Proton dependence

FIGURE 13.18 The appearance of the splitting pattern of two coupled protons depends on their coupling constant J and the chemical shift difference Av between them. As the ratio Av/J decreases, the doublets become increasingly distorted. When the two protons have the same chemical shift, no splitting is observed. 

Snyder NJ, Tabas LB, Berry DM, Duckworth DC, Spry DO and Dantzig AH. Structure-activity relationship of carbacephalosporins and cephalosporins antibacterial activity and interaction with the intestinal proton-dependent dipeptide transport carrier of Caco-2 cells. Antimicrob Agents Chemother 1997 41 1649-57. 

An alternative to most of these mechanisms is the existence of efficient efflux systems, so that toxic concentrations of the drug are not achieved. There are three major families of proton-dependent multidrug efflux systems (1) the major facilitator superfamily, (2) the small multidrug resistance family, and (3) the resistance/nodulation/cell division family (Paulsen et al. 1996). It should be emphasized that several of these systems are involved not with antibiotic efflux but with, for example, acriflavine, chlorhexidine, and crystal violet. An attempt is made only to outline a few salient features of the resistance/nodulation/cell division family that mediates antibiotic efflux, and these are given in Table 3.3 (Nikaido 1996). They consist of a transporter, a linker, and an outer membrane channel. 

Paulsen IT, MH Brown, RA Skurray (1996) Proton-dependent multidrug efflux systems. Microbiol Rev 60 575-608. 

Note that since the profile of the lower adiabatic potential energy surface for the proton depends on the coordinates of the medium molecules, the zeroth-order states and the diabatic potential energy surfaces depend also on the coordinates of the medium molecules. The double adiabatic approximation is essentially used here the electrons adiabatically follow the motion of all nuclei, while the proton zeroth-order states adiabatically follow the change of the positions of the medium molecules. 

This technique is also used as a very common diagnostic tool, since the chemical shift of water protons depends also on the mobility of the water molecules. Therefore, it is possible to discriminate between different tissues. A computer analyses the emissions from the hydrogen nuclei of water molecules... 

Xiao, G., et al. A novel proton-dependent nucleoside transporter, CeCNT3, from Caenorhabditis elegans. Mol. Pharmacol. 2001, 59, 339-348. 

The recovery of neurotransmitters from synaptic clefts and their storage in cytoplasmic vesicles is accomplished by the tandem actions of the secondary transporters in plasma and vesicular membranes. Sodium-dependent symporters mediate neurotransmitter reuptake from synaptic clefts into neurons and glia, whereas proton-dependent antiporters concentrate neurotransmitters from neuronal cytoplasm into synaptic vesicles (Fig. 5-13). 

So what about aromatic protons (<56.0-9.5) aldehyde protons (<59.5—9.6), or even protons oh double, nay triple bonds (<52.5-3.1) All these protons are attached to carbons with n bonds, double or triple bonds, or aromatic systems. The electrons in these n bonds generate their own little local magnetic field. This local field is not spherically symmetric — it can shield or deshield protons depending on where the protons are — it s anisotropic. In Fig. 137, the shielding regions have plusses on them, and deshielding regions have minuses. 

The energy level of hydrated proton depends on the proton concentration. For an acidic proton in Eqn. 3-32 and a basic proton in Eqn. 3-34, the proton levels Hh- are, respectively, given in Eqns. 3-37 and 3-38 ... 

The ability to catalyse the evolution or oxidation of H2 may have been exploited by the earliest life forms as H2 would have been present in the early prebiotic environments. The origins of the proton-dependent chemiosmotic mechanism for ATP synthesis may also reflect the formation of proton gradients created by hydrogenases on either side of the cytoplasmic membrane. In addition, it has been speculated that the coupling of H2 and S metabolisms was also of fundamental importance in the origin of life. These two processes seem intimately coupled in the bifunctional sulfhydrogenase found in Pyrococcus furiosus (a combination of subunits for hydrogenase and sulfite reductase) which can dispose of excess reductant either by the reduction of protons to H2 or S° to H2S (Ma et al. 1993 Pedroni et al. 1995). 

As previously mentioned, allenes can only be obtained by 1,6-addition to acceptor-substituted enynes when the intermediate allenyl enolate reacts regioselectively with an electrophile at C-2 (or at the enolate oxygen atom to give an allenyl ketene acetal see Scheme 4.2). The regioselectivity of the simplest trapping reaction, the protonation, depends on the steric and electronic properties of the substrate, as well as the proton source. Whereas the allenyl enolates obtained from alkynyl enones 22 always provide conjugated dienones 23 by protonation at G-4 (possibly... 

Such a dual reactivity toward protons depends on the difference between proton affinity to an electron and the first ionization potential of an anion-radical. This difference may not be very strong. The fate of the competition between directions a and b in Scheme 1.10 also depends on relative stability of the reaction products. It is reasonable to illustrate the duality with two extreme examples from real synthetic practice. 

For fused silica the magnitude of the EOF is controlled by the pH value of the electrophoretic buffer used. At high pH where the silanol groups are predominantly deprotonated, the EOF is significantly greater than at low pH (pH < 4) where they become protonated. Depending on the specific conditions, the EOF can vary by more than one order of magnitude between pH 2 and pH 12. In nonionic materials such as Teflon and other polymers, electroosmotic flow is also encountered. The electrical double layer in this case results from adsorption of buffer anions to the polymer surface. 

Charge on the oxide surface is established by dissociation (ionization) of the surface hydroxyl groups. The situation corresponds to adsorption or desorption of protons depending on the pH of the solution (Fig. 10.4). These reactions can be trea-... 

The results described above indicate that some of the amino groups were protonated when y-APS films were adsorbed onto most metals from aqueous solutions at pH 10.4. Moreover, the extent of protonation depended on the... 

Scheme 17). The starting material, intermediates, and products may be in various states of protonation, depending on the acidity of the medium and the nature of X and Y. An alternative mechanism, observed much less commonly, is a unimolecular SN1 dissociation (Chapter 5) to an acylium ion (25, Scheme 18), which then reacts with a nucleophile to yield the same final result. 

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