Protonation loci

For very neutron-deficient (i.e., proton-rich) nuclei, the Q value for proton emission, Qp, becomes positive. One estimate, based on the semiempirical mass equation, of the line that describes the locus of the nuclei where Qp becomes positive for ground-state decay is shown in Figure 7.11. This line is known as the proton-drip line. Our ability to know the position of this line is a measure of our ability to describe the forces holding nuclei together. Nuclei to the left of the proton dripline in Figure 7.11 can decay by proton emission. 

Figure 7.11 Locus of neutron and proton driplines as predicted by the liquid drop model.

Cam G, Roller DL, Peacock M, Hui SL, Evans WE, Conneally PM, Johnston CC Jr, Foroud T, Econs MJ. 2002. Sibling pair linkage and association studies between peak bone mineral density and the gene locus for the osteoclast-specific subunit of the vacuolar proton pump on chromosome 1 lpl2—13. J Clin Endocrinol Metab 87 3819—24. 

Hirschler (43) and Planck (71) proposed that hydroxyl protons are the locus of carboniogenic activity. Hirschler also proposed that cations influence the geometry and the acidity of protonic sites, and Plank suggested that protons are introduced through hydrolysis of the cation ... 

One of the most intriguing aspects of ABA signal transduction is also one of the most fundamental. Despite intense research, the locus and nature of the guard cell ABA receptor remains elusive. ABA is a weak acid and its protonated form readily diffuses through the lipid bilayer [103]. Thus, in theory, ABA receptors could be intracellular and/or extracellular. Based largely on the results of Hartung [104] and Homberg and Weiler... 

The commonly used methods of pK, determination, such as potentiometric or spectrophotometric titrations, do not generally give information on the sites of protonation. Since it is well known that the chemicals shifts of all atoms, e.g. hydrogen, fluorine, carbon or phosphorus, are related to the amount of charge present at the atom [3], they can be used to determine both the pK, values and the locus of protonation. 

The usefulness of pH-dependent NMR measurements for the determination of pA a values, the order and locus of protonation, and information about structural changes upon protonation has often been shown. Instead of the time-consuming pA"a and NMR measurements in a series of samples of varying pH, the automated NMR-controlled titration method using P, Cd, C, H and F atoms can be employed. 

An atom with an unshared electron pair is not the only locus that confers basicity on an organic compound. The tt bond of an alkene can have the same effect. Later we shall study many reactions in which, as a first step, alkenes react with a strong acid by accepting a proton in the following way ... 

These reactions were further studied at miCTO-ITIES by Quinn et al. who found the reactions to be reversible [204]. In the early days, most electron-transfer reactions were considered heterogeneous, but, as discussed, the locus of the ET step may occur in one of the adjacent phases. More recently, Sugihara et al. showed that the oxidation of ascorbic acid and chlorogenic acid in water by a zinc porphyrin (5,10,15,20-tetraphenylporphirinato zinc(II)) in nitrobenzene occurs on the organic side of the interface, accompanied by a back proton transfer reaction [205]. According to Osakai et al., one case of truly heterogeneous ET reactions was observed with a cadmium tetraphenylporphyrin in nitrobenzene and ferricyanide in water [206]. 

The DCNB-sensitized addition of ammonia to the 1-arylbutadiene 29 occurs selectively to afford the allylamine 29a (Scheme 18). The locus of amine addition is determined, as expected, by formation of the most stable radical intermediate, in this case the 1-phenylallyl radical. Reduction of this radical by DCNB-yields the 1-phenylaUyl anion, which is selectively protonated at the locus of highest charge density, adjacent to the phenyl ring. The nonsymmetric 1,4-diphenylbutadiene 30 affords a 5 1 mixture of the adducts 30a and 30b. 

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