"The Sulfur Dance" Around Arenes and Heteroarenes - the Reversible Nature of Nucleophilic Aromatic Substitutions.

We disclose the features of a category of reversible nucleophilic aromatic substitutions in view of their significance and generality in dynamic aromatic chemistry. Exchange of sulfur components surrounding arenes and heteroarenes may occur at 25 °C, in a process that one may call a "sulfur dance". These SN Ar systems present their own features, apart from common reversible reactions utilized in dynamic covalent chemistry (DCC). By varying conditions, covalent dynamics may operate to provide libraries of thiaarenes with some selectivity, or conversion of a hexa(thio)benzene asterisk into another one. The reversible nature of SN Ar is confirmed by three methods: a convergence of the products distribution in reversible SN Ar systems, a related product redistribution between two per(thio)benzenes by using a thiolate promoter, and from kinetic/thermodynamic data. A four-component dynamic covalent system further illustrates the thermodynamically-driven formation of a thiacalix[2]arene[2]pyrimidine by sulfur component exchanges. This work stimulates the implementation of reversible SN Ar in aromatic chemistry and in DCC.

The Effect of Acidic and Alkaline Seawater on the F-Actin-Dependent Ca2+ Signals Following Insemination of Immature Starfish Oocytes and Mature Eggs.

In starfish, the addition of the hormone 1-methyladenine (1-MA) to immature oocytes (germinal vesicle, GV-stage) arrested at the prophase of the first meiotic division induces meiosis resumption (maturation), which makes the mature eggs able to respond to the sperm with a normal fertilization response. The optimal fertilizability achieved during the maturation process results from the exquisite structural reorganization of the actin cytoskeleton in the cortex and cytoplasm induced by the maturing hormone. In this report, we have investigated the influence of acidic and alkaline seawater on the structure of the cortical F-actin network of immature oocytes of the starfish (Astropecten aranciacus) and its dynamic changes upon insemination. The results have shown that the altered seawater pH strongly affected the sperm-induced Ca2+ response and the polyspermy rate. When immature starfish oocytes were stimulated with 1-MA in acidic or alkaline seawater, the maturation process displayed a strong dependency on pH in terms of the dynamic structural changes of the cortical F-actin. The resulting alteration of the actin cytoskeleton, in turn, affected the pattern of Ca2+ signals at fertilization and sperm penetration.

Effects of Dithiothreitol on Fertilization and Early Development in Sea Urchin.

The vitelline layer (VL) of a sea urchin egg is an intricate meshwork of glycoproteins that intimately ensheathes the plasma membrane. During fertilization, the VL plays important roles. Firstly, the receptors for sperm reside on the VL. Secondly, following cortical granule exocytosis, the VL is elevated and transformed into the fertilization envelope (FE), owing to the assembly and crosslinking of the extruded materials. As these two crucial stages involve the VL, its alteration was expected to affect the fertilization process. In the present study, we addressed this question by mildly treating the eggs with a reducing agent, dithiothreitol (DTT). A brief pretreatment with DTT resulted in partial disruption of the VL, as judged by electron microscopy and by a novel fluorescent polyamine probe that selectively labelled the VL. The DTT-pretreated eggs did not elevate the FE but were mostly monospermic at fertilization. These eggs also manifested certain anomalies at fertilization: (i) compromised Ca2+ signaling, (ii) blocked translocation of cortical actin filaments, and (iii) impaired cleavage. Some of these phenotypic changes were reversed by restoring the DTT-exposed eggs in normal seawater prior to fertilization. Our findings suggest that the FE is not the decisive factor preventing polyspermy and that the integrity of the VL is nonetheless crucial to the egg's fertilization response.

Acyl Transfer Catalytic Activity in De Novo Designed Protein with N-Terminus of α-Helix As Oxyanion-Binding Site.

The design of catalytic proteins with functional sites capable of specific chemistry is gaining momentum and a number of artificial enzymes have recently been reported, including hydrolases, oxidoreductases, retro-aldolases, and others. Our goal is to develop a peptide ligase for robust catalysis of amide bond formation that possesses no stringent restrictions to the amino acid composition at the ligation junction. We report here the successful completion of the first step in this long-term project by building a completely de novo protein with predefined acyl transfer catalytic activity. We applied a minimalist approach to rationally design an oxyanion hole within a small cavity that contains an adjacent thiol nucleophile. The N-terminus of the α-helix with unpaired hydrogen-bond donors was exploited as a structural motif to stabilize negatively charged tetrahedral intermediates in nucleophilic addition-elimination reactions at the acyl group. Cysteine acting as a principal catalytic residue was introduced at the second residue position of the α-helix N-terminus in a designed three-α-helix protein based on structural informatics prediction. We showed that this minimal set of functional elements is sufficient for the emergence of catalytic activity in a de novo protein. Using peptide-αthioesters as acyl-donors, we demonstrated their catalyzed amidation concomitant with hydrolysis and proved that the environment at the catalytic site critically influences the reaction outcome. These results represent a promising starting point for the development of efficient catalysts for protein labeling, conjugation, and peptide ligation.

Evolution of a constitutional dynamic library driven by self-organisation of a helically folded molecular strand.

Conversion of macrocyclic imine entities into helical strands was achieved through three- and four-component exchange reactions within constitutionally dynamic libraries. The generation of sequences of the intrinsic helicity codon, based on the hydrazone-pyrimidine fragment obtained by condensation of pyrimidine dialdehyde A with pyrimidine bis-hydrazine B, shifted the equilibrium between all the possible macrocycles and strands towards the full expression (>98%) of helical product [A/B]. Furthermore, it was shown that chain folding accelerated the dynamic exchange reactions among the library members. Lastly, in four-component experiments (involving A, B, E and either C or D), even though the macrocyclic entities ([A/C], [B/E]; [A/D], [B/E]) were the kinetically preferred products, over time dialdehyde A relinquished its initial diamine partners C or D to opt for bis-hydrazine B, which allowed the preferential formation of the helically folded strand. The present results indicate that self-organisation pressure was able to drive the dynamic system towards the selective generation of the strand undergoing helical folding.

Driven evolution of a constitutional dynamic library of molecular helices toward the selective generation of [2 x 2] gridlike arrays under the pressure of metal ion coordination.

Constitutional dynamics, self-assembly, and helical-folding control are brought together in the efficient Sc(OTf)3/microwave-catalyzed transimination of helical oligohydrazone strands, yielding highly diverse dynamic libraries of interconverting constituents through assembly, dissociation, and exchange of components. The transimination-type mechanism of the ScIII-promoted exchange, as well as its regioselectivity, occurring only at the extremities of the helical strands, allow one to perform directional terminal polymerization/depolymerization processes when starting with dissymmetric strands. A particular library is subsequently brought to express quantitatively [2 x 2] gridlike metallosupramolecular arrays in the presence of ZnII ions by component recombination generating the correct ligand from the dynamic set of interconverting strands. This behavior represents a process of driven evolution of a constitutional dynamic chemical system under the pressure (coordination interaction) of an external effector (metal ions).

Scandium(III) catalysis of transimination reactions. Independent and constitutionally coupled reversible processes.

Sc(OTf)(3) efficiently catalyzes the self-sufficient transimination reaction between various types of C=N bonds in organic solvents, with turnover frequencies up to 3600 h(-)(1) and rate accelerations up to 6 x 10(5). The mechanism of the crossover reaction in mixtures of amines and imines is studied, comparing parallel individual reactions with coupled equilibria. The intrinsic kinetic parameters for isolated reactions cannot simply be added up when several components are mixed, and the behavior of the system agrees with the presence of a unique mediator that constitutes the core of a network of competing reactions. In mixed systems, every single amine or imine competes for the same central hub, in accordance with their binding affinity for the catalyst metal ion center. More generally, the study extends the basic principles of constitutional dynamic chemistry to interconnected chemical transformations and provides a step toward dynamic systems of increasing complexity.