Modified nucleotides

A modified nucleotide or nucleoside molecule comprising a purine or pyrimidine base and a ribose or deoxyribose sugar moiety having a removable 3′-OH blocking group covalently attached thereto, such that the 3′ carbon atom has attached a group of the structure

• —O-Z
• wherein Z is any of —C(RIV)2—O—R″, —C(R′)2—N(R″)2, —C(R′)2—N(H)R″, —C(RIV)2—S—R″ and —C(R″)2—F,
• wherein —C(RIV)2—O—R″ is of the formula —CR4(R5)—O—CR4(R5)—OR6 or of the formula —CR4(R5)—O—CR4(R5)—SR6;
• and wherein —C(RIV)2—S—R″ is of the formula —CR4(R5)—S—CR4(R5)—OR6 or of the formula —CR4(R5)—S—CR4(R5)—SR6;
• wherein each R″ is or is part of a removable protecting group;
• each R′ is independently a hydrogen atom, an alkyl, substituted alkyl, arylalkyl, alkenyl, alkynyl, aryl, heteroaryl, heterocyclic, acyl, cyano, alkoxy, aryloxy, heteroaryloxy or amido group, or a detectable label attached through a linking group;
• or (R′)2 represents an alkylidene group of formula ═C(R′″)2 wherein each R′″ may be the same or different and is selected from the group comprising hydrogen and halogen atoms and alkyl groups;
• each R4 and R5 is independently a hydrogen atom or an alkyl group;
• R6 is alkyl, cycloalkyl, alkenyl, cycloalkenyl or benzyl;
• and
• wherein said molecule may be reacted to yield an intermediate in which each R″ is exchanged for H or, where Z is —C(R′)2—F, the F is exchanged for OH, SH or NH2, preferably OH, which intermediate dissociates under aqueous conditions to afford a molecule with a free 3′OH;
• with the proviso that where Z is —C(RIV)2—S—R″, both RIV groups are not H.