Aldimines, cross-condensation

An exceptionally mild procedure for the cross-condensation of aldimines and enolsilanes has been described (eq. [67]) (80). This titanium tetrachloride-mediated reaction is predicated on the previous analogies provided by Mukaiyama for related aldol condensations (73a). Depending on aldimine structure and reaction time, either -lactams or their penultimate amino esters may be isolated from the reaction. The authors postulate that these reactions are proceeding via titanium enolates derived from ligand exchange by... 

Cross-linking is initiated by the formation of allysine and/or hydroxyallysine from specific Lys residues in the telopeptides of the interstitial collagens (Fig. 6). This step requires the action of the specific enzyme, lysyl oxidase. These aldehydes may then form initial bifunctional cross-links, termed reducible cross-links as they can be reduced by sodium borohydride. These are either aldol condensation products or aldimine or ketoamine links. The aldimine and ketoamine involve specific Lys or Hyl residues either from the telopeptides or from the helical domain of the collagen. The sites of these specific residues have been determined (Fig. 6) (50). 

Incubation of protein with peroxidase/H202/cate-chol also results in cross-linking. The reactions in this case are the oxidative deamination of lysine residues, followed by aldol and aldimine condensations, i. e. reactions analogous to those catalyzed by lysyl oxidase in connective tissue ... 

Tryptophan synthase catalyzes the last two steps of the biosynthesis of tryptophan. It is typically found as a tetramer. The a subunits catalyze the reversible formation of indole and G3P from InGP. The p subunits catalyze the irreversible condensation of indole and serine to form tryptophan in a PLP-dependent reaction. Each a active site is connected to a p active site in a process known as substrate channeling [18]. Eigure 14.7 represents the stereo view of one ap heterodimer of tryptophan [19]. The a subunit contains bound indole-3-propanol phosphate and p subunit contains L-serine, which forms aldimine with the coenzyme PEP The orange sphere represents the course of tunnel running from the a to the p subunits. This tunnel is a 25 A long hydrophobic channel contained within the enzyme allowing for the diffusion of indole. If the channel did not exist, the indole formed at a active site would quickly diffuse always and be lost to the cell as it is hydrophobic and can easily cross membranes. 

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