The net electrostatic potential and hydration of ABCG2 affect substrate transport.

ABCG2 is a medically important ATP-binding cassette transporter with crucial roles in the absorption and distribution of chemically-diverse toxins and drugs, reducing the cellular accumulation of chemotherapeutic drugs to facilitate multidrug resistance in cancer. ABCG2's capacity to transport both hydrophilic and hydrophobic compounds is not well understood. Here we assess the molecular basis for substrate discrimination by the binding pocket. Substitution of a phylogenetically-conserved polar residue, N436, to alanine in the binding pocket of human ABCG2 permits only hydrophobic substrate transport, revealing the unique role of N436 as a discriminator. Molecular dynamics simulations show that this alanine substitution alters the electrostatic potential of the binding pocket favoring hydration of the transport pore. This change affects the contact with substrates and inhibitors, abrogating hydrophilic compound transport while retaining the transport of hydrophobic compounds. The N436 residue is also required for optimal transport inhibition of ABCG2, as many inhibitors are functionally impaired by this ABCG2 mutation. Overall, these findings have biomedical implications, broadly extending our understanding of substrate and inhibitor interactions.

Membrane cholesterol regulates inhibition and substrate transport by the glycine transporter, GlyT2.

Membrane cholesterol binds to and modulates the function of various SLC6 neurotransmitter transporters, including stabilizing the outward-facing conformation of the dopamine and serotonin transporters. Here, we investigate how cholesterol binds to GlyT2 (SLC6A5), modulates glycine transport rate, and influences bioactive lipid inhibition of GlyT2. Bioactive lipid inhibitors are analgesics that bind to an allosteric site accessible from the extracellular solution when GlyT2 adopts an outward-facing conformation. Using molecular dynamics simulations, mutagenesis, and cholesterol depletion experiments, we show that bioactive lipid inhibition of glycine transport is modulated by the recruitment of membrane cholesterol to a binding site formed by transmembrane helices 1, 5, and 7. Recruitment involves cholesterol flipping from its membrane orientation, and insertion of the 3' hydroxyl group into the cholesterol binding cavity, close to the allosteric site. The synergy between cholesterol and allosteric inhibitors provides a novel mechanism of inhibition and a potential avenue for the development of potent GlyT2 inhibitors as alternative therapeutics for the treatment of neuropathic pain and therapeutics that target other SLC6 transporters.

Hydroxy Groups Enhance [2]Rotaxane Anion Binding Selectivity.

We report the synthesis of two [2]rotaxanes containing an interlocked three dimensional binding cavity formed from a pyridinium bis(amide) axle component containing two phenol donors, and an isophthalamide based macrocycle. In the competitive solvent mixture 1 : 1 CDCl3 : CD3 OD, one of the receptors exhibits a much higher selectivity preference for chloride than an analogous rotaxane without the hydroxy groups. X-ray crystal structures reveal the chloride anion guest encapsulated within the interlocked binding cavity, though not all of the hydrogen bond donors are utilised. Computational semi-empirical simulations indicate that secondary intermolecular interactions occur between the axle hydroxy hydrogen bond donors and the [2]rotaxane macrocycle components, contributing to a more preorganised binding pocket, which may be responsible for the observed enhanced selectivity.

Using Synergistic Multiple Dynamic Bonds to Construct Polymers with Engineered Properties.

Dynamic bonds have achieved significant attention for their ability to impart fascinating properties to polymeric materials, such as high mechanical strength, self-healing, shape memory, 3D printability, and conductivity. Incorporating multiple dynamic bonds into polymer systems affords an attractive and efficient approach to endow multiple functionalities. This mini-review focuses on the use of complementary dynamic interactions to control the properties of soft materials. Owing to the diversity in dynamic chemistries that can be explored, the scope of this article is restricted to polymers and does not include colloids, amphiphiles, liquid crystals, or biological soft matter.

The Fate of the Peroxyl Radical in Autoxidation: How Does Polymer Degradation Really Occur?

Bolland and Gee's basic autoxidation scheme (BAS) for lipids and rubbers has long been accepted as a general scheme for the autoxidation of all polymers. This scheme describes a chain process of initiation, propagation, and termination to describe the degradation of polymers in the presence of O2. Central to this scheme is the conjecture that propagation of damage to the next polymer chain occurs via hydrogen atom transfer with a peroxyl radical. However, this reaction is strongly thermodynamically disfavored for all but unsaturated polymers, where the product allylic radical is resonance-stabilized. Paradoxically, there is no denying that the autocatalytic degradation and oxidation of saturated polymers still occurs. Critical analysis of the literature, described herein, has begun to unravel this mystery. One possibility is that the BAS still holds for saturated polymers but only at unsaturated defect sites, where H transfer is thermodynamically favorable. Another is that peroxyl termination rather than H transfer is dominant. If this were the case, tertiary peroxyl radicals (formed at quaternary centers or quaternary branching defects) may terminate to form alkoxy radicals, which can much more readily undergo chain transfer. This process would lead to the creation of hydroxy groups on the degraded polymer. On the other hand, primary and secondary peroxyl radicals would terminate to form nonradical products and halt further degradation. As a result, under this scenario the degree of branching and substitution would have a major effect on polymer stability. Herein we survey studies of polymer degradation products and of the effect of polymer structure on stability and show that indeed peroxyl termination is competitive with peroxyl transfer and possibly dominant under some conditions. It is also feasible that oxygen may not be the only reactive atmospheric species involved in catalyzing polymer degradation. Herein we outline plausible mechanisms involving ozone, hydroperoxyl radical, and hydroxyl radical that have all been suggested in the literature and can account for the experimentally observed formation of hydroperoxides without invoking peroxyl transfer. We also show that oxygen itself has even been reported to slow the degradation of poly(methyl methacrylate)s, which might be expected if peroxyl radicals are unreactive toward hydrogen transfer. Discrepancies between the rate of oxidation and the rate of degradation have been observed for polyolefins and also support the counterintuitive notion that oxygen stabilizes these polymers against degradation. We show that together these studies support alternative mechanisms for polymer degradation. A thorough assessment of kinetic studies reported in the literature indicates that they are limited by their propensity to use models based on the BAS, disregarding the chemical differences intrinsic to each class of polymer. Thus, we propose that further work must be done to fully grasp the complex mechanism of polymer degradation under ambient conditions. Nonetheless, our analysis of the literature points to measures that can be used to enhance or prevent polymer degradation and indicates that we should focus beyond just the role of oxygen toward the specific chemical nature and environment of the polymer at hand.

Harnessing electrostatic catalysis in single molecule, electrochemical and chemical systems: a rapidly growing experimental tool box.

Static electricity is central to many day-to-day practical technologies, from separation methods in the recycling of plastics to transfer inks in photocopying, but the exploration of how electrostatics affects chemical bonding is still in its infancy. As shown in the Companion Tutorial, the presence of an appropriately-oriented electric field can enhance the resonance stabilization of transition states by lowering the energy of ionic contributors, and the effect that follows on reaction barriers can be dramatic. However, the electrostatic effects are strongly directional and harnessing them in practical experiments has proven elusive until recently. This tutorial outlines some of the experimental platforms through which we have sought to translate abstract theoretical concepts of electrostatic catalysis into practical chemical technologies. We move step-wise from the nano to the macro, using recent examples drawn from single-molecule STM experiments, surface chemistry and pH-switches in solution chemistry. The experiments discussed in the tutorial will educate the reader in some of the viable solutions to gain control of the orientation of reagents in that field; from pH-switchable bond-dissociations using charged functional groups to the use of surface chemistry and surface-probe techniques. All of these recent works provide proof-of-concept of electrostatic catalysis for specific sets of chemical reactions. They overturn the long-held assumption that static electricity can only affect rates and equilibrium position of redox reactions, but most importantly, they provide glimpses of the wide-ranging potential of external electric fields for controlling chemical reactivity and selectivity.

Can electrostatic catalysis of Diels-Alder reactions be harnessed with pH-switchable charged functional groups?

Quantum-chemical calculations at the M06-2X/6-31+G(d,p) and G3(MP2)CC levels of theory are used to assess the feasibility of harnessing charged functional groups to electrostatically catalyse Diels-Alder reactions and alter their regio selectivity. For the reaction of the polar diene 2-pyrone with substituted cyclopentene, pH switches of nearly 60 kJ mol-1 are observed in the gas-phase. To switch regioselectivity however it is necessary to toggle between negatively and positively charged functional groups. With the 6-membered cyclohexene derivatives, similar pH-switches are observed but this time an opportunity to pH-switch diastereomeric selectivity is also observed due to the asymmetry of the transition state. When 2-pyrone was replaced with a non-polar diene, cyclopentadiene, pH switches were understandably smaller but still substantial (ca. 15 kJ mol-1). Likewise pH switches are attenuated by solvent but remain substantial (ca. 30 kJ mol-1) in toluene and synthetically useful (ca. 15 kJ mol-1) even in moderately low polar solvents such as dichloromethane.

7-Selenabicyclo[2.2.1]heptane.

Thermolysis of a benzene solution of N-[4-(p-(methoxybenzyl)seleno)cyclohexanoyl]-N,S-dimethyldithiocarbonate affords the hitherto unknown 7-selenabicyclo[2.2.1]heptane in 48% conversion and in 20% yield after chromatography. G3(MP2)-RAD calculations predict a rate constant of 5 × 10(4) s(-1) at 80 °C (3.8 × 10(6) s(-1) at 200 °C) for the intramolecular homolytic substitution process involved in this cyclization.

Understanding (the lack of) homolytic substitution chemistry of sulfones.

High level calculations suggest that homolytic substitution (S(H)2) by alkyl radicals at sulfur proceeds through a mechanism that is assisted and dominated by LP → SOMO interactions; in the absence of these interactions, S(H)2 chemistry at sulfur is predicted to be virtually impossible. G3(MP2)-RAD calculations suggest that cyclization of the tert-butylsulfonylbutyl radical 2 (n = 2) proceeds with a rate constant of 1.7 × 10(-24) s(-1) at 80°, some 28 orders of magnitude slower than its sulfide cousin (n = 0).

Intramolecular homolytic substitution of sulfinates and sulfinamides--a computational study.

Ab initio and density functional theory (DFT) calculations predict that intramolecular homolytic substitution by alkyl radicals at the sulfur atom in sulfinates proceeds through a smooth transition state in which the attacking and leaving radicals adopt a near collinear arrangement. When forming a five-membered ring and the leaving radical is methyl, G3(MP2)-RAD//ROBHandHLYP/6-311++G(d,p) calculations predict that this reaction proceeds with an activation energy (ΔE(1)(‡)) of 43.2 kJ mol(-1). ROBHandHLYP/6-311++G(d,p) calculations suggest that the formation of five-membered rings through intramolecular homolytic substitution by aryl radicals at the sulfur atom in sulfinates and sulfinamides, with expulsion of phenyl radicals, proceeds with the involvement of hypervalent intermediates. These intermediates further dissociate to the observed products, with overall energy barriers of 45-68 kJ mol(-1), depending on the system of interest. In each case, homolytic addition to the phenyl group competes with substitution, with calculated barriers of 51-78 kJ mol(-1). This computational study complements and provides insight into previous experimental observations.