Peptides amino group

Arg residues in RNase-A causes a marked loss of activity. The half-time for the loss is about 5 min at pH 8 and slower in more acid solution. Arginines 39 and 85, the most accessible arginines, are the principal sites of modification, and the activity loss appears to correlate with the reaction at Arg-39. Arginines 10 and 33 seem to be quite un-reactive toward the reagent. (Deamination of peptide -amino groups occurs with this reagent.)... 

Palladium-carbon/hydrogen chloride Protection of peptide amino groups as formamides Selective removal of the protective group... 

Yellow liquid m.p. 26" C, b.p. l37 "C/2mm. Reagent introduced by Sanger for identification of the terminal amino group of peptides with which it forms an /V-2,4-dlni-irophenyl derivative under mild conditions. 

A major trend in organic synthesis, however, is the move towards complex systems. It may happen that one needs to combine a steroid and a sugar molecule, a porphyrin and a carotenoid, a penicillin and a peptide. Also the specialists in a field have developed reactions and concepts that may, with or without modifications, be applied in other fields. If one needs to protect an amino group in a steroid, it is advisable not only to search the steroid literature but also to look into publications on peptide synthesis. In the synthesis of corrin chromophores with chiral centres, special knowledge of steroid, porphyrin, and alkaloid chemistry has been very helpful (R.B. Woodward, 1967 A. Eschenmoser, 1970). 

In these cases, it is better to protect the carboxyl group. Optimized conditions for A/-acetylation have been studied (78). A/-Acylation can be utilized for protecting the amino group in the reaction of amino acids, for example in peptide synthesis. 

Amino Acids. Chloroformates play a most important role for the protection of the amino group of amino acids (qv) during peptide synthesis (32). The protective carbamate formed by the reaction of benzyl chloroformate and amino acid (33) can be cleaved by hydrogenolysis to free the amine after the carboxyl group has reacted further. The selectivity of the amino groups toward chloroformates results in amino-protected amino acids with the other reactive groups unprotected (34,35). Methods for the preparation of protected amino acids on an industrial scale have been developed (36,37). A wide variety of chloroformates have been used that give various carbamates that are stable or cleaved under different conditions. 

Two disadvantages are associated with the use of S-acetyl or 5-benzoyl derivatives in peptide syntheses (a) base-catalyzed hydrolysis of 5-acetyl- and 5-benzoylcys-teine occurs with /S-elimination to give olefinic side products, CH2=C-(NHPG)CO—(b) the yields of peptides formed by coupling an unprotected amino group in an 5-acylcysteine are low because of prior S-N acyl migration. ... 

Many protective groups have been developed for the amino group, including carbamates (>NCO,R), used for the protection of ammo acids in peptide and protein syntheses, and amides (>NCOR). used more widely in syntheses of alkaloids and for the protection of the nitrogen bases adenine, cytosine, and guanine in nucleo-... 

Figure 1.2 Proteins are built up by amino acids that are linked by peptide bonds to form a polypeptide chain, (a) Schematic diagram of an amino acid. Illustrating the nomenclature used in this book. A central carbon atom (Ca) is attached to an amino group (NH2), a carboxyl group (COOH), a hydrogen atom (H), and a side chain (R). (b) In a polypeptide chain the carboxyl group of amino acid n has formed a peptide bond, C-N, to the amino group of amino acid + 1. One water molecule is eliminated in this process. The repeating units, which are called residues, are divided into main-chain atoms and side chains. The main-chain part, which is identical in all residues, contains a central Ca atom attached to an NH group, a C =0 group, and an H atom. The side chain R, which is different for different residues, is bound to the Ca atom.

Amino-4,6-dimethyl-3-oxo-3//-phenoxazine-l,9-dicarboxylic acid also named actinocin is the chromophor of the red antineoplastic chromopeptide aetinomyein D (formula A). Two cyclopenta-peptide lactone rings (amino acids L-threonine, D-valine, L-proline, sarcosine, and 7V-methyl-L-valine) are attached to the carboxy carbons of actinocin by two amide bonds involving the amino groups of threonine. 

To direct the synthesis so that only Phe-Gly is formed, the amino group of phenylalanine and the car boxyl group of glycine must be protected so that they cannot react under the conditions of peptide bond formation. We can represent the peptide bond formation step by the following equation, where X and Y are amine- and carboxyl-protecting groups, respectively ... 

Section 27.17 Peptide bond fonnation between a protected amino acid having a free carboxyl group and a protected amino acid having a free amino group can be accomplished with the aid of A, A/ -dicyclohexylcaibodiimide (DCCI). 

In addition to illustrating the mechanics of translation. Figure 28.12 is important in that it shows the mechanism of peptide bond formation as a straightforward nucleophilic acyl substitution. Both methionine and alanine are attached to their respective tRNAs as esters. The amino group of alanine attacks the methionine carbonyl, displacing methionine from its tRNA and converting the carbonyl group of methionine from an ester to an anide function. 

N terminus (Section 27.7) The amino acid at the end of a peptide or protein chain that has its a-amino group intact that is, the a-amino group is not part of a peptide bond. 

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