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.     

The effect of molecular shape on the thermotropic liquid crystal behavior of monolauroylated amino acid glyceride conjugates

Monoacylglycerol amino acid conjugates constitute a novel class of specific biocompatible surfactants that can be considered analogues to partial glycerides and lysophospholipids. They consist of one aliphatic chain and one polar head, i.e., the amino acid, linked through a glycerol moiety. In a previous work, we synthesized monolauroylated amino acid glyceride conjugates, 1-O-lauroyl-rac-glycero-3-O-(N(alpha)-acetyl-L-amino acid), changing the amino acid headgroup systematically: arginine (compound 2), aspartic acid (compound 3), glutamic acid (compound 4), asparagine (compound 5), glutamine (compound 6), and tyrosine (compound 7), to elucidate the structure-properties relationship governing the occurrence of their polymorphism. The thermotropism of the new compounds was measured with polarizing light microscopy, differential scanning calorimetry, and X-ray diffraction and compared with the classical monoglyceride rac-1-lauroylglycerol (compound 1). The experiments were performed for a sequence of heating, cooling, and reheating scans. The results showed that compounds 1-6 exhibit a thermotropic smectic phase. As a consequence, the substitution of the polar head did not engender any curvature into the system, which might lead to the formation of cubic or columnar phases. Interestingly, liquid crystalline phases were not found in the case of compound 7. Small-angle X-ray diffraction data in the gel phase revealed that the substitution of the polar head by the different amino acid structures did not modify significantly the lamellar repeat distance relative to that of the reference one. The observed area per molecule, however, was larger for the new compounds. Consequently, interdigitation was promoted in compounds 2-7. The diffraction patterns were analyzed in terms of electron density profiles, using a modified Caillé theory plus a Gaussian electron density representation (MCG method) on X-ray diffraction data.     

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.     

Digital key for chaos communication performing time delay concealment

We introduce a scheme that integrates a digital key in a phase-chaos electro-optical delay system for optical chaos communications. A pseudorandom binary sequence (PRBS) is mixed within the chaotic dynamics in a way that a mutual concealment is performed; e.g., the time delay is hidden by the binary sequence, and the PRBS is also masked by the chaos. In addition to bridging the gap between algorithmic symmetric key cryptography and chaos-based analog encoding, the proposed approach is intended to benefit from the complex algebra mixing between a (pseudorandom) Boolean variable, and another continuous time (chaotic) variable. The scheme also provides a large flexibility allowing for easy reconfigurations to communicate securely at a high bit rate between different systems.     

Long‐Range Effects in Anion–π Interactions: Their Crucial Role in the Inhibition Mechanism of Mycobacterium Tuberculosis Malate Synthase

The glyoxylate shunt is an anaplerotic bypass of the traditional Krebs cycle. It plays a prominent role in Mycobacterium tuberculosis virulence, so it can be exploited for the development of antitubercular therapeutics. The shunt involves two enzymes: isocitrate lyase (ICL) and malate synthase (GlcB). The shunt bypasses two steps of the tricarboxylic acid cycle, allowing the incorporation of carbon, and thus, refilling oxaloacetate under carbon‐limiting conditions. The targeting of ICL is complicated; however, GlcB, which accommodates the pantothenate tail of acetyl‐CoA in the active site, is easier to target. A catalytic Mg²⁺ unit is located at the bottom of the cavity, and plays a very important role. Recently, the development of effective antituberculosis drugs based on phenyldiketo acids (PDKAs) has been reported. Interestingly, all the crystal structures of GlcB–inhibitor complexes exhibit close contact between the carboxylate of Asp633 and the face of the aromatic ring of the inhibitor. Remarkably, the replacement of the phenyl ring in PDKA by aliphatic moieties yields inactive inhibitors, suggesting that the aromatic moiety is crucial for inhibition. However, the aromatic ring of PDKA is not electron‐deficient, and consequently, the anion–π interaction is expected to be very weak (dominated only by polarization effects). Herein, through a combination analysis of the recent X‐ray structures of GlcB–PDKA complexes retrieved from the protein data bank (PDB) and computational ab initio studies (RI‐MP2/def2‐TZVP level of theory), we demonstrate the prominent role of the Mg²⁺ ion in the active site, which promotes long‐range enhancement of the anion–π interaction.