Insights on the Synthesis of N-Heterocycles Containing Macrocycles and Their Complexion and Biological Properties

Macrocyclic chemistry has been extensively developed over the past several decades. In fact, the architecture of new macrocyclic models has undergone exponential growth to offer molecules with specific properties. In this context, an attempt is made in this study to provide an overview of some synthetic methods allowing the elaboration of N-heterocycles containing macrocycles (imidazole, triazole, tetrazole, and pyrazole), as well as their applications in the complexation of metal cations or as pharmacological agents.

A computational approach on the stereoselective binding of peptides from aqueous medium with endo-functionalized molecular tubes

The need to obtain enantiomerically pure isomers of amino acids and peptides is often realized in the field of biology and in the pharmaceutical industry. Research is underway to devise simple methods for the chiral resolution of amino acids from their racemic mixtures. Inspired by this objective, in our present work, we have computationally shown the possibility of chiral separation of the enantiomeric pairs of two model peptides, namely, (D,L)-aspargine and (D,L)-phenylalanine, in the presence of water. For this purpose, we have used two synthetic supramolecular receptors named host-1a and host-1b, respectively. Molecular dynamics simulations and quantum chemical methods are employed to analyze the structural features and the energy aspects involved in the separation process. The information obtained at the molecular level helps us gain better insights into the key interactions that operate to produce such enantioselectivity. We have also investigated the dynamics and changes in the water structure in the vicinity of the host molecules, both in the presence and absence of the model peptides. The D- and L-isomers of the same peptide undergo complexation with a particular host molecule registering a difference of more than 1.5 kcal mol^-1 (obtained from PMF and MM-PBSA analyses) in their respective energies. This indicates that the chiral separation of the peptides with the help of these endo-fuctionalized molecular tube receptors may be energetically feasible. The connection between the peptide stereochemistry and its interaction with the endo-functionalized hosts would be instrumental in designing novel segregation techniques that can be further extended to separate larger model peptides or proteins.

Gas sensors based on mass-sensitive transducers. Part 2: Improving the sensors towards practical application

Within the framework outlined in the first part of the review, the second part addresses attempts to increase receptor material performance through the use of sensor systems and chemometric methods, in conjunction with receptor preparation methods and sensor-specific tasks. Conclusions are then drawn, and development perspectives for gravimetric sensors are discussed.

Noncovalent Complexes of Cyclodextrin with Small Organic Molecules: Applications and Insights into Host-Guest Interactions in the Gas Phase and Condensed Phase

Cyclodextrins (CDs) have drawn a lot of attention from the scientific communities as a model system for host–guest chemistry and also due to its variety of applications in the pharmaceutical, cosmetic, food, textile, separation science, and essential oil industries. The formation of the inclusion complexes enables these applications in the condensed phases, which have been confirmed by nuclear magnetic resonance (NMR) spectroscopy, X-ray crystallography, and other methodologies. The advent of soft ionization techniques that can transfer the solution-phase noncovalent complexes to the gas phase has allowed for extensive examination of these complexes and provides valuable insight into the principles governing the formation of gaseous noncovalent complexes. As for the CDs’ host–guest chemistry in the gas phase, there has been a controversial issue as to whether noncovalent complexes are inclusion conformers reflecting the solution-phase structure of the complex or not. In this review, the basic principles governing CD’s host–guest complex formation will be described. Applications and structures of CDs in the condensed phases will also be presented. More importantly, the experimental and theoretical evidence supporting the two opposing views for the CD–guest structures in the gas phase will be intensively reviewed. These include data obtained via mass spectrometry, ion mobility measurements, infrared multiphoton dissociation (IRMPD) spectroscopy, and density functional theory (DFT) calculations.

Chiral recognition of carboxylates by a static library of thiourea receptors with amino acid arms

Chiral recognition is based on a large network of very subtle interactions whose outcome is difficult to predict. A combinatorial approach is therefore the most suitable to search for the most efficient receptor and obtain a structure-enantioselectivity correlation. We synthesized a set of 12 receptors constructed with 1,9-diaminoantracene and α-amino acid esters, linked via thiourea groups. The association constants and enantioselectivities for the complexes with mandelate and N-acetylphenylalanine were determined by competitive NMR titrations. Association constants quite regularly depend on the substituents in the receptor structure, but the distribution of enantioselectivities across the library could not easily be rationalized.

Building on Cram's legacy: stimulated gating in hemicarcerands

Donald Cram’s pioneering Nobel Prize-winning work on host–guest molecules led eventually to his creation of the field of container molecules. Cram defined two types of container molecules: carcerands and hemicarcerands. Host–guest complexes of carcerands, called carceplexes, are formed during their synthesis; once a carceplex is formed, the trapped guest cannot exit without breaking covalent bonds. Cram defined a quantity called constrictive binding, arising from the mechanical force that prevents guest escape. The constrictive binding in carceplexes is high. In contrast, hemicarcerands have low constrictive binding and are able to release the incarcerated guests at elevated temperatures without breaking covalent bonds. We have designed molecules that can switch from carcerand to hemicarcerand through a change in structure that we call gating.

The original discovery of gating in container molecules involved our computational studies of a Cram hemicarceplex that was observed to release a guest upon heating. We found that the side portals of this hemicarceplex have multiple thermally accessible conformations. An eight-membered ring that is part of a portal changes from a “chair” to a “boat” structure, leading to the enlargement of the side portal and the release of the guest. This type of gating is analogous to phenomena often observed with peptide loops in enzymes. We refer to this phenomenon as thermally controlled gating.

We have also designed and synthesized redox and photochemically controlled gated hemicarceplexes. Gates are built onto host molecules so that the opening or closing of such gates is stimulated by reducing or oxidizing conditions, or by ultraviolet irradiation. In both cases, the appropriate stimuli can produce a carceplex (closed gates) or hemicarceplex (open gates). A hemicarceplex with closed gates behaves like a carceplex, due to its very high constrictive binding energy. When the gates are opened, constrictive binding is dramatically lowered, and guest entrance and exit become facile. This stimulated switching between open and closed states controls access of the guest to the binding site. The experimental and computational investigations of gated hemicarcerands and several potential applications of gated hemicarceplexes are described in this Account.

A hypothesis for the mechanism of sodium channel opening by batrachotoxin and related toxins

The mechanism of action of one class of sodium channel opening agents (batrachotoxin, veratridine, aconitine and grayanotoxin) is proposed to involve complexation of a triad of agent oxygen atoms with the epsilon-ammonium ion of a channel lysing side chain, holding open the mouth or exit of the ion channel. This idea complements the oxygen triad model derived by structural considerations (Masutani, T., Seyama, I., Narahashi, T. and Iwasa, J. (1981) J. Pharm. Exp. Therap. 217, 812) and extended by crystal structure comparisons (Codding, P.W. (1983) J. Am. Chem. Soc. 105, 3172). The mechanism is based on results for acetylcholine receptor ion channel gating, structure and function, using single group rotation (SGR) theory (cf. Kosower, E.M. (1983) Biochem. Biophys. Res. Commun. 111, 1022 and in press (1983); FEBS Lett. (1983) 155, 245; ibid. 157, 144; Biophys. J. (1983) 45, in press).