The measurement of the rotational state distribution of a velocity-selected, buffer-gas-cooled beam of ND3 is described. In an apparatus recently constructed to study cold ion-molecule collisions, the ND3 beam is extracted from a cryogenically cooled buffer-gas cell using a 2.15 m long electrostatic quadrupole guide with three 90° bends. (2+1) resonance enhanced multiphoton ionization spectra of molecules exiting the guide show that beams of ND3 can be produced with rotational state populations corresponding to approximately T(rot) = 9-18 K, achieved through manipulation of the temperature of the buffer-gas cell (operated at 6 K or 17 K), the identity of the buffer gas (He or Ne), or the relative densities of the buffer gas and ND3. The translational temperature of the guided ND3 is found to be similar in a 6 K helium and 17 K neon buffer-gas cell (peak kinetic energies of 6.92(0.13) K and 5.90(0.01) K, respectively). The characterization of this cold-molecule source provides an opportunity for the first experimental investigations into the rotational dependence of reaction cross sections in low temperature collisions.

The time-and concentration-dependence of agonist-induced ion currents through postsynaptic receptors is often remarkably complex, as is the modulation of these currents by other solutes such as anesthetics. Traditionally, kinetic models have been developed that involve agonist binding and conformational transitions among a manifold of protein conformational states engineered to reproduce the complexity of the electrophysiological results. In the present work, an alternative model is proposed that minimizes the number of conformational states while additionally incorporating effects of adsorption of agonist and nonbinding compounds such as anesthetics to the bilayer in which these intrinsic membrane proteins are embedded. Adsorption of these aqueous solutes alters bilayer physical properties, which in turn can distort the protein conformational free energy landscape, and thus alter the rate constants of protein conformational transitions. The complexity of the predicted ion currents then arises from the time-dependence of solute adsorption, resulting in strongly time-dependent transition rate ''constants''. If only nonbinding solutes are present, the model simplifies considerably. Best fits with respect to a small set of parameters of the predicted current traces are found to be in excellent agreement with previously measured currents in GABA A receptors induced by a broad range of supraclinical concentrations of isoflurane and sevoflurane.

No abstract

No abstract