Merrifield’s synthesis

The solid support in Merrifield s synthesis of ribonuclease (see Section 25.18) was prepared by incorporating —CH2CI groups into a styrene// -divinylbenzene copolymer by electrophilic aromatic substitution. 

In the incipience of the methodical development, after some tests with different types of supports, Merrifield had already selected from the gigantic palette of organic and inorganic polymers the commercially available beaded polystyrene (200-400 mesh, 80—20 /i diameter), cross-linked by 2% divinylbenzene as the most suitable up to that time for the purposes of peptide synthesis on solid phase. Today there are series of investigations to find better supports for use in Merrifield s synthesis (for comparison, see the review articles of Merrifield [16, 35] and Meienhofer [33]). But in all cases the improved properties of novel carriers or modified polystyrenes concern only one or two of the above-mentioned necessary parameters — e.g., mechanical stability or strengthened C-terminal bond of an amino acid to the carrier — whereas nearly all the other characteristics for a suitable solid phase turn out to be less favourable, compared to the original Merrifield resin. 

Describe the synthesis of the dipeptide Lys-Ala by Merrifield s solid phase chemical method of peptide synthesis. What pitfalls might be encountered if yon attempted to add a leucine residue to Lys-Ala to make a tripeptide ... 

Initial solid phase synthesis25 was carried out on Merrifield s resin (1 % crosslinked chloromethylated styrene/divinylbenzene copolymer, 200-400 mesh) because of its track record in solid-phase peptide synthesis.26 Unfortunately, the Merrifield resin has limitations as a carbohydrate carrier to study interactions between the carbohydrates and relevant binding proteins. The hydrophobic nature of the resin leads to nonspecific, irreversible protein adsorption.27 Later work utilized Rapp s TentaGel, an amphiphilic, polyethylene glycol resin.28... 

Modern combinatorial chemistry involves a number of steps that begin with the creation of a library of molecules that are closely related in structure. The library can be created in two ways (a) parallel synthesis, which is simultaneous synthesis of numerous products in separate discrete reaction vessels (b) combinatorial synthesis, of numerous reactions within one single reaction vessel followed by separations. The initial successes in parallel synthesis have been in solid peptide synthesis of proteins, which was based on Merrifield s solid-phase peptide synthesis. 

Merrifield s concept of a solid-phase method for peptide synthesis and his development of methods for carrying it out set the stage for an entirely new way to do chemical reactions. Solid-phase synthesis has been extended to include numerous other classes of compounds and has helped spawn a whole new field called combinatorial chemistry. Combinatorial synthesis allows a chemist, using solid-phase techniques, to prepare hundreds of related compounds (called libraries) at a time. It is one of the most active areas of organic synthesis, especially in the pharmaceutical industry. 

The traditional and established way of conducting chemical transformations employs one reaction vessel for each compound synthesized. To prepare 50 different acetamides, 50 reaction vessels are needed, one for each amide. When synthesis of 50 amides is performed at the same time, this approach is referred to as a parallel synthesis. It is, of course, always desirable to simplify the process by reducing the number of vessels used in the synthesis. However, it is not feasible just to mix 50 amines into one reaction vessel and acetylate the mixture of amines because the isolation of 50 amides from the resultant reaction mixture could be complicated and time consuming. Simple separation of individual components was made possible by Merrifield s solid-phase synthesis.1 Solid support-bound substrates can... 

Despite the breakthrough associated with Merrifield s approach, there are several limitations such as the discontinuous nature of the reaction, the need for large excesses of reagent and the mechanical instability of the polymer matrix. An early solution to the restrictions imposed by Merrifield s polystyrene supported batch process was the use of commercially available benzyl alcohol-functionalized silica (used for H PLC columns). This was initially derivatized with the first member of the peptide chain to be propagated. The synthesis of a tetrapeptide in flow was completed in half the time required for the equivalent batch mode assembly and required significantly smaller excesses of the solution-phase reagent [92],... 

The split-mix method was developed for preparing peptide libraries. The method is based on Merrifield s solid phase procedure, published in 1963 [8], Each coupling cycle of the solid phase synthesis is replaced by the following simple operations ... 

In this way, it is possible to synthesize peptides with the desired sequence of amino acids. Although the process has been completely automated, this approach is only used to prepare peptides of relatively low molar mass. Merrifield s approach, called solid-phase synthesis, has been expanded as a more general tool for synthesizing a wide range of organic compounds. 

The chemical steps involved in the Merrifield s peptide synthesis using chloromethylated polystyrene is outlined in scheme 1. After the incorporation of the first amino add to the polymer through a benzyl ester linkage, the terminal amino group is deprotected under conditions which do not cleave the resin-amino add ester bond. 

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