Internal rotation in squaramide and related compounds. A theoretical ab initio study

The structural and energetic changes associated with C–N bond rotation in a squaric acid derivative as well as in formamide, 3-aminoacrolein and vinylamine have been studied theoretically using ab initio molecular orbital methods. Geometry optimizations at the MP2(full)/6-31+G* level confirmed an increase in the C–N bond length and a smaller decrease in the C=O length on going from the equilibrium geometry to the twisted transition state. Other geometrical changes are also discussed. Energies calculated at the QCISD(T)/6-311+G** level, including zero-point-energy correction, show barrier heights decreasing in the order formamide, squaric acid derivative, 3-aminoacrolein and vinylamine. The origin of the barriers were examined using the atoms-in-molecules approach of Bader and the natural bond orbital population analysis. The calculations agree with Pauling's resonance model, and the main contributing factor of the barrier is assigned to the loss of conjugation on rotating the C–N bond. Finally, molecular interaction potential calculations were used to study the changes in the nucleophilicity of N and O (carbonyl) atoms upon C–N rotation, and to obtain a picture of the abilities of the molecules to act in nonbonded interactions, in particular hydrogen bonds. The molecular interaction potential results confirm the suitability of squaramide units for acting as binding units in host–guest chemistry. Received: 13 March 2002 / Accepted: 23 June 2002 / Published online: 21 August 2002     

A ‘naked-eye’ chemosensor system for phytate

We report the synthesis of two new anion receptors of a covalently linked 1,3,5-triarylbenzoamido-crown ether. Our results show that combined with a picrate salt they act by means of an intermolecular charge transfer process (EDA complex), as naked-eye sensors for basic anions, especially for sodium phytate in DMSO/H₂O (1:1).     

Magneto-Structural Correlations in Trinuclear Cu(II) Complexes: A Density Functional Study

Summary.  Density functional theoretical methods have been used to study magneto-structural correlations for linear trinuclear hydroxo-bridged copper(II) complexes. The nearest-neighbor exchange coupling constant shows very similar trends to those found earlier for dinuclear compounds for which the Cu–O–Cu angle and the out of plane displacement of the hydrogen atoms at the bridge are the two key structural factors that determine the nature of their magnetic behavior. Changes in these two parameters can induce variations of over 1000 cm⁻¹ in the value of the nearest-neighbor coupling constant. On the contrary, coupling between next-nearest neighbors is found to be practically independent of structural changes with a value for the coupling constant of about −60 cm⁻¹. The magnitude calculated for this coupling constant indicates that considering its value to be negligible, as usually done in experimental studies, can lead to considerable errors, especially for compounds in which the nearest-neighbor coupling constant is of the same order of magnitude. Corresponding author. E-mail: eliseo.ruiz@qi.ub.es Received June 28, 2002; accepted July 22, 2002     

Excitability mediated by localized structures in a dissipative nonlinear optical cavity

We find and characterize an excitability regime mediated by localized structures in a dissipative nonlinear optical cavity. The scenario is that stable localized structures exhibit a Hopf bifurcation to self-pulsating behavior, that is followed by the destruction of the oscillation in a saddle-loop bifurcation. Beyond this point there is a regime of excitable localized structures under the application of suitable perturbations. Excitability emerges from the spatial dependence since the system does not exhibit any excitable behavior locally. We show that the whole scenario is organized by a Takens-Bogdanov codimension-2 bifurcation point.     

Evaluation of anion selectivity in protic media by squaramide-Cresol Red ensembles

A new series of positively charged squaramide-ammonium compounds 1-6 bind sulfate and hydrogen phosphate in ethanol-water mixtures by a combination of electrostatic and hydrogen bonding interactions. A sensing ensemble composed of Cresol Red and an squaramide receptor signals the association events of these anions. Competitive experiments in ethanol-water mixtures (9:1) reveal a moderate selectivity for sulfate over hydrogen phosphate and suggest that squaramide 4, featuring an intermediate distance between squaramide subunits, is better suited for complexation of sulfate.     

Nucleophile Promiscuity of Engineered Class II Pyruvate Aldolase YfaU from E. Coli

Pyruvate‐dependent aldolases exhibit a stringent selectivity for pyruvate, limiting application of their synthetic potential, which is a drawback shared with other existing aldolases. Structure‐guided rational protein engineering rendered a 2‐keto‐3‐deoxy‐l ‐rhamnonate aldolase variant, fused with a maltose‐binding protein (MBP‐YfaU W23V/L216A), capable of efficiently converting larger pyruvate analogues, for example, those with linear and branched aliphatic chains, in aldol addition reactions. Combination of these nucleophiles with N‐Cbz‐alaninal (Cbz=benzyloxycarbonyl) and N‐Cbz‐prolinal electrophiles gave access to chiral building blocks, for example, derivatives of (2S,3S,4R)‐4‐amino‐3‐hydroxy‐2‐methylpentanoic acid (68 %, d.r. 90:10) and the enantiomer of dolaproine (33 %, d.r. 94:6) as well as a collection of unprecedented α‐amino acid derivatives of the proline and pyrrolizidine type. Conversions varied between 6–93 % and diastereomeric ratios from 50:50 to 95:5 depending on the nucleophilic and electrophilic components.