Left-handed gamma-peptide nucleic acids, methods of synthesis and uses therefor

A composition for use in detection of a target nucleic acid, comprising:

• a) a sensor comprising:
• i) a protecting (P) strand of achiral peptide nucleic acid and/or right-handed gamma-peptide nucleic acid (RγPNA) having an N-terminal end and a C-terminal end, comprising, in an N-terminal to C-terminal direction, a first section having nucleobase sequence n, second section having nucleobase sequence m comprising unnatural nucleobases, third section having nucleobase sequence a and fourth section having nucleobase sequence b and comprising an N-terminal cysteine, sulfhydryl or protected sulfhydryl group, and a thioester bond linking sections m and n;
• ii) a sensing (S) strand of RγPNA or achiral peptide nucleic acid (achiral PNA) having an N-terminal end and a C-terminal end, hybridized to the P strand, comprising, in an N-terminal to C-terminal direction a first section having nucleobase sequence b′ complementary to sequence b, a second section having nucleobase sequence a′ complementary to sequence a, a third section having nucleobase sequence n′ complementary to sequence n and a fourth, toe-hold section having nucleobase sequence o′, the S strand having a sequence complementary to a nucleotide sequence of a nucleic acid and comprising the target sequence, having a 5′ end and a 3′ end, comprising, in a 5′ to 3′ direction, without intervening nucleobases a first section having nucleobase sequence 0 complementary to sequence o′, a second section having nucleobase sequence N complementary to sequence n′, a third section having nucleobase sequence A complementary to sequence a′ and a fourth section having nucleobase sequence B complementary to sequence b′, wherein hybridization of the target nucleic acid to the S strand displaces the P strand, and the first section of the displaced P strand having nucleobase sequence n is removed by self-splicing;
• b) a converter comprising,
• i) an achiral peptide nucleic acid (achiral PNA) having an N-terminal end and a C-terminal end, comprising, in an N-terminal to C-terminal direction, a first section having nucleobase sequence b′ complementary to sequence b, a second section having nucleobase sequence a′ complementary to sequence a, and a third section having nucleobase sequence m′ having unnatural nucleobases complementary to sequence m;
• and
• ii) a left-handed γPNA (LγPNA) having an N-terminal end and a C-terminal end, hybridized to the achiral PNA, comprising, in an N-terminal to C-terminal direction a first section having nucleobase sequence b complementary to sequence b′ of the PNA, and a second section having nucleobase sequence a′ complementary to sequence a of the PNA, wherein the displaced P strand hybridizes to the achiral PNA of the converter, displacing the LγPNA from the converter;
• and
• wherein, other than where sequences are indicated as being complementary, the sequences A, a, a′, B, b, b′, N, n, n′, 0, o′, m or m′ are not complementary to each other.

A method of detecting a nucleic acid comprising a specific nucleic acid sequence in a sample, comprising:

• a) hybridizing the nucleic acid to a duplex right-handed and/or achiral gamma-peptide nucleic acid (γPNA) comprising a protecting (P) strand comprising, in order a first section comprising the specific nucleic acid sequence, a second section comprising a sequence of unnatural nucleobases, and a third section comprising a sequence of the nucleic acid adjacent to the sequence of the first section, and further comprising a thioester bond between the second and third sections and an N-terminal sulfhydryl group, hybridized to a sensing (S) strand comprising, in order a sequence complementary to the first section and the third section, but not the second section, such that the P strand comprising the specific nucleic acid sequence is displaced by the nucleic acid, and the third section of the P strand is removed by self-splicing between the sulfydryl and thioester;
• b) converting the P strand to an LγPNA by hybridizing the P strand to a duplex converter PNA comprising a first strand of an achiral PNA having a sequence complementary to the P strand hybridized to a second strand of an LγPNA having the same sequence as the first section of the P strand, thereby displacing the LγPNA;
• and
• c) amplifying the displaced LγPNA by concatenization of a pair of single-stranded LγPNA amplifiers having a hairpin structure, the first amplifier having, in order, a sequence complementary to the displaced LγPNA, a sequence that is not present in the displaced LγPNA, and a portion of (not the complete sequence) the sequence of the displaced LγPNA such that the hairpin of the first amplifier is opened by binding to the displaced LγPNA, leaving a single-stranded tail comprising the sequence not present in the displaced LγPNA and the portion of the sequence of the displaced LγPNA, the second amplifier comprising a hairpin structure comprising, in order, a first portion having a sequence complementary to the sequence of the single-stranded tail formed by the binding of the LγPNA to the first amplifier and a second portion having the sequence of the LγPNA such that binding of the second amplifier to the single-stranded tail formed by the first amplifier binding to the LγPNA opens the hairpin of the second amplifier, leaving a single-stranded tail having the same sequence as the LγPNA, so that the first amplifier can bind to the single stranded tail resulting from binding of the second amplifier to the single-stranded tail formed by the first amplifier binding to the LγPNA.