Proton selective

Figure 5.14 Pulse sequence for selective indirecty-spectroscopy. The three proton pulses at the center of the evolution period flip attached protons selectively, resulting in decoupling between distant and attached protons. (Reprinted from J. Magn. Reson. 60, V. Rutar, et ai, 333, copyright (1984), with permission from Academic Press, Inc.)...

Assuming a y3-orientation of the C-1 methine proton (8 4.8), the stereochemistry at other centers can be assigned. For instance, )8-orientation of the C-2 methine proton (8 3.2) is confirmed by nOe interaction between C-2 and C-1 protons. Selective irradiation of C-1 proton (8... 

A potentially useful variant of the synthesis of some Mo(CHR)(NAr)(OR )2 complexes (R=CMe3 or CMe2Ph) consists of addition of two equivalents of a relatively acidic alcohol (R OH=a fluorinated alcohol or phenol) to Mo(NAr)(N-t-Bu)(CH2R)2 [78,79], a variation of the reaction of that type that was first reported in 1989 [80]. The more basic t-butylimido ligand is protonated selectively. This synthesis avoids the addition of triflic acid to Mo(NAr)2(CH2R)2 to give Mo(CHR)(N-t-Bu)(triflate)2(dimethoxyethane), the universal precursor to any Mo(CHR)(NAr)(OR )2 complex. Unfortunately, the method does not appear to succeed when R OH does not have a relatively high pK,. 

Irradiation of long narrowband RF-pulses with frequency offsets between 1 and 20 kHz relative to the resonance frequency of the free protons selectively influences transitions which correspond to the slopes of the broad resonance lines from the bound pool of spins. Therefore, only the spins of protons with restricted motion are saturated, whereas the free protons remain unaffected (e.g., see Refs. 13 and 43). 

The protonation mechanism includes Coulomb electrostatic forces resulting from charged surfaces. The development of surface acidity by the solid phase of the subsurface offers the possibility that solutes having proton-selective organic functional groups can be adsorbed through a protonation reaction. 

Family 10 xylanase Cex from Cellulomonas fimi and family 11 xylanase Bex from Bacillus circulans198,199 (the first being an anti-, the second a syn-protonator) were exposed to deoxynojirimycin-related (80, Scheme 22) and isofagomine analogous (81) xylosidase inhibitors, along with the anti-protonation-selective xylobiosyl... 

Figure 28 Schematic presentation of the relative situation of the different types of radiations used in therapy. Two criteria are considered the physical selectivity and the LET (or radiobiological properties). For the low-LET radiations, the physical selectivity was improved from the historical 200-kV x-rays to cobalt-60 gamma rays and the modern linacs. Even with the linacs today, significant improvement is continuously achieved (IMRT, etc.). Among the low-LET radiation, the proton beams have the best physical characteristics, but one of the issues is the proportion of patients who will benefit from proton irradiation. A similar scale can be drawn for high-LET radiation the heavy-ion beams have a physical selectivity similar to protons. Selection between low- and high-LET radiation is a biological/medical problem it depends on the tumor characteristics, and reliable criteria still need to be established (see text). (From Ref 54.)...

An analogous study has been carried out on ylide formation in cyclic sulfonium salts. Using the conformationally rigid systems in Table 12, relative kinetic acidities for Ho and Heq were determined. The results show that it is possible to remove one proton selectively, and thereby transfer chirality at sulfur to the neighbouring carbon atom (78JA200). As rigidity increases the differential kinetic acidity also increases but this is due not to an increase in overall acidity, rather to a decrease in lability of the axial proton in these... 

The behavior of the influenza A M2 proton-selective channel has been investigated by MD simulation using tetrameric bundles of the transmembrane domains [91]. 

The genome in influenza A and B types is enclosed within an outer lipoprotein envelope (Fig. 17.1). The Ml protein lines the inside of the envelope and is chemically bound to the ribonucleoprotein [14], The Ml protein plays an important role in the mediation of nuclear export of viral ribonucleoproteins and also in virus assembly and budding during the infectious cycle [5, 14, 15], An antigenic protein M2, which functions as a proton-selective ion channel, is present in the viral membrane of influenza A viruses [6, 13], In influenza B, the ion channel activity to aid virus uncoating in the endosome is carried out by the similar integral membrane protein BM2 [11],... 

Under the same conditions, the alcohol 3a is protonated selectively to give 4, bat the corresponding acetate (3b) is protonated with reverse selectivity. 

Struyk AF, Cannon SC. A Na+Channel mutation linked to hypokalemic periodic paralysis exposes a proton-selective gating pore. J. Gen. Physiol. 2007 130 11-20. 

More difficult in BLMs, refined HPTS assays exist to address the special cases of selective transport of protons [11] and electrons [17] in LUVs. In the conventional HPTS assay (Fig. 11.5c), the apparent activity of proton channels decreases with increasing proton selectivity because the rate ofthe disfavored cation (M ) influx influences the detected velocity more than the favored proton efflux. Disfavored potassium influx can, however, be accelerated with the potassium carrier vaiinomycin (Fig. 11.8). Increasing activity in the presence of vaiinomycin identifies proton channels with H > K+ selectivity being at least as high as the maximal measurable increase (in unpolarized LUVs of course, compare Section 11.3.4). Important controls include evidence for low enough vaiinomycin concentrations to exclude activity without the proton channel (due to disfavored H+ efflux). The proton carrier FCCP is often used as complementary additive to confirm M+ > H+ selectivity (e.g. amphotericin B). 

Reduction of the achiral ketodiester 139 gives the racemic lactone 140. Hydrolysis of both ester groups then gives the again achiral hydroxydicarboxylate 141. This compound is prochiral and the two C02 groups are enantiotopic. If one could be protonated selectively by an enantiomerically pure acid, one enantiomer of the monoacid would be formed. This sounds like an improbable event. 

Further evidence of the strength of the chelating agent-cation interaction comes from the H NMR spectra of chelated LiBr systems which have an excess of chelating agent. We have already noted the large up-field shift of the N-CH2- protons of iso-HMTT LiBr in benzene relative to free iso-HMTT (Table II). In several experiments distinct peaks for free and complexed iso-HMTT could be observed. In order to study this system without the interference of the N-CH3 protons, selectively deu-terated iso-HMTT-di8 was used (> 99% N-CD3). Figure 7 shows the... 

The last question to be considered here is whether hydrogen-bonded water molecules account for the extreme proton selectivity (107) observed in F0. First, it is important to note that the proton selectivity observed by Lill et al. (25) in CF0 is not due to an anomalously high proton conductance, but rather to the apparent failure of Na+ or K+ to permeate the channel even at 300-mM electrolyte concentration. CF0 effectively excludes all ions except protons. Exclusion of this sort could result from any of several underlying characteristics of the conductance mechanism. For instance, ions larger than the diameter of the channel might be rejected, or there may be specific binding to a site in the channel (an energy well) and failure of that site to bind other ions (47, 48). 

Rejection of an ion by size seems unlikely for the F0 subunit. That is, such a channel must be at least 2.5 A in diameter to accommodate water molecules, but then would resemble gramicidin, which does not exclude Na+ or K+ ions with diameters significantly less than 2.5 A. A proton selectivity ratio of 100 would be expected rather than 107 (CF0). 

At high FMN concentrations the potential corresponding to the electron transport to ferricyanide decreases because, in addition to the transmembrane electron transport, there also takes place peroxidation of lipid, which leads to the enrichment in protons of the layer adjacent to the membrane. Simultaneously, lipid peroxides formed in BLM and also a reduced form of Q6, which makes the bilayer proton selective. The presence of such compounds in BLM decreases the potential connected with electron transport and transforms the pH gradient into a potential of the opposite sign. 

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