)-(H(2)O)(n) (n = 1-6) complexes by means of DFT methods.

This paper analyses the behavior of different density functionals in the description of the most stable structures of Cu(2+)-(H(2)O)(n) complexes (n = 1-6). From n = 3 to n = 6, different coordination numbers of the metal cation were considered. The structures and energies of the complexes were theoretically determined by means of density functional methods that include different amounts of exact exchange: the BLYP functional (0% of exact exchange), the B3LYP functional (20% of exact exchange), the MPWB1K functional (44% of exact exchange), and BHLYP functional (50% of exact exchange). In addition, CCSD(T) calculations with a large basis set were carried out. It has been found that the functionals with lesser amount of exact exchange, especially BLYP, fail to describe the relative energies between the different structures of each cluster because these functionals tend to overestimate the stability of low-coordinated structures. The inclusion of the exact exchange into the functional improves the results, those obtained with MPWB1K and BHLYP being in very good agreement with the CCSD(T) ones. This behavior is related to the poor description of the second ionization energy of Cu by pure functionals, which leads to a too delocalized spin density in the complex with GGA functionals.

Atomistic fibrillar architectures of polar prion-inspired heptapeptides.

This article provides the computational prediction of the atomistic architectures resulting from self-assembly of the polar heptapeptide sequences NYNYNYN, SYSYSYS and GYGYGYG. Using a combination of molecular dynamics and a newly developed tool for non-covalent interaction analysis, we uncover the properties of a new class of bionanomaterials, including hydrogen-bonded polar zippers, and the relationship between peptide composition, fibril geometry and weak interaction networks. Our results, corroborated by experimental observations, provide the basis for the rational design of prion-inspired nanomaterials.

Coordination of (glycyl)(n)glycine (n = 1-3) to Co+ and Co2+.

This paper analyses the interaction of Co+ (d8, 1G, and 3F) and Co2+ (d7, 4F) cations with (glycyl)(n)glycine (n = 1-3) oligomers. The structure, relative energies, and binding energies of the complexes formed have been theoretically determined by means of density functional methods. For all Co+ complexes the ground spin state is the triplet one and the most stable structures show tricoordinated geometries. In contrast, for Co2+ systems the lowest energy structures are tricoordinated (n = 1), tetracoordinated (n = 2), and pentacoordinated (n = 3). For both cobalt cations, interaction energies increase with the peptide length. Differences in the coordination properties of the ligands are discussed according to their length as well as to the electronic configuration of the metal cation, and results are compared to those previously obtained for the analogous Cu(+/2+) systems. The IR spectra of the most stable and low energy conformers have been simulated, and a discussion of the main vibrational features is provided.

Structure, bonding, and relative stability of the ground and low-lying electronic states of CuO2. The role of exact exchange.

The C(2v) and C(s) ground and low-lying states of doublet CuO2 are examined for a series of different density functionals (pure, hybrid, and meta-hybrid) and CCSD(T) methods. The effect of changing the B3LYP functional a0 parameter is also explored. CCSD(T) results at the complete basis set limit show that the relative stability of the different electronic states is 2A(2)(C(2v)) < 2A''(Cs) < 2B(2)(C(2v)) < 2A'(C(s)) 2A(1)(C(2v)) < 2B(1)(C(2v)). Unlike CCSD(T), all DFT methods analyzed in this work erroneously predict the end-on 2A'' state as the ground state for CuO2 irrespective of the type of functional and percentage of Hartree-Fock (exact) exchange included in the B3LYP-like functional. Among the different functionals tested, B3LYP gives the best geometries and relative energies for the different electronic states when compared to CCSD(T) results. As for the effect of the a0 parameter, it is found that the B3LYP-like functional yielding better geometries contains 20% of exact exchange, although somewhat unexpectedly, the B3LYP-like functional with a larger contribution of exact exchange (90%) is the one that gives the smaller standard deviation for relative energies.