Merrifield syntheses peptide amides

NHCH2COH, which in oxytocin has been modified so that it appears as —NHCH2CNH2. Therefore, attach glycine to the solid support in the first step of the Merrifield synthesis. The carboxyl group can be modified to the required amide after all the amino acid residues have been added and the completed peptide is removed from the solid support. 

The recommended procedure for the synthesis of peptide amides is to use a resin (e.g., MBHA) that yields amides directly upon cleavage with HE Ammonolysis can be used to give amides of peptides on Merrifield or HMBA resins. Anunonolysis can also probably be used with PAM resins. 

Peptide synthesis requires the use of selective protecting groups. An N-protected amino acid with a free carboxyl group is coupled to an O-protected amino acid with a free amino group in the presence of dicydohexvlcarbodi-imide (DCC). Amide formation occurs, the protecting groups are removed, and the sequence is repeated. Amines are usually protected as their teit-butoxy-carbonyl (Boc) derivatives, and acids are protected as esters. This synthetic sequence is often carried out by the Merrifield solid-phase method, in which the peptide is esterified to an insoluble polymeric support. 

Formation of an amide bond (peptide bond) will take place if an amine and not an alcohol attacks the acyl enzyme. If an amino acid (acid protected) is used, reactions can be continued to form oligo peptides. If an ester is used the process will be a kinetically controlled aminolysis. If an amino acid (amino protected) is used it will be reversed hydrolysis and if it is a protected amide or peptide it will be transpeptidation. Both of the latter methods are thermodynamically controlled. However, synthesis of peptides using biocatalytic methods (esterase, lipase or protease) is only of limited importance for two reasons. Synthesis by either of the above mentioned biocatalytic methods will take place in low water media and low solubility of peptides with more than 2-3 amino acids limits their value. Secondly, there are well developed non-biocatalytic methods for peptide synthesis. For small quantities the automated Merrifield method works well. 

Solid-phase methodology was established in 1963 in pioneering work conducted by Merrifield in the area of peptide synthesis [19]. Interest in this synthetic strategy continues unabated to this day, particularly in connection with the production of new active components for drugs, since the repetitive amide bond formation performed in automated synthesisers lends itself ideally to the construction of extensive substance libraries by combinatorial chemistry [20]. 

Peptide chemistry has made headlines this year with the publication of the first total synthesis of an enzyme. Ribonuclease A, a peptide or rather protein with 124 amino acid residues was prepared by two independent groups and by two quite different procedures, the now well-established Merrifield automated solid-state synthesis and a fragment condensation method with 19 preassembled parts. Regarding the Merrifield method, improved attachment and removal of the polymeric peptide support have been reported. A new peptide synthesis proceeds through sulfen-amides by an oxidation-reduction process . Peptides can also be obtained from N-protected aminothiolic aryl esters and sulfen-amino acids with selective removal of a protective o-nitrophenyl-sulfenyl group... 

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