PNA-DNA chimeras containing 5-alkynyl-pyrimidine PNA units. Synthesis, binding properties, and enzymatic stability

Three chimeric dimer synthons (oeg_t(NH)T, oeg_up(NH)T and oeg_uh(NH)T) containing thymine (t), 5-(1-propynyl)-uracil (up) and 5-(1-hexyn-1-yl)-uracil (uh) PNA units with N-(2-hydroxyethyl)glycine (oeg) backbone were synthesized in solution and incorporated into T20 oligonucleotide analogues, using standard P-amidite chemistry. Insertion of dimer blocks led to destabilization of duplexes with dA20 target. The smallest Tm drops were found for chimeras containing oeg_up(NH)T dimers. Incorporation of the chimeric synthons into the 3'-end of T20 brought about growing resistance to 3'-exonucleolytic (SV PDE) cleavage in the order of oeg_t(NH)T < oeg_up(NH)T < oeg_uh(NH)T. Due to different endonuclease activities of 3'- and 5'-exonucleases applied, placing of five consecutive dimers at the 5'-terminus resulted in a relatively smaller, but also side-chain dependent, stabilization towards the hydrolysis by 5'-exonuclease (BS PDE). Neither exonucleases (SV and BS PDE) nor an endonuclease (Nuclease P1) could hydrolyse the unnatural phosphodiester bond linking the 3'-OH of thymidine to the terminal OH of N-(2-hydroxyethyl)glycine PNA backbone.

Formation of oligonucleotide-PNA-chimeras by template-directed ligation

DNA sequences have previously been reported to act as templates for the synthesis of PNA, and vice versa. A continuous evolutionary transition from an informational replicating system based on one polymer to a system based on the other would be facilitated if it were possible to form chimeras, that is molecules that contain monomers of both types. Here we show that ligation to form chimeras proceeds efficiently both on PNA and on DNA templates. The efficiency of ligation is primarily determined by the number of backbone bonds at the ligation site and the relative orientation of template and substrate strands. The most efficient reactions result in the formation of chimeras with ligation junctions resembling the structures of the backbones of PNA and DNA and with antiparallel alignment of both components of the chimera with the template, that is, ligations involving formation of 3'-phosphoramidate and 5'-ester bonds. However, double helices involving PNA are stable both with antiparallel and parallel orientation of the two strands. Ligation on PNA but not on DNA templates is, therefore, sometimes possible on templates with reversed orientation. The relevance of these findings to discussions of possible transitions between genetic systems is discussed.

Peptide nucleic acid (PNA): its medical and biotechnical applications and promise for the future

Synthetic molecules that can bind with high sequence specificity to a chosen target in a gene sequence are of major interest in medicinal and biotechnological contexts. They show promise for the development of gene therapeutic agents, diagnostic devices for genetic analysis, and as molecular tools for nucleic acid manipulations. Peptide nucleic acid (PNA) is a nucleic acid analog in which the sugar phosphate backbone of natural nucleic acid has been replaced by a synthetic peptide backbone usually formed from N-(2-amino-ethyl)-glycine units, resulting in an achiral and uncharged mimic. It is chemically stable and resistant to hydrolytic (enzymatic) cleavage and thus not expected to be degraded inside a living cell. PNA is capable of sequence-specific recognition of DNA and RNA obeying the Watson-Crick hydrogen bonding scheme, and the hybrid complexes exhibit extraordinary thermal stability and unique ionic strength effects. It may also recognize duplex homopurine sequences of DNA to which it binds by strand invasion, forming a stable PNA-DNA-PNA triplex with a looped-out DNA strand. Since its discovery, PNA has attracted major attention at the interface of chemistry and biology because of its interesting chemical, physical, and biological properties and its potential to act as an active component for diagnostic as well as pharmaceutical applications. In vitro studies indicate that PNA could inhibit both transcription and translation of genes to which it has been targeted, which holds promise for its use for antigene and antisense therapy. However, as with other high molecular mass drugs, the delivery of PNA, involving passage through the cell membrane, appears to be a general problem.

Synthesis of a monocharged peptide nucleic acid (PNA) analog and its recognition as substrate by DNA polymerases

The preparation of a novel phosphoramidite monomer based on thyminyl acetic acid coupled to the secondary nitrogen of 2-(2-amino-ethylamino)ethanol is described. This monomer can be used to attach a deoxynucleotide to the carboxy terminus of a PNA oligomer by solid-phase synthesis. The resulting PNA primer is recognized as a substrate by various DNA polymerases.