Carboxylic acid oxygen coordination

Figure 9. Possible general acid interaction with Nien2 by (a) hydrogen bonding to a coordinated H20 molecule or (b) carboxylic acid oxygen coordination to Ni. In both cases the proton of the carboxylic add is available to be transferred to an amine N as proposed in Figure 8.

Figure 6 Possible coordination configurations of Cu by peptides. can be coordinated by amide nitrogens (E) or carboxylic acid oxygens (1) from acidic amino acids. The Cu center can be oxidized to Cu for amperometric detection.

The reactivity of mercury salts is a fimction of both the solvent and the counterion in the mercury salt. Mercuric chloride, for example, is unreactive, and mercuric acetate is usually used. When higher reactivity is required, salts of electronegatively substituted carboxylic acids such as mercuric trifiuoroacetate can be used. Mercuric nitrate and mercuric perchlorate are also highly reactive. Soft anions reduce the reactivity of the Hg " son by coordination, which reduces the electrophilicity of the cation. The harder oxygen anions leave the mercuric ion in a more reactive state. Organomercury compounds have a number of valuable synthetic applications, and these will be discussed in Chapter 8 of Part B. 

Electrochemical studies performed in the 7 x Cys-Aspl4 D. afri-canus Fdlll indicate that the reduced [3Fe-4S] center can react rapidly with Fe to form a [4Fe-4S] core that must include noncysteinyl coordination (101). The carboxylate side chain of Asp 14 was proposed as the most likely candidate, since this amino acid occupies the cysteine position in the typical sequence of a 8Fe protein as indicated before. The novel [4Fe-4S] cluster with mixed S and O coordination has a midpoint redox potential of 400 mV (88). This novel coordinated state with an oxygen coordination to the iron-sulfur core is a plausible model for a [4Fe-4S] core showing unusual spin states present in complex proteins (113, 114). 

The vibrational spectra of [ZnL L ] (LH = 2-pyridinecarboxylic acid A-oxide V = H20 or D20) indicate coordination through the A-oxide oxygen and the carboxylate group-504 In contrast, the complexes with 3-pyridinecarboxylic acid A-oxide and pyridine-4-carboxylic acid A-oxide are polymeric multinuclear species.505 Crystal structures of [ZnL4(NCS)2] (L = 3-pyridine-A,A-diethylcarboxamide)506 and [ZnL2(H20)2(HC02)2] (L = 3-pyridinecarbox-amide)507 have been reported. 

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