Oligonucleotides soluble polymer supports

Figure 19.16 Supports and linkers for the soluble polymer supported oligonucleotide synthesis.

Finally, the same Italian group studied monocarboxy-functionalized poly-N-acryloylmorpholine (PAcM) (MW 6000) as a new soluble polymer support for liquid-phase oligonucleotide synthesis [322] and a possible alternative to PEG, although its advantages over the latter were not immediately apparent. Preparation and antisense properties of oligonucleotide-polyacryloylmorpholine conjugates were also described [323]. 

The basic requirements for a suitaUe soluble polymer support in oligonucleotide synthesis are mechanical stability, chemical inertness, and a high solubilizing power. The solubility properties of the polymer carrier are especially important in this hdd because of the relatively large size and high polarity of oligonucleotides. 

Generally, two different strategies are used in conventional synthesis of oligonucleotides the diester and the triester method [131,132]. In syntheses with soluble polymer supports and in recent works in general the diester method is preferred. Thereby, the 5 -0 position is protected by the solutde polymer and the elongation of the oligonucleotide chain is carried out via the 3 -0 position. Most researchers used an ether, ester, or a carbonate bond for the S -nucleoside linkage. 

TaNe 13. Soluble polymer supports for oligonucleotide synthesis... 

The preparation and availability of suitable soluble polymer supports are basic requirements for the successful application of liquid-phase synthesis of peptides and nucleotides. Many parameters have to be considered when designing appropriate supports and the route of synthesis and the target product of synthesis must also be considered. Liquid-phase synthesis requires optimum solubility properties of the polymer supports and therefore the adaption of the functional capacity to the solubilizing power of the polymer backbone and side-chains is the determining step before synthesis. The main factors of evaluation are the solubility characteristics of the polypeptide or oligonucleotide to be synthesized. 

Several modifications of Letsinger s procedure and two novel proceduresl00 101 were reported for the synthesis of oligonucleotides on insoluble polymer supports. Khorana s procedure, using soluble polymer support, was published in detail . ... 

Different polyethylene glycol polymers were used in various papers and proved to be very reliable and useful for different classes of molecules their use for the synthesis of peptides [180, 181], of peptidomimetics [182] and of oligosaccharide libraries [183] was reported as the development and the use of a new PEG-linked traceless linker [184, 185], the selection of ligands for asymmetric Sharpless dihydroxylation [186-188], the use of PEG-linked triarylphosphines for LPCS requiring Mitsunobu or Staudinger conditions [189], the use of PEG-based supports to prepare a library of [l,4]oxazepine-7-ones [190] and the use of PEG-supported Schiff bases for the synthesis of a-substituted amino acids [191], Other examples of soluble polymers used for LPCS may include cellulose[192], polyacrylamide [193] polyvinyl alcohol [194, 195], various copolymers [196, 197] and NCPS [198-200]. Three excellent reviews [201-203] summarized the properties of PEG and other soluble polymers and their applications to the synthesis of peptides, oligonucleotides,... 

Seliger H, Aumann G, Polymer support synthesis, 5, Oligonucleotide synthesis on a polymer support soluble in water and pyridine, Tetrahedron Lett., 2911-2914, 1973. 

While PEG-based supports are widely used for liquid-phase combinatorial chemistry, other non-PEG-based soluble polymers have also been reported for combinatorial applications. A recent review (276) contains an exhaustive list of homo- and copolym-eric soluble supports used in peptide, oligonucleotide, and oligosaccharide synthesis, including combinatorial chemistry. Two of these supports have also been used for small organic molecule synthesis. Homopolymeric polyvinyl alcohol was used in conjunction with PEG for a protection/derivatization strategy in solution (284), and the copolymer between isopropylacrylamide and acrylic acid was used in the catalytic hydrogenation of a Cbz group (285). 

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. 

The first nucleotide is attached to the functional soluble polymer via the phosphate group. After cleavage of the protecting group, reagents and other low-molecular compounds are separated by membrane filtration. Then, the product can be controlled analytically and the next nucleotide units are coupled repeatedly in the same cycle until the desired length of the oligonucleotide is attained. FinaUy, the product is cleaved chemically or enzymatically from the polymer support... 

The selection of the polymer support is the most important step in liquid-phase synthesis of oligonucleotides. The number of soluble polymers, which could serve here as carriers, is very limited due to many restrictions. In most cases synthetic macromolecules are preferred because they fulfill the requirements better than many biopolymers. 

The majority of the studies in this field stress the synthesis of oligonucleotides and not the polymer support. An optimization of the factors in the application of soluble carriers needs more intensive investigation because of the complex interaction of many parameters during polymer-supported reactions. 

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