The thermal stability of RNA duplexes containing modified base pairs placed at internal and terminal positions of the oligoribonucleotides.

The presence of various modifications within oligomers changes their thermodynamic stability. To get more systematic data, we measured effects of 5- and 6-substituted uridine on thermal stability of (AUCU(Mod.)AGAU)2 and (AUCUAGAU(Mod.))2. Collected results lead to the following conclusions: (i) 5-halogenated and 5-alkylated substituents of the uridine affect thermal stability of the RNA duplexes differently. Moreover, the 5-fluorouridine changes stability of the RNA duplexes opposite to remaining 5-halogenouridines; (ii) for oligomers containing 5-chloro, 5-bromo or 5-iodouridine stronger hydrogen bond formed between oxygen-4 of the 5-halogenated uracil and 6-amino group of the adenine is presumably responsible for stabilizing effect; (iii) placing of A-U(5R) base pairs closer to the end of the duplex enhance thermal stability relatively to oligomer with central position of this base pair; (iv) the effects of 5-substituents are additive, particularly for substituents which stabilize RNA duplexes; (v) 6-methyluridines (N1 and N3 isomers) as well as 3N-methyluridine present at internal position of A-U(Mod.) inhibit duplexes formation; (vi) 6-methyluridines (N1 and N3 isomers) as well as 3N-methyluridine placed as terminal base pairs stabilize the duplexes mostly via 3'-dangling end effect.