Containing metal-oxygen bonds hydroxylation

In the third step, proteins act as amides and react with the free hydroxyl of the citric acid. The hydroxyl of the citric acid reacts like the hydroxyl of an alcohol in a standard esterification process. During the reaction of the amide with the citric acid, the substitution of the amide hydroxyl by an alkoxy radical (-OR) takes place. At the end of the process, the oxygen in the -OH group of the citric acid remains in the chain whereas the hydroxyl oxygen in the amino acids is eliminated in the form of water. This mechanism results in long chains containing metal cations tightly bond and evenly distributed. 

The influence of crystalline orientation and surface structure on water reactivity was clearly shown in a study performed by ESCA, LEED, and STM on NiO thin films [123], After oxidation at 300 K at low oxygen pressure of a single crystal of Ni(lll), the thin film, three to four layers thick, consists mainly of NiO( 111) grains in parallel epitaxy with the substrate. This film, otherwise unstable, is stabilized by a complete layer of OH formed by reaction with the residual water contained in oxygen gas. The hydroxyl groups are ionic (OH ) and singly bonded to the surface metal atoms that terminate the polar NiO(l 11) plane [124]. The ionic character of the hydroxyl groups would compensate at least partially the repulsive interaction between Ni cations in the outermost plane of NiO [124], A further exposure (150 L) at room temperature to water leads to the formation of a nearly complete layer of nickel hydroxide with the terminal sequence OH-Ni-OH. 

By far the most important metal containing dyes are derived from OjO-dUiydroxyazo stmctures in which one of the two azo nitrogen atoms and the two hydroxyl oxygen atoms are involved in bonding with the metal ion. Thus these dyes serve as terdentate ligands. In the case of metal ions with a coordination number of four, eg, Cu(H), the fourth position is usuaUy occupied by a solvent molecule (47). 

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