Polarization of protons

The polarity of proton translocation during these transients is as predicted, namely extrusion from mitochondria and uptake into thylakoid membranes and... 

Fio. 3.3.3 Selectivity of cross-polarization demonstrated for polyethylene terephthalate at room temperature and a spinning frequency of = 4090 Hz. (a) Polarization of protonated and unprotonated C nuclei with tcp = 5 ms. (b) Polarization of protonated C nuclei for cp = 50 xs. (c) Polarization of unprotonated nuclei for fcp = 5 ms and subsequent dephasing for fa = 100 p.s under the influence of the dipole-dipole coupling. Adapted... 

Figure 20 Polarizations of protons 1, 7, and / as labels in the sensitized photoreaction of quadricyclane with benzoquinone. The polarizations arise in biradical BR,. Ring closure of BR) yields the product P, rearrangement the biradicals BRj and BR2 from the latter, the product Pj is formed. The CIDNP spectrum is shown at the bottom. Further explanation, see text.

During the polarization of protons in the permanent magnets field Bq (Figure 7) the spin temperature of the proton system approaches lattice temperature Tl- After the Tl temperature is reached, the entropy of the proton system equals ... 

This interpretation has however been questioned by Stehlick and Hausser who have observed the polarization of protons in free radicals by using n.m.r. pulse techniques. They have obtained results for the t-butyl protons in the radicals TTBP, CR and DBNO. Measurement of the ratio of the relaxation times and of the magnitude of the... 

A number of methods can be used to enhance the signals of nuclei with low gyromagnetic ratios through polarization transfer. These methods exploit the nuclear spin polarization of protons which are characterized by much larger Boltzmann population differences than nitrogen. (See also atoms (B 71)). 

Respiratory and photoredox chains are chemiosmotic membrane-located protonmotive systems, having characteristic ->H /2e stoichiometry, and having the same polarity of proton translocation across the membrane for normal forward redox activity as the ATPase has for ATP hydrolysis. 

In the case of proteins, a special variant of the ion polarization, namely, the polarization of protons, may provide an important contribution to this complexity. However, proteins that have so far been investigated at high time resolution do not reveal any contribution from ion or proton polarization. Both... 

Scholes P, Mitchell P and Moyle J (1969) The polarity of proton translocation in some photosynthetic microorganisms, Eur. J. Biochem. 8, 450-454. 

One of the unshared pairs of the hydroxide oxygen is used to form a covalent bond to the positively polar ized proton of hydrogen fluoride The covalent bond betwen H and F in hydrogen fluoride breaks with the pair of electrons in this bond becoming an unshared pair of fluoride ion... 

The S—H bond is less polar than the O—H bond as is clearly seen m the elec trostatic potential maps of Figure 15 7 The decreased polarity of the S—H bond espe cially the decreased positive character of the proton causes hydrogen bonding to be absent m thiols Thus methanethiol (CH3SH) is a gas at room temperature (bp 6°C) whereas methanol (CH3OH) is a liquid (bp 65°C)... 

The ablated vapors constitute an aerosol that can be examined using a secondary ionization source. Thus, passing the aerosol into a plasma torch provides an excellent means of ionization, and by such methods isotope patterns or ratios are readily measurable from otherwise intractable materials such as bone or ceramics. If the sample examined is dissolved as a solid solution in a matrix, the rapid expansion of the matrix, often an organic acid, covolatilizes the entrained sample. Proton transfer from the matrix occurs to give protonated molecular ions of the sample. Normally thermally unstable, polar biomolecules such as proteins give good yields of protonated ions. This is the basis of matrix-assisted laser desorption ionization (MALDI). 

According to a kinetic study which included (56), (56a) and some oxaziridines derived from aliphatic aldehydes, hydrolysis follows exactly first order kinetics in 4M HCIO4. Proton catalysis was observed, and there is a linear correlation with Hammett s Ho function. Since only protonated molecules are hydrolyzed, basicities of oxaziridines ranging from pii A = +0.13 to -1.81 were found from the acidity rate profile. Hydrolysis rates were 1.49X 10 min for (56) and 43.4x 10 min for (56a) (7UCS(B)778). O-Protonation is assumed to occur, followed by polar C—O bond cleavage. The question of the place of protonation is independent of the predominant IV-protonation observed spectroscopically under equilibrium conditions all protonated species are thermodynamically equivalent. 

The reversed polarity of the double bond is induced by a n electron-accepting substituent A (A = C=0, C=N, NO2) the carbon and proton in the p-position are deshielded (-A/effect, larger shifts). These substituents have analogous effects on the C atoms of aromatic and heteroaromatic rings. An electron donor D (see above) attached to the benzene ring deshields the (substituted) a-C atom (-/ effect). In contrast, in the ortho and para positions (or comparable positions in heteroaromatic rings) it causes a shielding +M effect, smaller H and C shifts), whereas the meta positions remain almost unaffected. 

Nuclear dipole-dipole interaction is a veiy important relaxation mechanism, and this is reflected in the relationship between 7, and the number of protons bonded to a carbon. The motional effect is nicely shown by tbe 7 values for n-decanol, which suggest that the polar end of the molecule is less mobile than the hydrocarbon tail. Comparison of iso-octane with n-decanol shows that the entire iso-octane molecule is subject to more rapid molecular motion than is n-decanol—compare the methyl group T values in these molecules. 

It has been proposed that protonation or complex formation at the 2-nitrogen atom of 14 would enhance the polarization of the r,6 -7i system and facilitate the rearrangement leading to new C-C bond formation. The equilibrium between the arylhydrazone and its ene-hydrazine tautomer is continuously promoted to the right by the irreversible rearomatization in stage II of the process. The indolization of arylhydrazones on heating in the presence of (or absence of) solvent under non-catalytic conditions can be rationalized by the formation of the transient intermediate 14 (R = H). Under these thermal conditions, the equilibrium is continuously pushed to the right in favor of indole formation. Some commonly used catalysts in this process are summarized in Table 3.4.1. 

Figure 8. Three types of polarization of Mn02 (1) J]c (H+ solid), due to proton diffusion in solid (2) rja, due to the solution-solid interface (3) 7/t (ApH), due to a pH change of the electrolyte in the pores.

Provided that polarization arises predominantly as a result of the difference in the gr-factors of the components of the radical pair, similar considerations apply to systems where several coupled nuclei are polarized. Net polarization, A or E, results. This is indicated in Fig. 3b for the AB system of protons in a hypothetical product R2CHA. CHbRj... 

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