Linking groups imine

Excess diamine (ODA) component on the other hand leads to undesirable chemical reactions. For instance, it was proposed that under conditions of excess ODA, following imidization, the free amino groups react with imide carbonyl groups resulting in the formation of imine links. These imine links are unstable beyond 300°C, consequently, the films also exhibit poor thermal stability. The optimum relative fluxes need to be determined for each polymer separately taking into consideration the differences in the surface residence time of the species. 

The actual pKa value of an active site catalytic group will be influenced by the particular microenvironment of the active site, which could raise or lower the pKa. For example, the enzyme acetoacetate decarboxylase contains an active site lysine residue that forms an imine link with its substrate its pKa value was found to be 5.9, which is much less than the expected value of 9. Adjacent to this residue in the active site is a second lysine residue, which in protonated form destabflizes the protonated amine and, therefore, reduces the pKa. Conversely, aspartic acid or glutamic acid residues that are positioned in hydrophobic active sites can have increased pKa values near 7 because the anionic form of the side chain is destabilized. 

The e-amino group of lysine is used in several enzymes to form imine links with ketone groups for example, it is used in acetoacetate decarboxylase, shown in Fig. 11. 

The effect of the nature of the terminal group and the position of the imine link has also been investigated. Thus, when hexyl chains are substituted at each end of a rhenium complex with the same motif as 12, a nematic phase was observed (Cr 129N 167 1). Moreover, when one of the two hexyl chains were replaced by one (Figure 28) or two perfluorinated chains, the mesophase was changed to SmA phase, and occurred at higher temperatures, with decomposition taking place in the mesophase. ... 

Pham et al. [1100] electropolymerized 5-amino-l-naphthol from both aqueous and nonaqueous solutions onto glassy carbon and platinum electrodes. The electrochemically active and conducting films were found to be formed via the amino group based on the infrared spectra recorded in situ during the polymerization. With in situ Raman spectroscopy, the structure of the film that contained amine and imine links similar to the PANI structure was identified [1101]. Upon oxidation, some amine units are transformed into imine groups. 

High thermostabilizing efficiency of polyamine disulphides relative to chemically cross-linked polyethylene is conditioned by the ability to accept macroradicals at the disulphide bridge and imine group. Besides, the presence of paramagnetic centers causes the adherence of macroradicals providing for an extra stabilizing effect [49]. 

Figure 12.3 Hydroxymethyl adducts from formaldehyde fixation form highly reactive imines in the presence of ethanol during tissue processing, giving off a molecule of water (upper). While still in alcohol, imines may either form more complex ethoxymethyl adducts or will cross-link to neighboring reactive groups (lower).

The importance of leaving groups linking up the chemistry of amides with imine formation... 

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