Anion Binding Modes In Meso-substituted Hexapyrrolic Calix[4]pyrrole Isomers

We report on the synthesis of a new receptor for anions, meso-substituted hexapyrrolic calix[4]pyrrole 1. The calix[4]pyrrole\u27s core features two additional pyrrole side-arms suspended above or below the calix[4]pyrrole core. This hexapyrrolic calix[4]pyrrole 1 is formed as cis- and trans-configurational isomers, the structures of which have been determined by single crystal X-ray diffraction. The anion binding experiments revealed interesting difference in the binding mode: The cis-1 isomer binds anions in a mixed binding mode featuring a combination of hydrogen bonding and anion-pi interactions resulting in an unexpected strong binding. On the other hand, the trans-1 isomer displays only hydrogen bonding and lower affinity for anions. This is unexpected as one would assume both isomers to display the same binding modes. Overall, the titrations of 1 using UV spectrophotometry and NMR titrations by anions reveal that cis-isomer 1 displays higher affinity (10(5)-10(6) M-1) and cross-reactivity for anions, while the trans-isomer 1 shows a more selective response to anions. Such differences in binding mode in configurational isomers are so far unexplored and a feature deserving further study

Anion Binding Modes in <i>meso</i>-Substituted Hexapyrrolic Calix[4]pyrrole Isomers

We report on the synthesis of a new receptor for anions, <i>meso</i>-substituted hexapyrrolic calix[4]­pyrrole <b>1</b>. The calix[4]­pyrrole’s core features two additional pyrrole side-arms suspended above or below the calix[4]­pyrrole core. This hexapyrrolic calix[4]­pyrrole <b>1</b> is formed as <i>cis</i>- and <i>trans</i>-configurational isomers, the structures of which have been determined by single crystal X-ray diffraction. The anion binding experiments revealed interesting difference in the binding mode: The <i>cis</i>-<b>1</b> isomer binds anions in a mixed binding mode featuring a combination of hydrogen bonding and anion−π interactions resulting in an unexpected strong binding. On the other hand, the <i>trans</i>-<b>1</b> isomer displays only hydrogen bonding and lower affinity for anions. This is unexpected as one would assume both isomers to display the same binding modes. Overall, the titrations of <b>1</b> using UV spectrophotometry and NMR titrations by anions reveal that <i>cis</i>-isomer <b>1</b> displays higher affinity (10⁵–10⁶ M^–1) and cross-reactivity for anions, while the <i>trans</i>-isomer <b>1</b> shows a more selective response to anions. Such differences in binding mode in configurational isomers are so far unexplored and a feature deserving further study

A photoreactive analog of allopregnanolone enables identification of steroid-binding sites in a nicotinic acetylcholine receptor

Many neuroactive steroids potently and allosterically modulate pentameric ligand-gated ion channels, including GABAA receptors (GABAAR) and nicotinic acetylcholine receptors (nAChRs). Allopregnanolone and its synthetic analog alphaxalone are GABAAR-positive allosteric modulators (PAMs), whereas alphaxalone and most neuroactive steroids are nAChR inhibitors. In this report, we used 11β-(p-azidotetrafluorobenzoyloxy)allopregnanolone (F4N3Bzoxy-AP), a general anesthetic and photoreactive allopregnanolone analog that is a potent GABAAR PAM, to characterize steroid-binding sites in the Torpedo α2βγδ nAChR in its native membrane environment. We found that F4N3Bzoxy-AP (IC50 = 31 μm) is 7-fold more potent than alphaxalone in inhibiting binding of the channel blocker [3H]tenocyclidine to nAChRs in the desensitized state. At 300 μm, neither steroid inhibited binding of [3H]tetracaine, a closed-state selective channel blocker, or of [3H]acetylcholine. Photolabeling identified three distinct [3H]F4N3Bzoxy-AP-binding sites in the nAChR transmembrane domain: 1) in the ion channel, identified by photolabeling in the M2 helices of βVal-261 and δVal-269 (position M2-13'); 2) at the interface between the αM1 and αM4 helices, identified by photolabeling in αM1 (αCys-222/αLeu-223); and 3) at the lipid-protein interface involving γTrp-453 (M4), a residue photolabeled by small lipophilic probes and promegestone, a steroid nAChR antagonist. Photolabeling in the ion channel and αM1 was higher in the nAChR-desensitized state than in the resting state and inhibitable by promegestone. These results directly indicate a steroid-binding site in the nAChR ion channel and identify additional steroid-binding sites also occupied by other lipophilic nAChR antagonists

Allyl <i>m</i>-Trifluoromethyldiazirine Mephobarbital: An Unusually Potent Enantioselective and Photoreactive Barbiturate General Anesthetic

We synthesized 5-allyl-1-methyl-5-(<i>m</i>-trifluoromethyl-diazirynylphenyl)­barbituric acid (<b>14</b>), a trifluoromethyldiazirine-containing derivative of general anesthetic mephobarbital, separated the racemic mixture into enantiomers by chiral chromatography, and determined the configuration of the (+)-enantiomer as <i>S</i> by X-ray crystallography. Additionally, we obtained the ³H-labeled ligand with high specific radioactivity. <i>R</i>-(−)-<b>14</b> is an order of magnitude more potent than the most potent clinically used barbiturate, thiopental, and its general anesthetic EC₅₀ approaches those for propofol and etomidate, whereas <i>S</i>-(+)-<b>14</b> is 10-fold less potent. Furthermore, at concentrations close to its anesthetic potency, <i>R</i>-(−)-<b>14</b> both potentiated GABA-induced currents and increased the affinity for the agonist muscimol in human α1β2/3γ2L GABA_A receptors. Finally, <i>R</i>-(−)-<b>14</b> was found to be an exceptionally efficient photolabeling reagent, incorporating into both α1 and β3 subunits of human α1β3 GABA_A receptors. These results indicate <i>R</i>-(−)-<b>14</b> is a functional general anesthetic that is well-suited for identifying barbiturate binding sites on Cys-loop receptors