Synthesis and Enzymic Hydrolysis of Oligoribonucleotides Incorporating 3‐Deazaguanosine: The Importance of the Nitrogen‐3 Atom of Single Conserved Guanosine Residues on the Catalytic Activity of the Hammerhead Ribozyme

Four base‐modified hammerhead ribozyme/substrate complexes were constructed in which single guanosine ( 1 ) residues were replaced by 3‐deazaguanosine ( 2 ) in the positions G₅, G₈, G_L2.1, and G₁₂. The base‐modified ribozyme complexes were prepared by solid‐phase synthesis of oligoribonucleotides employing the novel phosphoramidite 3 derived from 2 . Phosphoramidite 3 carried a phenoxyacetyl group at the amino function and a diphenylcarbamoyl residue at the oxo group of the nucleobase. The 2′‐hydroxy group was blocked with a triisopropylsilyl residue. Kinetic analysis of the phosphodiester hydrolysis showed a moderate decrease of the ribozyme catalytic activity when the residues G₅ or G₈ were replaced by 3‐deazaguanosine and a 200‐fold decrease when G₁₂ was substituted. A 6‐fold catalytic increase occurred when 3‐deazaguanosine was replacing G_L2.1 in the loop region. The data indicate that the N(3) atom of compound 2 , in particular at position G₁₂ is critical for the ribozyme activity.