Chiral recognition by resorcin[4]arene receptors: intrinsic kinetics and dynamics

Molecular recognition of representative amino acids (A) by a chiral amido[4]resorcinarene receptor (1(L)) was investigated in the gas phase by ESI-FT-ICR mass spectrometry. The ligand displacement reaction between noncovalent diastereomeric [1(L).H.A](+) complexes and the 2-aminobutane enantiomers (B) exhibits a distinct enantioselectivity with regard to both the leaving amino acid A and the amine reactant B. The emerging selectivity picture, discussed in the light of molecular mechanics and molecular dynamics calculations, points to chiral recognition by 1(L), as determined by the effects of the host asymmetric frame on the structure, stability, and rearrangement dynamics of the diastereomeric [1(L).H.A](+) complexes and the orientation of the amine reactant B in encounters with [1(L).H.A](+). The results contribute to the development of a dynamic model of chiral recognition of biomolecules by enzyme mimics in the unsolvated state.     

New high-nuclearity Ni-Pt carbonyl clusters: synthesis and X-ray structure of the ordered [HNi24Pt17(CO)46]5- and the substitutionally Ni/Pt disordered [Ni32Pt24(CO)56]6- cluster anions

Condensation between preformed Ni-Pt and Pt carbonyl clusters leads to the new [H(6-n)Ni(24)Pt(17)(CO)(46)](n-)(n= 5, 6) and the substitutionally Ni/Pt disordered [Ni(24)(Ni(12-x)Pt(x))Pt(20)(CO)(56)](6-) (x = 4) carbonyl clusters, the latter of which represents the highest nuclearity homoleptic carbonyl cluster anion so far reported.     

Fluorescence sensing of ionic analytes in water: from transition metal ions to vitamin B13

The fluorescence chemosensor ATMCA has been realised by appending an anthrylmethyl group to an amino nitrogen of TMCA (2,4,6-triamino-1,3,5-trimethoxycyclohexane), a tripodal ligand selective for divalent first-row transition metal ions in water. The ATMCA ligand can act as a versatile sensor for ZnII and CuII ions. Its sensing ability can be switched by simply tuning the operating conditions. At pH 5, ATMCA detects copper(II) ions in aqueous solutions by the complexation-induced quenching of the anthracene emission. Metal ion concentrations < 1 microM can be readily detected and very little interference is exerted by other metal ions. At pH 7, ATMCA signals the presence of ZnII ions at concentrations < 1 microM by a complexation-induced enhancement of the fluorescence. Again the sensor is selective for ZnII over several divalent metal ions, with the exception of CuII, CoII and HgII. Most interestingly, the [ZnII(atmca)]2+ complex can act as a fluorescence sensor for specific organic species, notably selected dicarboxylic acids and nucleotides, by the formation of ternary ligand/zinc/substrate complexes. The oxalate anion is detected in concentrations <0.1 mM; however, no effects on the system's fluorescence is observed in the presence of monocarboxylic acids and long-chain dicarboxylic acids. Among the nucleotides, those containing an imide or amide function are readily detected and an unprecedented high sensitivity for guanine derivatives allows the determination of this nucleotide for 0.05-0.5 mM solutions. Moreover, [ZnII(atmca)]2+ is a very effective and selective sensor in the case of vitamin B13 (orotic acid) in sub-micromolar concentrations. The operative features of the systems investigated are also clearly suitable for intracellular analyses. The factors at the source of organic substrate recognition, here briefly discussed, are of paramount importance for further developments in the applicability of these sensing systems.     

Experimental and theoretical studies on three spiromuscar-3-one M1 agonist derivatives

Muscarone analogues are compounds that have been proposed for the prevention or treatment of senile dementias. ARL-16607 (I), ARL-15467 (II), ARL-14995 (III) and YM-796 (IV) are spiromuscar-3-one derivatives and behave as muscarinic M₁ agonists, with different binding selectivity and efficacy at the M₁ receptors. In this work, we have elucidated the solid-state structures of I-III and compared the results obtained with the data available in the literature for IV.The solid-state arrangements of I-IV have then been used to input a series of theoretical calculations. For each molecule, eight conformations have been modeled and the obtained structures (1-32) have been submitted to a series of molecular dynamics/simulated annealing and molecular mechanics calculations aimed to explore the conformational freedom of the spiromuscar-3-one moiety. Some hints about the reactivity of I-IV have been obtained by performing Hartree-Fock, density functional theory and semiempirical quantum mechanics calculations. These studies analyzed the properties of the frontier orbitals and of the molecular electrostatic potential of I-IV.The information gained has been used to explain the better efficacy and poorer selectivity shown by III. Our results suggest that the behavior of III is due to its smaller size, the features of its molecular surface, the shape of its electrostatic potential and the orientation of its reactive domains.     

Spinon attraction in spin- 1/2 antiferromagnetic chains

We derive the representation of the two-spinon wave function for the Haldane-Shastry model in terms of the spinon coordinates. This result allows us to rigorously analyze spinon interaction and its physical effects. We show that spinons attract one another. The attraction gets stronger as the size of the system is increased and, in the thermodynamic limit, determines the power law with which the susceptibility diverges.     

Energy release rate and stress intensity factor in antiplane elasticity

In the setting of antiplane linearized elasticity, we show the existence of the stress intensity factor and its relation with the energy release rate when the crack path is a C1,1 curve. Finally, we show that the energy release rate is continuous with respect to the Hausdorff convergence in a class of admissible cracks.     

Time and Simple Existence

Sceptics about substantial disputes in ontology often argue that when two philosophers seem to disagree on a quantified claim, they are actually equivocating on the notion of existence that they are using. When temporal elements play a central role, as in the debate between presentists and eternalists, the hypothesis of an equivocation with respect to existence acquires more plausibility. However, the anti-sceptic can still argue that this hypothesis is unjustified.     

A Semisynthetic Approach to New Immunoadjuvant Candidates: Site‐Selective Chemical Manipulation of Escherichia coli Monophosphoryl Lipid A

A semisynthetic approach to novel lipid A derivatives from Escherichia coli (E. coli) lipid A is reported. This methodology stands as an alternative to common approaches based exclusively on either total synthesis or extraction from bacterial sources. It relies upon the purification of the lipid A fraction from fed‐batch fermentation of E. coli, followed by its structural modification through tailored, site‐selective chemical reactions. In particular, modification of the lipid pattern and functionalization of the phosphate group as well as of the sole primary hydroxyl group were accomplished, highlighting the unusual reactivity of the molecule. Preliminary investigations of the immunostimulating activity of the new semisynthetic lipid A derivatives show that some of them stand out as promising, new immunoadjuvant candidates.     

Instability of struts subject to radiant heat

Summary Two model struts subject to an axial force and to radiant heat parallel to the axis are studied dynamically. The elastic and thermal deformations of the strut are lumped in one or two cells; the heat absorbed by each cell and the consequential thermal deformation depend on its rotation. Thermal inertia is neglected, while a viscous damping is introduced.The possible exchange of energy between the strut and the surrounding world leads to a non-conservative mechanical problem. The stability of equilibrium of the original strut position is investigated by the kinetic method, and the zones of stability (bounded perturbed motion), static instability (divergent motion) and dynamic instability (flutter) are found and plotted in some cases, in function of axial load and radiant heat intensity.

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Sommario Si studiano due modelli di asta soggetti a una forza assiale ed a calore radiante parallelamente all'asse. Le deformazioni termiche ed elastiche dell'asta sono concentrate in uno o dueconci di lunghezza trascurabile rispetto a quella dei tratti rigidi costituenti l'asta stessa. La quantità di calore assorbita da ciascun concio deformabile e la sua conseguente deformazione termica dipende dalla sua rotazione. Si trascura l'inerzia termica, mentre si introduce uno smorzamento viscoso.La possibilità di scambio di calore tra l'asta ed il mondo circostante conduce a un problema meccanico non-conservativo. Pertanto, la stabilità dell'equilibrio dell'asta nella sua posizione iniziale è investigata con il metododinamico (ocinematico), cioè studiando le caratteristiche del moto susseguente ad unaperturbazione dell'equilibrio stesso. Si individuano così le zone di stabilità (moto limitato), instabilità statica (moto monotonicamente crescente o motodivergente) e instabilità dinamica (moto oscillatorio di ampiezza crescente, oflutter), riportandole in diagramma per alcuni casi numerici, in funzione del carico e dell'intensità della sorgente di calore.

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Paper presented at the 12th International Congress of Applied Mechanics; Stanford, California; 26–31 August 1968.

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(presently, on leave at the Division of Engineering, Brown University, Providence, R. I.).