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The synthesis of milligram quantities of lariat RNAs 1, 2, 5-7 and their cyclic analogs 3 and 4, modelling the lariat formed at the penultimate step of ligation of Group II and Nuclear mRNA introns in the Splicing reaction, have been achieved for the first time. These syntheses are highlighted by unique employment of various orthogonal protecting groups using phosphotriester and phosphoramidite chemistry. Some of the lariat-RNAs have been found to undergo site specific self-cleavage reaction to give an acyclic branched-RNA with 2',3'-cyclic phosphate and a 5'-hydroxyl termini, which is reminiscent of the products formed in some catalytic RI;JAs. These lariat-RNAs are much smaller than the natural catalytic RNAs such as the hammerhead ribozyme (k = -1 min-at 37"C), and their rates of the self-cleavage is also much slower (k=0.25~10-~ min-' for lariat hexamer 5, and 0 . 1 6~1 0 -~ min-' for lariat heptamer 6 at 22°C). We have shown that the trinucleotidyl loop in the tetrameric 1 and pentameric 2 lariat-RNAs is completely stable whereas the tetranucleotidyl or pentanucleotidyl loop in the hexameric 5 or heptameric 6 lariat-RNA does indeed have the required local and global conformation promoting the self-cleavage while the simple 2 ' 4 or 3'4-linked cyclic RNAs, 3 and 4, respectively, are completely stable and their structures are considerably different from the self-cleaving lariat-RNAs such as 5 or 6. The unique 3'-ethylphosphatefunction at the branch-point in 7, mimicking the 3'-tail of the lariat-hexamer 5, is the key structural feature that orchestrates its self-cleavage reaction (k =0.15~10-~ min" at 19°C) compared to the stable 2'+5'-linked cyclic RNA 3. The detailed conformational features of the self-cleaving lariat-RNAs 5, 6 and 7 by 500 MHz NMR spectroscopy and molecular dynamics simulations in the aqueous environment has been reviewed. A comparative study of the temperature dependence of the N o s equilibrium for the lariat tetrarner 7 and the 2'4-linked cyclic tetramer 3 shows that the A' residue in 7 is in 92% S-type conformation at 20°C whereas it is only in 55% S in 3 with a 3'-hydroxyl group. This displacement of the N o s pseudorotational equilibrium toward the S geometry is due to the enhanced gauche effect of the 3'-OP03Et' group at the branch-point adenosine in 7 compared to 3'-OH group in 3. This 3'-OP03Ef group promoted stabilisation of the S geometryat the branch-point by AH = 4 kcal.mol-' in 7 is contributing to the conformational driving force promoting its unique self-cleavage reaction. The comparison of AH" and AS" of the NoS pseudorotational equilibria in 7 and 3 clearly shows the remarkable effect of the 3'-ethylphosphate group in 7 in being able to dictate the conformational changes from the sugar moiety of the branch-point adenosine to the entire molecule (conformational transmission). Thus the S conformation in A', U2 and C6 sugar moieties is clearly thermodynamically more stabilised while it is considerably destabilised in G3 owing to the 3'-ethylphosphate group in 7 comqar...

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