Voltage-sensing domains (VSDs) of voltage-gated potassium (Kv) channels undergo a series of conformational changes upon membrane depolarization, from a down state when the channel is at rest to an up state, all of which lead to the opening of the channel pore. The crystal structures reported to date reveal the pore in an open state and the VSDs in an up state. To gain insights into the structure of the down state, we used a set of experiment-based restraints to generate a model of the down state of the KvAP VSD using molecular-dynamics simulations of the VSD in a lipid bilayer in excess water. The equilibrated VSD configuration is consistent with the biotin-avidin accessibility and internal salt-bridge data used to generate it, and with additional biotin-avidin accessibility data. In the model, both the S3b and S4 segments are displaced approximately 10 A toward the intracellular side with respect to the up-state configuration, but they do not move as a rigid body. Arginine side chains that carry the majority of the gating charge also make large excursions between the up and down states. In both states, arginines interact with water and participate in salt bridges with acidic residues and lipid phosphate groups. An important feature that emerges from the down-state model is that the N-terminal half of the S4 segment adopts a 3(10)-helical conformation, which appears to be necessary to satisfy a complex salt-bridge network.

The photooxidation of 0.6-0.9 ppm alpha-pinene in the presence of a deliquesced thin film of NaNO(3), and for comparison increasing concentrations of NO(2), was studied in a 100 L Teflon(R) chamber at relative humidities from 72-88% and temperatures from 296-304 K. The loss of alpha-pinene and the formation of gaseous products were followed with time using proton transfer mass spectrometry. The yields of gas phase products were smaller in the NaNO(3) experiments than in NO(2) experiments. In addition, pinonic acid, pinic acid, trans-sobrerol and other unidentified products were detected in the extracts of the wall washings only for the NaNO(3) photolysis. These data indicate enhanced loss of alpha-pinene at the NaNO(3) thin film during photolysis. Supporting the experimental results are molecular dynamics simulations which predict that alpha-pinene has an affinity for the surface of the deliquesced nitrate thin film, enhancing the opportunity for oxidation of the impinging organic gas during the nitrate photolysis. This new mechanism of oxidation of organics may be partially responsible for the correlation between nitrate and the organic component of particles observed in many field studies, and may also contribute to the missing source of SOA needed to reconcile model predictions and field measurements. In addition, photolysis of nitrate on surfaces in the boundary layer may lead to oxidation of co-adsorbed organics.

H v 1 voltage-gated proton channels appear to conduct H þ through a voltage sensor domain (VSD) that is homologous to that found in voltage-dependent cation channels and phosphatases. A conserved S4 transmembrane helix that contains a series of at least three Arg residues is integral to the voltage sensing function of all VSD proteins. In contrast to other VSD-containing proteins, voltagegated proton channels possess an additional unique biophysical property: coupling of the transmembrane pH gradient to voltage dependent activation. For both native voltage-gated H þ currents and expressed H v 1 channels, the apparent voltage threshold for H þ current activation (V thr ) shifts linearly~40 mV per log([H þ ]) over at least five pH units. The molecular mechanism of coupling between voltage and the pH gradients represents one of the central mysteries of proton channel function. What constitutes the pH sensor in proton channels and how does it interact with the voltage sensor? DeCoursey and colleagues previously proposed a model for H þ channel gating wherein protonation of discrete sites that are alternatively exposed to either the extra-or intra-cellular milieu regulates the voltage-dependence of channel opening (Cherny et al., 1996); the required first step in this model is deprotonation of an extracellular H þ binding site. In order to identify residues that mediate pH-dependent regulation of voltage sensitivity in H v 1, we performed sitedirected mutagenesis to convert each of the candidate H þ acceptors in the H v 1 VSD to either neutral (alanine or asparagine), basic (arginine) or H þ -titratable (histidine) amino acids. Mutant channels were expressed in HEK-293 cells and V thr was determined under a variety of imposed pH gradients using whole-cell voltage clamp. Surprisingly, charge-neutralizing mutations failed to abrogate pH gradient sensing in H v 1. Our findings are interpreted in the context of the Cherny and DeCoursey model for proton channel gating.