Water wire mechanism

A review of the hydrolysis of azo ethers indicates that they react by three mechanisms involving a ring-protonated azo ether. The most common is the concerted protonation and cleavage of the C-O bond transferring the OR group to an H+ q complex, described earlier. The second is the concerted water wire mechanism where three water molecules attack the RO oxygen of the protonated azo compound via the transition state shown in Scheme 13. The third is the three-step mechanism, which is shown in Scheme 14. 

Conduction along water wires may as well be the dominant mechanism in the permeation of protons in channels an MD study of proton transport through a gramicidin channel can be found in, for example, [156]. 

In contrast to the LS3 pore, the water molecules were frozen in the tetrameric LS2 pore, with diffusion coefficients of zero. They were found to be aligned antiparallel to the helix dipole caused by the orientation of the hydroxyl groups of the serine residues. This enabled formation of a water wire network important for the transport of protons by the proton wire or Grotthiis mechanism. 

The results summarized above suggest important mechanistic differences in the long-range relay mechanism of protons by water wires embedded in non-polar (NP) and polar (GA) channels, respectively. Whereas GA constitutes an example of of fast proton equilibration by water-filled pores, NP channels may... 

Figure 4. Leakage vs, conduction models for the permeation of protons. Left transient water wires form very infrequently in the nonpolar environment provided by lipid bilayers, but when they do, they could translocate just one proton very rapidly before breaking up only the hop step ofGrotthuss takes place. Right in polar channels such as GA, water wires are much more long-lived, which is consistent with their rapid relay of proton via a complete Grotthuss mechanism involving both hop and turn steps (l(f s ).

There have been several models for the permeation mechanism of ionic molecules. All models propose the ion permeation with a guide of water molecules. For example, the water wire is widely believed to help a proton transport across the membrane. Investigating the water wire is still a challenging subject for MD simulations, though a few studies have been reported [61, 62]. However, permeability of neutral small molecules across the bilayers can now be evaluated by MD simulations. Here, we demonstrate an evaluation of water permeability across DPPC and DPhPC bUayers. 

Induction step - the field induced membrane potential difference increase reached a critical value at the polar position facing the electrodes, and this gave local defects (may be due to kinks in the lipid chains). A mechanical stress was present with a magnitude that depends on the buffer composition. These defects could be associated with water wires. Due to the potential charging time, the structural transition of the membrane affected a defined cap size on the cell surface. 

Concerning the mechanisms in play in water wires inside protein cavities, it is sometimes difficult to decide whether protons are being transferred in one direction or the hydroxyl anion OH in the opposite direction. The difficulty lies in the specific and unknown physicochemical nature of the H" " (or OH ) pathways in a protein cavity that is obviously distinct from bulk water. In this context, it is difficult to define what proton transfer really means. Are protons transferred in a sequence of hopping steps between waters and (de)protonatable polar residues, or is it also possible that a protonated water cluster diffuses in a protein cavity for a relatively short distance What and where is the rate limiting step for transfer Are protons just transferred between water molecules in the water wire Can polar residues shuttle protons between water molecules These questions have been asked and are essential to the full understanding of proton transfer processes in this context [93]. 

C. Fumaric acid from furfural. Place in a 1-litre three-necked flask, fitted with a reflux condenser, a mechanical stirrer and a thermometer, 112 5 g. of sodium chlorate, 250 ml. of water and 0 -5 g. of vanadium pentoxide catalyst (1), Set the stirrer in motion, heat the flask on an asbestos-centred wire gauze to 70-75°, and add 4 ml. of 50 g. (43 ml.) of technical furfural. As soon as the vigorous reaction commences (2) bvi not before, add the remainder of the furfural through a dropping funnel, inserted into the top of the condenser by means of a grooved cork, at such a rate that the vigorous reaction is maintained (25-30 minutes). Then heat the reaction mixture at 70-75° for 5-6 hours (3) and allow to stand overnight at the laboratory temperature. Filter the crystalline fumaric acid with suction, and wash it with a little cold water (4). Recrystallise the crude fumaric acid from about 300 ml. of iif-hydrochloric acid, and dry the crystals (26 g.) at 100°. The m.p. in a sealed capillary tube is 282-284°. A further recrystaUisation raises the m.p. to 286-287°. 

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