Purine nucleotide derivatives

A nucleotide or, nucleoside, oligonucleotide or polynucleotide, or peptide nucleic acid containing the following structural moiety:

• wherein
• a is a bond and is present or absent,
• b is a bond and is present or absent,
• wherein when a is present, then b is absent and when b is present, then a is absent;
• Y1 is N or CR5,
• wherein R5 is aminoalkyl;
• Y2 is O, NH2, NHR6 or NH—PG1,
• wherein R6 is alkyl, aminoalkyl or —C(O)-alkyl, and
• PG1 is a suitable amine protecting group;
• Y3 is N or NH;
• X1 is N or CH;
• X2 is N or CH;
• X3 is N or CH;
• R2 is H, NH2 or NHC(O)—R7,
• wherein R7 is alkyl, aryl, alkylaryl or aminoalkyl;
• R3 is present or absent and when present is —C(O)—, —C(O)NH—, —NH(O)C— or —N═N—;
• and
• R4 is a substituted five or six-membered aryl or heteroaryl,
• wherein the substitution on the six-membered aryl is other than fluorine substitution.