Solid supports polyamide-based

The solid-phase preparation of hyperbranched dendritic polyamides, based on 3,5-di-aminobenzoic acid, was attempted but deemed to have severe limitations , such as the critical condensation reaction could not be forced to completion. The accessibility of chain ends within the solid support was suggested1381 to be the major obstacle to high yield conversion. 

The synthesis of peptides on a solid support, usually beads of either polystyrene (the Merrifield approach) or polyamide (the Sheppard approach) resins has become extremely important, because it allows peptides to be synthesized by machines, and a key feature of the Sheppard approach is the use of Fmoc-protected amino acid residues. The idea is that the C-terminus amino acid is tethered to the resin by means of a carbamate linker that is stable to mild acid or base. The peptide chain is then built up using the sorts of methods we have been discussing and, when complete, is released by cleaving the linker with strong acid. 

In 1971, Sheppard proposed that peptide synthesis would proceed more efficiently if the polymeric support was designed to have solvation properties similar to those of the peptide product.This line of reasoning led to the development of polyamide resins.The most successful of this type of support is based on cross-linked poly(dimethylacrylamide) (Pep-syn). Polyamide resins swell up to 10 times their dry volume in dimethylformamide and even more in water. On the other hand, they swell much less in dichloromethane. The few comparative studies that have been carried out between polystyrene and polyamide resins indicate that both give very similar results in routine solid-phase peptide synthesis. 

The solid support should be well solvated to facilitate reactions to take place involving the two phases. The original supports were based on polystyrene but have generally been superseded by polyamide resins, which have an advantage in that they have a similar polarity to the peptide backbone. More recently, resins based on polyethylene glycol (PEG) grafted onto low... 

Fig. 5. Solid-phase supports made of synthetic resins. Figs 1 and 2 The nucleoside linkages used on the early polystyrene popcorn supports (3,4). Fig. 3. The nucleoside linkage used on a polyamide based resin (5). Fig. 4 The structure of a polyamide/silica gel composite developed for continuous-flow synthesis (8) Fig. 5. The structure of a PEG-polystyrene grafted copolymer, tentacle support (25). The mean mol wt of die PEG side chain is 3000 daltons. Fig 6 The nucleoside linkage used on the recently introduced highly crosslinked polystyrene support (27).

Solid Phase Peptide Synthesis (SPPS). Pentafluorophenyl esters have been used in both solution and solid phase peptide synthesis. So far, only solution methods have been considered. The principal of SPPS is incorporation of single amino acid residues into peptide chains built on insoluble polymeric supports. The famous Merrifield paper started the tradition of SPPS using f-butoxycarbonyl (Boc) amino acids activated with 1,3-Dicyclohexylcarbodiimide, although this has now been superseded with the introduction of the Fmoc group in polystyrene-and polyamide-based SPPS. The theory of SPPS will not be discussed here. Needless to say, the use of pentafluorophenyl esters in SPPS was inevitable and was first reported by Atherton in 1985. Fmoc-amino acid pentafluorophenyl active esters were used in the polyamide solid phase series in polar DMF solutions in the presence of 1-Hydroxybenzotriazole as a catalyst. 

The choice of polymer support is the most crucial factor in solid-phase oligonucleotide synthesis. Various types of supports used by different workers are summarized in Tables 5-1 and 5-2. Although most reported uses of polymers in nucleotide syntheses are styrene-based supports (both insoluble and soluble), other polymers such as polyamides, poly(ethyIene glycol), vinylacetate-V-vinylpyrrolidone copolymer, poly(vinyl alcohol), Sephadex LH20, and silica gel have also been used. 

Since Laursen s first report of solid-phase sequencing using a polymer-bound peptide, many other polymeric supports have been developed for this purpose. These are listed in Table 8-1. These polymeric supports are of three types styrene-based polymers, silylated aminoalkyl glass, and polyamides. 

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