Cooperative enhancement of water binding to antiparallel β‐sheet models: Analysis by ab initio calculations

The cooperative enhancement of water binding to the antiparallel β‐sheet models has been studied by quantum chemical calculations at the MP2/6‐311++G**//MP2/6‐31G* level. The binding energies of the two antiparallel β‐sheet models consisting of two strands of diglypeptide are calculated by supermolecular approach. Then water molecules are gradually bonded to the diglypeptide by N? H···OH₂ and C?O···HOH hydrogen bonds. Our calculation results indicated that the hydrogen bond length and the atom charge distribution are affected by the addition of H₂O molecules. The binding energy of antiparallel diglypeptide β‐sheet models has a great improvement by the increasing of the hydrogen bond cooperativity and the more H₂O molecules added the more cooperativity enhancement can be found. The orbital interactions are calculated by natural bond orbital analysis, and the results indicate that the cooperative enhancement is closely related to the orbital interaction. © 2012 Wiley Periodicals, Inc.     

Synthetic receptors for the differentiation of phosphorylated peptides with nanomolar affinities

Artificial ditopic receptors for the differentiation of phosphorylated peptides varying in i+3 amino acid side chains were synthesized, and their binding affinities and selectivities were determined. The synthetic receptors show the highest binding affinities to phosphorylated peptides under physiological conditions (HEPES, pH 7.5, 154 mM NaCl) reported thus far for artificial systems. The tight and selective binding was achieved by high cooperativity of the two binding moieties in the receptor molecules. All receptors interact with phosphorylated serine by bis(ZnII-cyclen) complex coordination and a second binding site recognizing a carboxylate or imidazole amino acid side chain functionality.     

Supramolecular Control over Split‐Luciferase Complementation

Supramolecular split‐enzyme complementation restores enzymatic activity and allows for on–off switching. Split‐luciferase fragment pairs were provided with an N‐terminal FGG sequence and screened for complementation through host‐guest binding to cucurbit[8]uril (Q8). Split‐luciferase heterocomplex formation was induced in a Q8 concentration dependent manner, resulting in a 20‐fold upregulation of luciferase activity. Supramolecular split‐luciferase complementation was fully reversible, as revealed by using two types of Q8 inhibitors. Competition studies with the weak‐binding FGG peptide revealed a 300‐fold enhanced stability for the formation of the ternary heterocomplex compared to binding of two of the same fragments to Q8. Stochiometric binding by the potent inhibitor memantine could be used for repeated cycling of luciferase activation and deactivation in conjunction with Q8, providing a versatile module for in vitro supramolecular signaling networks.     

Differential Many‐Body Cooperativity in Electronic Spectra of Oligonuclear Transition‐Metal Complexes

In computational chemistry, non‐additive and cooperative effects can be defined in terms of a (differential) many‐body expansion of the energy or any other physical property of the molecular system of interest. One‐body terms describe energies or properties of the subsystems, two‐body terms describe non‐additive but pairwise contributions and three‐body as well as higher‐order terms can be interpreted as a measure for cooperativity. In the present article, this concept is applied to the analysis of ultraviolet/visible (UV/Vis) spectra of homotrinuclear transition‐metal complexes by means of a many‐body expansion of the change in the spectrum induced by replacing each of the three transition‐metal ions by another transition‐metal ion to yield a different homotrinuclear transition‐metal complex. Computed spectra for the triangulo‐complexes [M₃{Si(mt^Me)₃}₂] (M=Pd/Pt, mt^Me=methimazole) and tritopic triphenylene‐based N‐heterocyclic carbene Rh/Ir complexes illustrate the concept, showing large and small differential three‐body cooperativity, respectively.     

Weak σ-Hole Triel Bond between C5H5Tr (Tr=B,Al, Ga) and Haloethyne: Substituent and Cooperativity Effects

The replacement of a CH group of benzene by a triel (Tr) atom places a positive region of electrostatic potential near the Tr atom in the plane of the aromatic ring. This σ-hole can interact with an X lone pair of XCCH (X=F, Cl, Br, and I) to form a triel bond (TrB). The interaction energy between C₅H₅Tr and FCCH lies in the range between 2.2 and 4.4 kcal/mol, in the order Tr=B<Ga<Al. This bond is strengthened by halogen substituents on the ring, particularly on the site adjacent to Tr. There is a much stronger strengthening trend as the F of the FCCH nucleophile is replaced by the heavier halogen atoms, rising up to 22 kcal/mol for ICCH. Adding a Li⁺ cation above the ring pulls density toward itself and thus magnifies the Tr σ-hole. The TrB to the XCCH nucleophile is thereby magnified as is the strength of the TrB. This positive cooperativity is particularly large for Tr=B.     

New Bifunctional Bis(azairidacycle) with Axial Chirality via Double Cyclometalation of 2,2′-Bis(aminomethyl)-1,1′-binaphthyl

As a candidate for bifunctional asymmetric catalysts containing a half-sandwich C–N chelating Ir(III) framework (azairidacycle), a dinuclear Ir complex with an axially chiral linkage is newly designed. An expedient synthesis of chiral 2,2′-bis(aminomethyl)-1,1′-binaphthyl (1) from 1,1-bi-2-naphthol (BINOL) was accomplished by a three-step process involving nickel-catalyzed cyanation and subsequent reduction with Raney-Ni and KBH₄. The reaction of (S)-1 with an equimolar amount of [IrCl₂Cp*]₂ (Cp* = η⁵–C₅(CH₃)₅) in the presence of sodium acetate in acetonitrile at 80 °C gave a diastereomeric mixture of new dinuclear dichloridodiiridium complexes (5) through the double C–H bond cleavage, as confirmed by ¹H NMR spectroscopy. A loss of the central chirality on the Ir centers of 5 was demonstrated by treatment with KOC(CH₃)₃ to generate the corresponding 16e amidoiridium complex 6. The following hydrogen transfer from 2-propanol to 6 provided diastereomers of hydrido(amine)iridium retaining the bis(azairidacycle) architecture. The dinuclear chlorido(amine)iridium 5 can serve as a catalyst precursor for the asymmetric transfer hydrogenation of acetophenone with a substrate to a catalyst ratio of 200 in the presence of KOC(CH₃)₃ in 2-propanol, leading to (S)-1-phenylethanol with up to an enantiomeric excess (ee) of 67%.     

Breaking Symmetry Relaxes Structural and Magnetic Restraints,Suppressing QTM in Enantiopure Butterfly Fe2Dy2 SMMs**

The {Fe₂Dy₂} butterfly systems can show single molecule magnet (SMM) behaviour, the nature of which depends on details of the electronic structure, as previously demonstrated for the [Fe₂Dy₂(μ₃-OH)₂(Me-teaH)₂(O₂CPh)₆] compound, where the [N,N-bis-(2-hydroxyethyl)-amino]-2-propanol (Me-teaH₃) ligand is usually used in its racemic form. Here, we describe the consequences for the SMM properties by using enantiopure versions of this ligand and present the first homochiral 3d/4 f SMM, which could only be obtained for the S enantiomer of the ligand for [Fe₂Dy₂(μ₃-OH)₂(Me-teaH)₂(O₂CPh)₆] since the R enantiomer underwent significant racemisation. To investigate this further, we prepared the [Fe₂Dy₂(μ₃-OH)₂(Me-teaH)₂(O₂CPh)₄(NO₃)₂] version, which could be obtained as the RS-, R- and S-compounds. Remarkably, the enantiopure versions show enhanced slow relaxation of magnetisation. The use of the enantiomerically pure ligand suppresses QTM, leading to the conclusion that use of enantiopure ligands is a “gamechanger” by breaking the cluster symmetry and altering the intimate details of the coordination cluster's molecular structure.     

The enhancing effects of molecule X (X = PH2Cl,SHCl, ClCl) on chalcogen–chalcogen interactions in cyclic trimers Y···Y···X (Y = SHCl,SeHCl)

The enhancing effects of molecule X (X = PH₂Cl, SHCl, ClCl) on S···S and Se···Se chalcogen–chalcogen bonds in the cyclic trimers SHCl···SHCl···X and SeHCl···SeHCl···X were investigated by calculations at the MP2/aug‐cc‐pVTZ level. When molecule X is added to the dimer SHCl···SHCl (SeHCl···SeHCl), cyclic trimers are formed. Compared with the dimer, all the cyclic trimers have shorter S···S (Se···Se) lengths, greater electron densities, negative three‐body interaction energies, and larger second‐order perturbation energies. These results indicate that the addition of molecule X strengthens the original S···S (Se···Se) bond. For the SHCl···SHCl···X cyclic trimers, the S···S bond is strongest in SHCl···SHCl···PH₂Cl, weaker in SHCl···SHCl···SHCl, and weakest in SHCl···SHCl···ClCl. This same trend is observed for the Se···Se bond in SeHCl···SeHCl···X. This means that PH₂Cl provides the greatest enhancement to the S···S (Se···Se) interaction.     

Ab initio study of the structure,cooperativity, and vibrational properties in the mixed hydrogen‐bonded trimers of hydrogen isocyanide and water

The five trimers of H₂O···HNC···H₂O, H₂O···H₂O···HNC, HNC···H₂O···H₂O, H₂O···HNC···HNC, and HNC···HNC···H₂O have been studied with quantum chemical calculations. Their structures, harmonic vibrational frequencies and interaction energies have been calculated at the B3LYP and MP2 levels with the aug‐cc‐pVDZ and aug‐cc‐pVTZ basis sets. The cooperative effect on these properties has also been studied quantitatively. For HNC:(H₂O)₂ systems, the cyclic H₂O···H₂O···HNC trimer is most stable with an interaction energy of ?16.01 kcal/mol and a large cooperative energy of ?3.25 kcal/mol at the MP2/aug‐cc‐pVTZ level. For H₂O:(HNC)₂ systems, the interaction energy and cooperative energy in the H₂O···HNC···HNC trimer are larger than those in the HNC···HNC···H₂O trimer. The NH stretch frequency has a blue shift for the terminal HNC molecule in the HNC···H₂O···H₂O and HNC···HNC···H₂O trimers and a red shift in other cases. A many‐body analysis has also been performed to understand the interaction energies in these hydrogen‐bonded clusters. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Theoretical study of the reaction of H2 with a Cu2Pt2 cluster

The study of the interaction of a pyramidal tetramer of Cu₂Pt₂ with the H₂ is reported here through ab initio multiconfigurational self-consistent field (MC-SCF) calculations, plus extensive multireference configuration interaction (MR-CI), variational and perturbative calculations. The lowest three electronic states X ¹A′, a ³A′ and a ¹A′ of the bare cluster were considered in order to study this interaction. For the H₂ C_s approaching a Pt vertex, results show that the Cu₂Pt₂ pyramid cluster in its X ¹A′ and a ¹A′ states can spontaneously capture and dissociate the H₂. For the H₂ C_s approaching a Cu vertex, where H₂ is located in the C_s reflecting plane, the Cu₂Pt₂ cluster in its X ¹A′ electronic state shows capture of the hydrogen molecule after surmounting an energy barrier; moreover, in this approach the Cu₂Pt₂ cluster in its a ¹A′ electronic state shows spontaneous capture of the hydrogen molecule. For the H₂ approaching a Cu vertex, where the C_s reflecting plane bisects the H₂ molecule, the Cu₂Pt₂ cluster in its three lowest-lying states is able to capture the hydrogen molecule after surmounting a small barrier. The Cu₂Pt₂+H₂ C_s face-on interactions show a lower H₂ activation than that which was obtained in the equivalent Pt₄+H₂ interactions.