Carbonic acid bridging

We note that, with the lone participation of a carboxyl group in the coordination, the coordination modes examined earlier for the complexes of carbonic acids (Sec. 2.2.5.4, formulae 276 283) are possible. Additionally, a mixed binding amongst ligands is observed in many complexes of aminoacids, for example chelates 277, 414, and 0,0 -bridge 279 [714],... 

In this chapter, we introduced the reader to some basic principles of solution chemistry with emphasis on the C02-carbonate acid system. An array of equations necessary for making calculations in this system was developed, which emphasized the relationships between concentrations and activity and the bridging concept of activity coefficients. Because most carbonate sediments and rocks are initially deposited in the marine environment and are bathed by seawater or modified seawater solutions for some or much of their history, the carbonic acid system in seawater was discussed in more detail. An example calculation for seawater saturation state was provided to illustrate how such calculations are made, and to prepare the reader, in particular, for material in Chapter 4. We now investigate the relationships between solutions and sedimentary carbonate minerals in Chapters 2 and 3. 

The mechanism of proton transfer for carbon acids often differs in one further way from that for oxygen and nitrogen acids. Proton transfer for these latter can occur through reaction complexes Ij and I2, and a transition state (XXIX) in which the acid and base are separated by a solvent bridge. An alternative transition state is shown in (XXX), viz. 

The contributions made by these pathways in proton transfer between amines and their conjugate acids have been determined [193] and the results are shown in Table 13. The rate coefficient kt refers to the direct proton transfer mechanism and k2 is for proton transfer through a solvent bridge. The available evidence for carbon acids suggests that proton transfer occurs directly between acid and base and an intervening solvent molecule is not involved [123,194]. This evidence is mostly based on the magnitude of the solvent isotope effect, and results for reactions involving nitroparaffins, acetals, and diazocompounds have been reviewed [123]. In a different approach to this question, the rate expression for the acid catalysed decomposition of ethyl vinyl ether in water/dimethyl-sulphoxide was measured [195] and shown to consist of two terms (111). 

It is unknown how the fatty chain carbonic acids of the lipids originally formed. It is noteworthy that fatty acids with up to six and seven C-atoms were detectable in the Murchison meteorite. Thus, it might be possible that membranes that were even more primitive existed in the past. However, dicarboxylic acids of chain lengths up to 17 were detectable after hydrothermal treatment. It is also remarkable that isoprenoid derivatives - more precisely, phytanyl and biphytanyl, that is CIO and C20 chains - linked to each other by monoether or diether bridges are part of membranes of the Archaea (Fig. 6.2, bottom structure). 

Figure 2 Mechanism of urease. This proposed mechanism features binding of urea through its carbonyl group to one of the Ni ions, making the carbon subject to nucleophilic attack by the bridging hydroxide leading to liberation of NH2, which is protonated by to form ammonia, and carbamate, which hydrolyzes to another mole of ammonia and carbonic acid. The same mechanism with proton transfer from water (instead of histidine) to ammonia is another possibility. Another possibility would be urea binding in a monodentate manner with a terminal hydroxide on Ni2 as the nucleophile...

Zhu, G., Zhang, X., Gai, P., Zhang, X., and Chen, J. (2012) fi-Cyclodextrin non-covalently functionalized single-walled carbon nanotubes bridged by 3,4,9,10-perylene tetracarboxylic acid for ultrasensitive electrochemical sensing of 9-anthracenecarboxylic acid. Nanoscale, 4, 5703. 

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