NS-Peptides

In each step of the usual C-to-N peptide synthesis the N-protecting group of the newly coupled amino acid must be selectively removed under conditions that leave all side-chain pro-teaing groups of the peptide intact. The most common protecting groups of side-chains (p. 229) are all stable towards 50% trifluoroacetic acid in dichloromethane, and this reagent is most commonly used for N -deprotection. Only /ert-butyl esters and carbamates ( = Boc) are solvolyzed in this mixture. 

The peptide linkage is usually portrayed by a single bond between the carbonyl carbon and the amide nitrogen (Figure 5.3a). Therefore, in principle, rotation may occur about any covalent bond in the polypeptide backbone because all three kinds of bonds (N——C, and the —N peptide bond) are sin-... 

Chinnery P.E., Taylor R.W., Diekert K., Lill R., Turnbull D.M., Lightow-LERS R.N. Peptide nucleic acid delivery to human mitochondria. Gene Therapy 1999 6 1919-1928. 

In accordance with the autocatalytic process, matrices are again formed. It is surprising that the autocatalysis decreases when only 1 of the 15 building blocks of the peptide has the opposite handedness, e.g., when the N-peptide fragment contains one D-amino acid as well as the 14 L-amino acids. These experimental results show that such a system is able to form homochiral products via self-replication. It can be assumed that similar mechanisms influenced the origin of homochirality on Earth (Saghatelian et al., 2001 Siegel, 2001). 

R Rebek, D Feitler. Mechanism of the carbodiimide reaction, n. Peptide synthesis on the solid support. J Am Chem Soc 96, 1606, 1974. 

Zaveri N. Peptide and nonpeptide ligands for the noci-ceptin/orphanin FQ receptor ORLl research tools and potential therapentic agents. Life Sci 2003 73(6) 663-78. 

Figure 4.7 Fragment condensation of short, prebiotically formed, oligopeptides. The asterisk indicates the catalytically active peptide, which can induce the fragment condensation by reverse peptide hydrolysis n peptides (e.g., ten residues long) react with each other to build ideally 20-peptides, and of these, m react further (to build ideally 40-peptides, of course in practice all possible mixtures may be present), and m are eliminated because of being unusable (e.g., insoluble) under the contingent conditions - and so on.

Stability constants for Cu11 complexation of Gly—X and X—Gly dipeptides show that even relatively small alkyl side chains can influence coordination. 3 At low pH, for example, the bidentate chelates N(amino),0(peptide) of L-Leu-Gly and L-Ile-Gly are the thermodynamically less stable, presumably due to steric hindrance from the branched side chains. Similar coordination by Gly—X is virtually independent of the nature of X. This latter result is not unexpected since only the Gly moiety is coordinating. However, on raising the solution pH (> 4) the peptide deprotonation that occurs to give tridentate N(amino),N(peptide),0(carboxyl) chelation is slightly inhibited by the Leu and lie residues (by 0.7-0.8 log units). 

The predominant interest in the reactivity of the metal-N(peptide) bond has produced some interesting observations. Thus the reaction of this coordination group with H3Q+ is several orders... 

Whilst metal-N(peptide) bond formation inhibits hydrolysis of the peptide bond, coordination to O(peptide) has the opposite effect. These differences in reactivity can be readily demonstrated and put to practical use with the inert Co111 complexes. One of the first examples was the reaction of [Co(trien)(H20)(OH)]2+ with peptides to give hydrolysis of the peptide bond at the N-terminal end. The proposed mechanism involving nucleophilic attack by hydroxide at the peptide carbon is shown in Scheme 7.110 Similar selective hydrolyses of N-terminal peptide bonds have since been demonstrated with other Co111 amine complexes and the reaction has been examined as a method for determining the N-terminal amino acid residue in peptides and proteins.1"112... 

Amino acids can link together by a covalent peptide bond between the a-carboxyl end of one amino acid and the a-amino end of another. Formally, this bond is formed by the loss of a water molecule, as shown in figure 3.9. The peptide bond has partial double-bond character owing to resonance effects as a result, the C—N peptide linkage and all of the atoms directly connected to C and N lie in a planar configuration called the amide plane. In the following chap-... 

Amino acids in proteins are joined together in a specific way. These bonds constitute the peptide linkage. The formation of peptide linkages is a condensation process involving the loss of water. For example, consider the condensation of alanine, leucine, and tyrosine shown in Figure 3.3. When these three amino acids join together, two water molecules are eliminated. The product is a /n peptide since there are three amino acids involved. The amino acids in proteins are linked as shown for this tripeptide, except that many more monomeric amino acid groups are involved. 

The C-N peptide bond has an interesting property It is planar and very rigid. This special geometry of the peptide bond makes it very stable and ideal to maintain the structure of proteins. 

The proton donated from the OH group of Ser 195 to His 57 is then donated to the N atom of the scissile bond, cleaving the C-N peptide bond (or the C-O ester bond) to produce the amine and the acyl-enzyme intermediate. The amine is that part of the substrate which follows the scissile bond in the sequence the acyl-enzyme intermediate is the remaining fragment covalently bound to Ser 195. 

BB)n— Peptide/ AA—Resin BB building block, AA amino acid FIGURE 3.2 Convergent method and stepwise method. 

---