Peptides Schiff bases

Inoue et al. reported that a complex prepared from AlMes and peptide Schiff base 166 is available for asymmetric cyanosilylation of aldehydes (Scheme 10.239) [630]. The enantioselectivity observed is not as high, even with a stoichiometric amount of the complex (up to 71% ee). A more recent study by Snapper and Hoveyda has, however, revealed that a similar catalyst system using Al(()t-Pr) ), and peptide Schiff base 167 is quite effective in catalytic asymmetric cyanosilylation of both aromatic and aliphatic ketones (66->98%, 80-95% ee wifh 10-20 mol% of fhe catalyst) [631]. 

Jacobsen et al. have reported that peptide Schiff bases and an Al-salen complex are valuable for asymmetric hydrocyanation of imines with TBSCN [660] and TMSCN [661], respectively. It is, however, most likely that HCN arising from these cyanosilanes and adventitious water is the reactive nucleophile. Hoveyda et al. also have used TMSCN as a source of HCN in the hydrocyanation of imines catalyzed by a Ti-peptide Schiff base complex [662],... 

Scheme 5. Peptidic Schiff bases may be screened for identification of an effective chiral ligand for catalytic enantioselective addition of TMSCN to meso-epoxides...

Differential reactivity of a C—H bonds has also been reported for Ni" complexes of peptides and peptide Schiff bases in which it is the carboxyl-terminal bond that is the more reactive. However, iV-salicylideneglycyl-L-valinatocopper is found to react with aqueous formaldehyde to give serine-L-valine containing highly optically pure serine. In this case the reaction obviously occurs stereoselec-tively at the amino-terminal residue." ... 

Calcium-binding proteins, 6, 564, 572, 596 intestinal, 6, 576 structure, 6, 573 Calcium carbonate calcium deposition as, 6, 597 Calcium complexes acetylacetone, 2, 372 amides, 2,164 amino acids, 3, 33 arsine oxides, 3, 9 biology, 6, 549 bipyridyl, 3, 13 crown ethers, 3, 39 dimethylphthalate, 3, 16 enzyme stabilization, 6, 549 hydrates, 3, 7 ionophores, 3, 66 malonic acid, 2, 444 peptides, 3, 33 phosphines, 3, 9 phthalocyanines, 2,863 porphyrins, 2, 820 proteins, 2, 770 pyridine oxide, 3,9 Schiff bases, 3, 29 urea, 3, 9... 

Figure 15.1 Reactions of formaldehyde with peptides and amino acids. Shown are the four types of reaction products seen when peptides or amino acids are treated with formaldehyde in aqueous solution. These reaction products are methylol (hydroxymethyl) adduct (reaction 15.1), Schiff-base (reaction 15.2), 4-imidazolidinone adduct (reaction 15.3), and one type of methylene bridge [cross-link] (reaction 15.4).

H Gausepohl, U Pieles, RW Frank. Schiff base analog formation during in situ activation by HBTU and TBTU, in JA Smith, JR Rivier, eds. Peptides Chemistry and Biology. Proceedings of the 12th American Peptide Symposium, Escom, Leiden, 1992, pp 523-524. 

Binding of pyridoxal phosphate to peptide PP-42 also appears to be selective for lysine 30. As was indicated by NMR spectroscopy and UV/vis experiments, only one of three potential lysine Schiff bases appeared to form. To determine the site or sites of attachment, the aldimine peptide intermediates were reduced, proteolytically cleaved, and the fragments analyzed by mass spectroscopy. This... 

In the S-peptide design, the Pal residue was introduced at position 8, replacing the native phenylalanine (Fig. 9) [22]. Lysine residues, which have the potential to form Schiff bases with the pyridoxal functionahty, and a potentially oxidatively unstable methionine residue were replaced with either norleucine or glycine. These changes resulted in a modified S-peptide that associated with S-protein at levels comparable to the original S-peptide. 

Dissolve the required amount of peptide (1 mole peptide per about 50 moles of lysine residues of the carrier, e.g., 60 pmol of peptide per 5 mg ovalbumin) in PBS and mix with the activated carrier. Stir at RT for 1 h and block with 10 mg/ml of solid NaBH4 at RT for 20 min. Alternatively, reduce (block) the formed azome-thines (Schiff bases) to secondary amines by addition of ascorbic acid (final concentration 5 mM). 

In addition to the above-mentioned reactions, metal complexes catalyze decarboxylation of keto acids, hydrolysis of esters of amino acids, hydrolysis of peptides, hydrolysis of Schiff bases, formation of porphyrins, oxidation of thiols, and so on. However, polymer-metal complexes have not yet been applied to these reactions. 

The initial Schiff base is digestible but after the Amadori rearrangement, the products are not metabolically available. Since lysine is the amino acid most likely to be involved and is an essential amino acid, Maillard browning reduces the biological value of proteins. Interaction of lysine with lactose renders the adjacent peptide bond resistant to hydrolysis by trypsin, thereby reducing the digestibility of the protein. 

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