Enthalpy–Entropy Compensation Combined with Cohesive Free‐Energy Densities for Tuning the Melting Temperatures of Cyanobiphenyl Derivatives

This work illustrates how minor structural perturbations produced by methylation of 4′‐(dodecyloxy)‐4‐cyanobiphenyl leads to enthalpy–entropy compensation for their melting processes, a trend which can be analyzed within the frame of a simple intermolecular cohesive model. The transformation of the melting thermodynamic parameters collected at variable temperatures into cohesive free‐energy densities expressed at a common reference temperature results in a novel linear correlation, from which melting temperatures can be simply predicted from molecular volumes.     

The first self-assembled trimetallic lanthanide helicates driven by positive cooperativity

The segmental tris-tridentate ligand L7 reacts with stoichiometric quantities of Ln(III) (Ln=La-Lu) in acetonitrile to give the complexes [Ln(2)(L7)(3)](6+) and [Ln(3)(L7)(3)](9+). Formation constants point to negligible size-discriminating effects along the lanthanide series, but Scatchard plots suggest that the self-assembly of the trimetallic triple-stranded helicates [Ln(3)(L7)(3)](9+) is driven to completion by positive cooperativity, despite strong intermetallic electrostatic repulsions. Crystallization provides quantitatively [Ln(3)(L7)(3)](CF(3)SO(3))(9) (Ln=La, Eu, Gd, Tb, Lu) and the X-ray crystal structure of [Eu(3)(L7)(3)](CF(3)SO(3))(9).(CH(3)CN)(9).(H(2)O)(2) (Eu(3)C(216)H(226)N(48)O(35)F(27)S(9), triclinic, P1, Z=2) shows the three ligand strands wrapped around a pseudo-threefold axis defined by the three metal ions rigidly held at about 9 A. Each metal ion is coordinated by nine donor atoms in a pseudo-trigonal prismatic arrangement, but the existence of terminal carboxamide units in the ligand strands differentiates the electronic properties of the terminal and the central metallic sites. Photophysical data confirm that the three coordination sites possess comparable pseudo-trigonal symmetries in the solid state and in solution. High-resolution luminescence analyses evidence a low-lying LMCT state affecting the central EuN(9) site, so that multi-metal-centered luminescence is essentially dominated by the emission from the two terminal EuN(6)O(3) sites in [Eu(3)(L7)(3)](9+). New multicenter equations have been developed for investigating the solution structure of [Ln(3)(L7)(3)](9+) by paramagnetic NMR spectroscopy and linear correlations for Ln=Ce-Tb imply isostructurality for these larger lanthanides. NMR spectra point to the triple helical structure being maintained in solution, but an inversion of the magnitude of the second-rank crystal-field parameters, obtained by LIS analysis, for the LnN(6)O(3) and LnN(9) sites with respect to the parameters extracted for Eu(III) from luminescence data, suggests that the geometry of the central LnN(9) site is somewhat relaxed in solution.     

L-arginine binding to human inducible nitric oxide synthase: an antisymmetric funnel route toward isoform-specific inhibitors?

Nitric oxide (NO) is an important signaling molecule produced by a family of enzymes called nitric oxide synthases (NOS). Because NO is involved in various pathological conditions, the development of potent and isoform-selective NOS inhibitors is an important challenge. In the present study, the dimer of oxygenase domain of human iNOS (iNOSoxy) complexed to its natural substrate L-arginine (L-Arg) and both heme and tetrahydro-L-biopterin (BH4) cofactors was studied through multiple molecular dynamics simulations. Starting from the X-ray structure available for that complex (PDB: 1NSI ), a 16 ns equilibration trajectory was first obtained. Twelve dynamics of slow extraction of L-Arg out from the iNOSoxy active site were then performed. The steered molecular dynamics (SMD) approach was used starting from three different points of the reference trajectory for a total simulation time of 35 ns. A probable unbinding/binding pathway of L-Arg was characterized. It was suggested that a driving force directed the substrate toward the heme pocket. Key intermediate steps/residues along the access route to the active site were identified along this "funnel shape" pathway and compared to existing data. A quasi-normal mode analysis performed on the SMD data suggested that large collective motions of the protein may be involved in L-Arg binding and that opening the route to the active site in one monomer promoted an inverse, closing motion in the second monomer. Finally, our findings might help to rationalize the design of human iNOS isoform competitive inhibitors.     

Capture of the complex [Ni(dto)2]2- (dto2- = dithiooxalato ligand) in a Mo12 ring: synthesis, characterizations, and application toward the reduction of protons

The encapsulation of the complex [Ni(dto)(2)](2-) within an oxothiododecamolybdic cyclic cluster has been investigated. The resulting molybdenum ring, [Mo(12)O(12)S(12)(OH)(12)(Ni(dto)(2))](2-), corresponds to the first example of the {Mo(2)O(2)S(2)}-based assembly arranged around a 3d transition-metal complex. It was unambiguously characterized in the solid state and in solution by FT-IR spectroscopy, single-crystal X-ray diffraction, NMR, UV-visible spectroscopy, and electrospray ionization-high-resolution mass spectrometry (ESI-HRMS). The latter technique revealed to be a powerful tool for the characterization of templated molybdenum ring systems in solution and gave excellent results in high resolution. The electronic spectrum of [Mo(12)O(12)S(12)(OH)(12)(Ni(dto)(2))](2-) evidenced a strong red shift of the LMCT bands attributed to the complex [Ni(dto)(2)](2-), suggesting significant variations of the electronic properties upon its encapsulation within the Mo(12) ring. These differences were demonstrated by electrochemical studies in CH(3)CN, which also revealed, for both compounds [Ni(dto)(2)](2-) and [Mo(12)O(12)S(12)(OH)(12)(Ni(dto)(2))](2-), electrocatalytic properties for the reduction of protons. These results evidence the ability of dithioxalato complexes to act as electrocatalysts for the hydrogen evolution reaction (HER) and confirm such a property for oxothiomolybdenum rings.     

Supramolecular Adduct of γ-Cyclodextrin and [{Re<Subscript>6</Subscript>Q<Subscript>8</Subscript>}(H<Subscript>2</Subscript>O)<Subscript>6</Subscript>]<Superscript>2+</Superscript> (Q=S,Se)

Slow evaporation of water solution of [{Re₆S₈}(H₂O)₆]²⁺ generated in situ from [{Re₆S₈}(OH)₆]^4– in presence of γ-cyclodextrin (CD) leads to crystallization of {[{Re₆S₈}(H₂O)₆] ? [γ-CD]}(NO₃)₂·12H₂O (1·12H₂O) supramolecular complex, which was characterized by single-crystal X-ray diffraction crystallography, IR-spectroscopy, thermogravimetric and elemental analyses. X-ray analysis confirms the formation of 1:1 {[{Re₆S₈}(H₂O)₆] ? [γ-CD]}²⁺ inclusion compound in the solid state. However, no adduct formation was detected between [{Re₆S₈}(H₂O)₆]²⁺ and γ-cyclodextrin in solution, according to ¹H NMR spectroscopy. In the case of in situ generated [{Re₆Se₈}(H₂O)₆]²⁺ the reaction solution with γ-cyclodextrin is unstable and during the crystallization only amorphous precipitate has been obtained.     

Spin crossover of ferric complexes with catecholate derivatives. Single-crystal X-ray structure, magnetic and Mössbauer investigations

Complexes of general formula [(TPA)Fe(R-Cat)]X.nS were synthesised with different catecholate derivatives and anions (TPA = tris(2-pyridylmethyl)amine, R-Cat2- = 4,5-(NO2)2-Cat2- denoted DNC(2-); 3,4,5,6-Cl4-Cat2- denoted TCC2-; 3-OMe-Cat(2-); 4-Me-Cat(2-) and X = BPh4-; NO3-; PF6-; ClO4-; S = solvent molecule). Their magnetic behaviours in the solid state show a general feature along the series, viz., the occurrence of a thermally-induced spin crossover process. The transition curves are continuous with transition temperatures ranging from ca. 84 to 257 K. The crystal structures of [(TPA)Fe(DNC)]X (X = PF6-; BPh4-) and [(TPA)Fe(TCC)]X.nS (X = PF6-; NO3- and n= 1, S = H2O; ClO4- and n= 1, S = H2O; BPh4- and n= 1, S = C3H6O) were solved at 100 (or 123 K) and 293 K. For those two systems, the characteristics of the [FeN(4)O(2)] coordination core and those of the dioxolene ligands appear to be consistent with a prevailing Fe(III)-catecholate formulation. This feature is in contrast with the large quantum mixing between Fe(III)-catecholate and Fe(II)-semiquinonate forms recently observed with the more electron donating simple catecholate dianion. The thermal spin crossover process is accompanied by significant changes of the molecular structures as shown by the average variation of the metal-ligand bond distances which can be extrapolated for a complete spin conversion from ca. 0.123 to 0.156 A. The different space groups were retained in the low- and high-temperature phases.     

Dissection of human tropoelastin: solution structure, dynamics and self-assembly of the exon 5 peptide

The elastic properties of elastin have essentially been discussed in terms of dominant entropic components, with questions still remaining about whether the basic mechanism is compatible with the classical theory of rubber elasticity. A better understanding of the structure-function relationships in terms of the protein's elastic properties remains an important goal in elastin science. Recently, we succeeded in the exon-by-exon synthesis of all polypeptide sequences encoded by the so-called hydrophobic exons and almost all of the cross-linking exons of human tropoelastin. Among these, the peptide encoded by exon 5 (PGGLAGAGLGA) has been extensively studied by classical spectroscopic methods, such as CD and NMR spectroscopy, and by molecular dynamics simulations. The results obtained clearly evidenced a large flexibility of the polypeptide chain, which oscillates between rather extended conformations, such as PPII, and folded ones, such as beta turns. At the supramolecular level, we obtained evidence by TEM that shows that the peptide encoded by exon 5 is able to self-assemble in fibrillar structures, a result indicating that the "information" for self-assembly is also contained within a small domain of tropoelastin.     

Molecular mobility of confined phases in model mesoporous (MCM-41) and microporous (AlPO4-5 zeolite) host materials

This paper discusses the self-assembly of block copolymers into vesicular morphology. After a brief state of art of the field, a system based on an amphiphilic poly(butadiene)-b-poly(-L-glutamic acid) (PB-b-PGA) diblock copolymer in aqueous solution is discussed in detail. The aggregation behavior of this block copolymer has been investigated by means of fluorescence spectroscopy, dynamic (DLS) and static (SLS) light scattering as well as transmission electron microscopy (TEM). The diblock copolymer was found to form well-defined vesicles in water. The size of these so-called polymersomes or peptosomes could be reversibly manipulated as a function of both pH and ion strength. Depending on the pH of the aqueous solution, the hydrodynamic radii of these vesicles were found to vary from 100 nm to 150 nm. By cross-linking the 1,2-vinyl double bonds present in the polybutadiene block, the ability to transform a transient supramolecular self-organized aggregate into a permanent “shape-persistent stimuli-responsive nanoparticle” has been demonstrated. Received 25 June 2002 and Received in final form 22 October 2002 Published online: 11 March 2003     

Molecular mobility of confined phases in model mesoporous (MCM-41) and microporous (AlPO(4)-5 zeolite) host materials

The considered host materials are well suited to confine quasi-(1d) molecular phases, seeing that their porosities are composed of parallel unconnected cylindrical pores. For such a simple geometry, confinement effects can be simply described by a single parameter, the pore diameter phi. Our study concerns medium and ultra confinement ranges ( 40 A >or= ? >or= 7.3 A). The primary effect of such confinements is the decrease of the molecular interactions within the confined phase. As a consequence, we have observed strong triple point depressure Delta T(3t) effects for hydrogen and water confined phases in MCM-41 samples. In the limit case of (1d) phase (the neopentane/AlPO(4)-5 system) it seems that a molecular mobility is observed even at very low temperature T=5 K. The secondary confinement effect is an increase of the interactions between the host inner surface and the confined molecular assembly induced by the pore diameter decreasing. Such host material influence gives rise, for medium range confinement to the physisorption of a curved solid film on the inner surface before the capillary phase condensation (hydrogen/MCM-41 (24 A)) and for ultra confinement to the solidification of the confined phase when the molecular species are commensurate with the inner surface sites (methane/AlPO(4)-5).