Peptides synthesis on polymers

Fig. 9. The schematic peptide synthesis on polymer by C-terminal chain elongation according to Letsin-ger and Kornet...

Series of reagents are in use to deprotect specifically peptide side functions following a Merrifield synthesis. Since this aspect, however, is not a problem in connection with the peptide synthesis on polymer phase, the discussion is omitted here. Questions relating to cleavage of protected peptides from the polymer support are discussed in Sect. 3.5. 

A library of 63 peptides, each containing one of the two phosphine-substituted amino acids 8 or 9, was prepared by Gilbertson and coworkers in a parallel synthesis on polymer beads [13]. After complexation of all peptides with rhodium, the obtained polymer-bound complexes were employed in the asymmetric hydrogenation of 10 (Fig. 5). 

The methodology of solid phase peptide synthesis (SPPS) [65, 66] has been credited with the award of 1984 Nobel Prize in chemistry [67] to its inventor, Bruce R. Merrifield of the Rockefeller University. At the heart of the SPPS lies an insoluble polymer support or gel , which renders the synthetic peptide intermediates insoluble, and hence readily separable from excess reagents and by-products. In addition to peptide synthesis, beaded polymer gels are also being studied for a number of other synthetic and catalytic reactions [2]. Ideally, the polymer support should be chemically inert and not interfere with the chemistry under investigation. The provision of chemical inertiKss presents no difficulty, but the backbone structure of the polymer may profoundly influence the course of the reaction on the polymer support. This topic has attracted considerable interest, particularly in relation to the properties of polystyrene (nonpolar, hydrophobic), polydimethylacrylamide (polar, hydrophilic), and copoIy(styrene-dimethylaciylamide) (polar-nonpolar, amphiphilic) (see later). 

Fig. 17. Chemical structures of polymer supports based on styrene (nonpolar, hydrophobic), polydimethylacryiamide (polar, hydrophilic) and styiene dimethylacrylamide (amphiphilic Structures of nonpdar (18-19) and polar (20-21) pq)tide residues are also shown to illustrate the basis of polymer-pepAide incompatiUIity during peptide synthesis on polystyrene and polydimethylacryl-amifte. Am philk polymer supports are expected to be compatible with both nonpolar and polar peptUe resMlnes...

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. 

The homogeneous growth of a peptide during synthesis on polymer phase is determined by the completion of all of the chemical reactions repeated alternately on each stage of the synthesis, namely liberation of the N-terminal amino functions and their condensation with the next amino acid to elongate the sequence. Therefore the control for completion of these reactions is the principal demand in a carefully performed Merrifield synthesis. 

Besides this fundamental reason one has to realize that the result of even a most accurate multistep synthesis on polymer can be questioned by the chemical reaction necessary to release the final product from its support. Parts of the target sequence can be destroyed and desired protecting groups on side functions may be cleaved. These facts furthermore multiply the possibility for contaminations. Therefore, the overall success of any Merrifield peptide synthesis is inherently related also to the quality of isolation and purification procedures (see for example [168]). 

The major disadvantage of solid-phase peptide synthesis is the fact that ail the by-products attached to the resin can only be removed at the final stages of synthesis. Another problem is the relatively low local concentration of peptide which can be obtained on the polymer, and this limits the turnover of all other educts. Preparation of large quantities (> 1 g) is therefore difficult. Thirdly, the racemization-safe methods for acid activation, e.g. with azides, are too mild (= slow) for solid-phase synthesis. For these reasons the convenient Menifield procedures are quite generally used for syntheses of small peptides, whereas for larger polypeptides many research groups adhere to classic solution methods and purification after each condensation step (F.M. Finn, 1976). 

At the time of writing this book, SPOS is in an area of reladve infancy but has considerable potential. One of the main difficulties in SPOS lies in the lack of techniques available to monitor reacdons carried out on polymer supports. Unlike reacdons in solution phase, reactions on solid support cannot be monitored with relative ease and this has hindered the progress as well as the efficacy of solid supported synthesis of small non-peptidic molecules. Despite these difficulties, a large body of studies is available for SPOS. Recent reviews incorporate... 

Polymer-supported esters are widely used in solid-phase peptide synthesis, and extensive information on this specialized protection is reported annually. Some activated esters that have been used as macrolide precursors and some that have been used in peptide synthesis are also described in this chapter the many activated esters that are used in peptide synthesis are discussed elsewhere. A useful list, with references, of many protected amino acids (e.g., -NH2, COOH, and side-chain-protected compounds) has been compiled/ Some general methods for the preparation of esters are provided at the beginning of this chapter conditions that are unique to a protective group are described with that group/ Some esters that have been used as protective groups are included in Reactivity Chart 6. 

When the polymer was prepared by the suspension polymerization technique, the product was crosslinked beads of unusually uniform size (see Fig. 16 for SEM picture of the beads) with hydrophobic surface characteristics. This shows that cardanyl acrylate/methacry-late can be used as comonomers-cum-cross-linking agents in vinyl polymerizations. This further gives rise to more opportunities to prepare polymer supports for synthesis particularly for experiments in solid-state peptide synthesis. Polymer supports based on activated acrylates have recently been reported to be useful in supported organic reactions, metal ion separation, etc. [198,199]. Copolymers are expected to give better performance and, hence, coplymers of CA and CM A with methyl methacrylate (MMA), styrene (St), and acrylonitrile (AN) were prepared and characterized [196,197]. 

More recently, Somfai and coworkers have reported on the efficient coupling of a set of carboxylic acids suitable as potential scaffolds for peptide synthesis to a polymer-bound hydrazide linker [24]. Indole-like scaffolds were selected for this small library synthesis as these structures are found in numerous natural products showing interesting activities. The best results were obtained using 2-(7-aza-l H-benzo-triazol-l-yl)-l,l,3,3-tetramethyluronium hexafluoride (HATU) and N,N-diisopropyl-ethylamine (DIEA) in N,N-dimethylformamide as a solvent. Heating the reaction mixtures at 180 °C for 10 min furnished the desired products in high yields (Scheme 7.4). In this application, no Fmoc protection of the indole nitrogen is required. 

Preparative applications of these reactions have included work on peptide synthesis from amino-acids (suitably protected) using triphenyl phosphine (120)10 7 >108 or a polymer-bound aryldiphenylphosphine (128).109 Coupling is generally very efficient,107 but racemization problems occur in some reactions.108 109 A typical coupling reaction using (128) is outlined in Scheme 9. 

Initially, the term Hquid-phase synthesis was used to contrast the differences between soHd-phase peptide synthesis and a method of synthesis on soluble polyethylene glycol (PEG) [5, 6]. Although soluble polymer-supported synthesis is less ambiguous than Hquid-phase synthesis, the latter term is more prevalent in the Hterature. In-keeping with previous reviews [7-12], the phrases classical or solution synthesis will be used to describe homogeneous reaction schemes that do not employ polymer supports while liquid-phase synthesis will be reserved... 

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