A- and B-type hydrogen bond modifications of potassium hydrogen tartronate
ACCORDING to Speakman1 two types of intermolecular hydrogen bonds between carboxyl groups can be discerned in the crystal structures of acid salts of carboxylic acids. The first, type B, occurs when two carboxyl groups are not symmetry related, and have structural differences sufficient to characterise them, separately, as unionised (−CO2H) and ionised (−CO2−). Thus, any hydrogen bond between these groups must be asymmetric. The second, type A, occurs when two carboxyl groups are related by a symmetry element of the crystal, so that they are effectively equivalent. Thus any hydrogen bond between the groups must be, at least formally, symmetrical.
Electrodiffusion of synaptic receptors: a mechanism to modify synaptic efficacy?
We analysed physical forces that act on synaptic receptor‐channels following the release of neurotransmitter. These forces are: 1) electrostatic interaction between receptors, 2) stochastic Brownian diffusion in the membrane, 3) transient electric field force generated by currents through open channels, 4) viscous drag force elicited by the flowing molecules and 5) strong in‐membrane friction. By considering α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA) type receptors, we show that, depending on the size and electrophoretic charge of the extracellular receptor domain, release of an excitatory neurotransmitter (glutamate) can induce receptor clustering towards the release site on a fast time scale (8–100 ms). This clustering progresses whenever repetitive synaptic activation exceeds a critical frequency (20–100 s‐1, depending on the currents through individual channels). As a result, a higher proportion of the receptors is exposed to higher glutamate levels. This should increase by 50–100% the peak synaptic current induced by the same amount of released neurotransmitter. In order for this mechanism to contribute to long‐term changes of synaptic efficacy, we consider the possibility that the in‐membrane motility of the AMPA receptors is transiently increased during synaptic activity, e.g., through the breakage of receptor anchors in the postsynaptic membrane due to activation of N‐methyl‐d‐aspartic acid receptors. Synapse 35:26–38, 2000. © 2000 Wiley‐Liss, Inc.
Effects of the mineral phase and valence of vanadium on vanadium extraction from stone coal
The influence of roasting on the leaching rate and valence of vanadium was evaluated during vanadium extraction from stone coal. Vanadium in stone coal is hard to be leached and the leaching rate is less than 10% when the raw ore is leached by 4 mol/L H2SO4 at 90°C for 2 h. After the sample is roasted at 900°C for 2 h, the leaching rate of vanadium reaches the maximum, and more than 70% of vanadium can be leached. The crystal of vanadium-bearing mica minerals decomposes and the content of V(V) increases with the rise of roasting temperature from 600 to 900°C, therefore the leaching rate of vanadium increases significantly with the decomposition of the mica minerals. Some new phases, anorthite for example, form when the roasting temperature reaches 1000°C. A part of vanadium may be enwrapped in the sintered materials and newly formed phases, which may impede the oxidation of low valent vanadium and make the leaching rate of vanadium drop dramatically. The leaching rate of vanadium is not only determined by the valence state of vanadium but also controlled by the decomposition of vanadium-bearing minerals and the existence state of vanadium to a large extent.