Proton deficiency

Stmcture Elucidator is not used for every unknown impurity or degradant problem in the author s laboratory, the program package does serve as a very useful tool when particularly challenging unknown stmctures have to be identified, especially if those stmctures are relatively proton deficient in nature. 

Three-center bonding in the carbohydrates and the amino acids is therefore a result of excess acceptors over donors, or of proton deficiency. Hence the analogy with the three-center electron bond from electron deficiency in the boron hydrides [76]. 

Due to proton deficiency, crystal structures of amino acids display a much higher proportion of three-center hydrogen bonds. Their geometries, given in Th-bles 8.6 and 8.7, are based on neutron diffraction data, of which a relatively large number is available for this class of biological molecules. 

The three-center bonds represent — 70% of the total number of hydrogen bonds in the crystal structures surveyed (Thble 2.3). This is a significantly higher proportion than in the other biological molecules, and was attributed to proton deficiency, which occurs because the amino acids form zwitterionic crystal structures where the predominant hydrogen bonding is between the -NH3 and the... 

Proteins contain, on average, more acceptor than donor sites [596]. Similar proton deficiency in the amino acid zwitterion crystal structures results in the formation of three-center hydrogen bonds rather than in unsatisfied acceptor sites [74] (Part I A, Chap. 2.6). There is less flexibility in the orientation of hydrogen-bond donor and acceptor groups in proteins which would lead to a relatively larger number of unsatisfied acceptor sites. Some of the more unsymmetrical three-center bonds which might have been missed in the survey [596] because of the 3.5 A X - A cut-off limit will also contribute to reduce the number of unsatisfied acceptors in side-chains. 

There are alternative algebraic functions. Instead of writing the electro-neutrality equation, we can derive a relation called the proton condition. If we made our solution from pure H2O and HB, after equilibrium has been reached the number of excess protons must be equal to the numtier of proton deficiencies. Excess of deficiency of protons is counted with respect to a zero level reference condition representing the species that were added, that is, H2O and HB. The number of excess protons is equal to [H ] the number of proton deficiencies must equal [B ] -I-[OH ]. This proton condition gives, as in equation iva, [H" ] = [B ] -f lOH-]. 

The reference level is defined by the composition of a pure solution of HA in H2O (/ = 0 [ANC] = 0), which is defined by the proton condition, [H l = [A-] -i- [OH ]. (In this and subsequent equations, the charge type of the eicid is unimportant the equation defining the net proton excess or deficiency can always be derived from a combination of the concentration condition and the condition of electroneutrality.) Thus in a solution containing a mixture of HA and NaA, [ANC] is a conservative capacity parameter. It must be expressed in concentrations (and not activities). Addition of HA (a species defining the reference level) does not change the proton deficiency and thus does not affect [ANC]. 

When nuclei are very proton-deficient or very neutron-deficient, an excess particle may boil off, that is, be ejected directly from the nucleus. These decay modes are called neutron emission and proton emission, respectively, and move nuclides down or to the left in Figure 19.1. Finally, certain unstable nuclei undergo spontaneous fission, in which they split into two nuclei of roughly equal size. Nuclear fission will be discussed in more detail in Section 19.5. 

The primary fission products A and B are formed with a proton-to-neutron ratio which is less than that for stable nuclei of the same mass. The fragments are therefore proton deficient and undergo a series of j3 -decays in a chain until they attain stability. Each mode of fission, therefore, leads to two such chains. For example, mass chains 133 and 101 may be complementary masses in the above fission process where v the average number of neutrons emitted per fission is 2. In this case, the mass chains are as follows... 

The proton deficiency between the anion and the neighbouring water molecule means that there can be no repulsion between opposed O—H bonds, as there is before H30 -H20 proton transfers, although lone-pair repulsion should play a part. If the only driving force for rotation is taken to be the derivative of the energy of ion-dipole interaction with respect to the angular displacement, then numerical calculations sug-gestMi29 proton-jumping contribution of OH" will be approximately half that of as is indeed the case. No estimates have been... 

At first, the proton potential changes only slightly as titrator is continuously added. However, as the point is approached where a stoichiometrically equivalent amount of hydrochloric acid (in this case 10 mmol) has been added to the sodium hydroxide solution, a drastic increase of proton potential occurs. At the equivalence point, there is no proton deficiency anymore, and the proton reservoir is completely filled. There is only an aqueous solution of Na" and Cl ions that has almost no influence upon the proton potential which is then equal to the neutral value of —40 kG of pure water. If we continue to add hydrochloric acid to the neutralized... 

Let us now turn to the titration of the base of a weak alkaline pair with the acid of a strong acidic pair using the example of titration of 100 mL of a 0.1 M ammonia solution with the standard solution of hydrochloric acid already used above. At first, the proton potential is —64 kG, which we have already calculated in Sect. 7.3 with the help of Eq. (7.7). The very low proton fill level of 1.2 % in the reservoir NH4 /NH3 is just compensated for by the proton deficiency (which is caused by the OH ions produced by the proton transfer according to NH3 -I- H2O NH + OH ) so that the total fill level in the aqueous solution equals zero. The relatirai in Fig. 7.4, or more exactly, a section of it (Fig. 7.6a), is now what determines the form of the titration curve. 

A possible realization of an OC is to construct a symplast, shown in Figure 1, consisting of two cells, Mq and M, joined by a plasmodesma without a desmotubule as the OC. The bathing media, and may contain a 1 mM KCI solution and are kept at a low and relatively high pH, respectively. The symplast contains a 100 mM solution at a pH of 7. The cell membranes are assumed to be permeable to K" ", Q , and water. The whole internal surface of the symplast is assumed to be a continuous proton conducting system. Protons are supposed to enter at the external surface of Mq at a proton deficient defect and cross into the internal surface of Afo- At the other side of the symplast H" cross through the membrane of Ml and go into 5, via an excess proton defect. In this manner a flux of H" " can occur from Bq to Bi down the pmf [=2.3 RTjF) dpH] if some arrangement is made to complete the circuit within the symplast since Bq and Bi are not physically linked otherwise. 

Acid A and its conjugate base B are two different but interrelated states in which a chemical system may exist. The acid is the proton-rich (and proton donor) state, while the base is the proton-deficient (and proton acceptor) state. 

Because free protons do not exist in solution, the above scheme is only a simplification. A complete acid-base reaction actually takes place when two systems, let them be Systems 1 and 2, of conjugate acid-base pairs interact with each other. If most of the species of system 1 are in the proton-rich (acidic) state, their acidity can be manifested only if most of the species in system 2 are in the proton-deficient (basic) state. During the course of their interaction, a proton is transferred from the acidic species of system 1, Ai, to the basic species of system 2, B2 ... 

Another consequence of membrane contamination by cationic impurity can be a decrease in the limiting current on polarization curve measured at high contamination levels. Due to proton deficiency at the cathode, the ORR current may become limited by diffusion of protons, but not oxygen diffusion through the CCL. This effect observed in experimental systems (Uribe et al, 2002 Halseid et al, 2006b) was qualitatively described using model assumptions proposed by Kienitz et al. (2009). 

In 2009, Kobayashi and coworkers [43] isolated and characterized a severely proton-deficient dimeric bromopyrroie alkaloid, benzosceptrin C (13), from a marine sponge, Agelas sp. The authors employed an H4—N1 correlation in the HMBC correlations to establish the dibromopyrrole and the cor-... 

In 2013, the complex indolocarbazole alkaloid staurosporine (4) was employed as a model compound by Senior et al. [29] to evaluate the abibty of the 1ADEQUATE experiment to probe long-range carbon—carbon coupling pathways as a supplement to more traditional HMBC data in severely proton-deficient molecules. It was also interesting to note that while not all of the Jcc correlations were observed in the conventional 1,1-ADEQUATE spectrum, there were a number of the missing correlations that were observed in the dual-optimized inverted Jcc. l. -ADEQUATE spectrum that was also acquired for the sample. 

Insenion of — SOsNa groups would cause break-up of the network, prevent the sharing of the proton deficiency and strongly reduce the proton donating tendency. Sorption of water or alcohol molecules may give a similar effect. 

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