Synthesis of conformationally preorganized and cell-permeable guanidine-based gamma-peptide nucleic acids (gammaGPNAs).

A general method for preparing optically pure guanidine-based gamma-peptide nucleic acid (gammaGPNA) monomers for all four natural nucleobases (A, C, G, and T) is described. These second-generation gammaGPNAs differ from the first-generation GPNAs in that the guanidinium group is installed at the gamma- instead of the alpha-position of the N-(2-aminoethyl)glycine backbone unit. This positional switch enables GPNAs to be synthesized from relatively cheap L- as opposed to D-amino acids. Unlike their alpha-predecessors, which are randomly folded, gammaGPNAs prepared from L-amino acids are preorganized into a right-handed helix and bind to DNA and RNA with exceptionally high affinity and sequence selectivity and are readily taken up by mammalian cells.