Self-association of organic solutes in solution: a NEXAFS study of aqueous imidazole.

N K-edge near-edge X-ray absorption fine-structure (NEXAFS) spectra of imidazole in concentrated aqueous solutions have been acquired. The NEXAFS spectra of the solution species differ significantly from those of imidazole monomers in the gas phase and in the solid state of imidazole, demonstrating the strong sensitivity of NEXAFS to the local chemical and structural environment. In a concentration range from 0.5 to 8.2 mol L(-1) the NEXAFS spectrum of aqueous imidazole does not change strongly, confirming previous suggestions that imidazole self-associates are already present at concentrations more dilute than the range investigated here. We show that various types of electronic structure calculations (Gaussian, StoBe, CASTEP) provide a consistent and complete interpretation of all features in the gas phase and solid state spectra based on ground state electronic structure. This suggests that such computational modelling of experimental NEXAFS will permit an incisive analysis of the molecular interactions of organic solutes in solutions. It is confirmed that microhydrated clusters with a single imidazole molecule are poor models of imidazole in aqueous solution. Our analysis indicates that models including both a hydrogen-bonded network of hydrate molecules, and imidazole-imidazole interactions, are necessary to explain the electronic structure evident in the NEXAFS spectra.

Edit, cut and paste in the nicotinic acetylcholine receptor gene family of Drosophila melanogaster.

Nicotinic acetylcholine receptors (nAChRs) are important for fast synaptic cholinergic transmission. They are targets of drugs/chemicals for human and animal health as well as for pest control. With the advent of genome sequencing, entire nAChR gene families have now been described for vertebrates and invertebrates. Mostly, these are extensive with a large number of distinct subunits, making possible many nAChR subtypes differing in transmitter affinity, channel conductance, ion selectivity, desensitization, modulation and pharmacology. The smallest nAChR gene family to date is that of the fruit fly, Drosophila melanogaster, with only 10 members. This apparently compact family belies its true diversity as 4 of the 10 subunits show alternative splicing. Also, using Drosophila, A-to-I pre-mRNA editing has been demonstrated for the first time in nAChRs. Such is the extent of this variation, that one subunit alone (Dalpha6) can potentially generate far more isoforms than seen in entire gene families from other species. We present here three-dimensional models constructed for insect nAChRs, which show that many variations introduced by alternative splicing and RNA editing may influence receptor function.

Wild-type and insecticide-resistant homo-oligomeric GABA receptors of Drosophila melanogaster stably expressed in a Drosophila cell line.

RDL is an ionotropic GABA receptor subunit, a product of the Rdl gene, originally identified in the Maryland strain of Drosophila melanogaster. Here, we report the generation of a Drosophila melanogaster cell line (S2-RDLA302S) stably expressing a mutated, dieldrin-resistant (A302S) form of RDL. The properties of this dieldrin-resistant, homo-oligomeric receptor have been compared with those of the stably expressed, wild-type form (S2-RDL). Using these stable lines, a striking reduction in sensitivity to both picrotoxinin and dieldrin was observed for responses to GABA of S2-RDLA302S compared to S2-RDL. To determine if these stable insect cell lines generate results similar to those obtained by transient expression in Xenopus laevis oocytes, we have examined the actions of two widely used convulsants, EBOB and TBPS, and a recently developed convulsant BIDN, on RDL-mediated GABA responses in the two expression systems. In both oocytes and S2 cells, the three convulsants suppressed the amplitude of responses to GABA. Thus, in accord with earlier work on agonist and allosteric sites, the S2-RDL cell line is found to yield similar pharmacological results to those obtained in transient expression studies. Stable cell lines are now available expressing susceptible and resistant forms of an ionotropic receptor by GABAergic insecticides.