Physico-chemical studies on Deoxyoligoyuanlic acids and their complexes with Polyribocytidylic acids

Abstract

Venom and spleen exonuclease are capable of cleaving guanosine oligonucleotides when they are not aggregated. At 22°C, in presence of sodium ions aggregation is rapid even at 0.5 × 10?? M concentration. On the other hand, bulky cations such as tetraethylammonium ion (Et?N?) prevent aggregation under the conditions studied. Self-association is found highly co-operative. ?G° for formation of a G·G pair is calculated to be about –1.5 kcal/mole. Determination of Km and Vmax shows that the affinity of enzymes for longer oligonucleotides is higher although shorter ones are cleaved faster. Stacking interaction in deoxyguanosine oligonucleotides is found chain-length dependent. Nearest-neighbour analysis of CD shows qualitative agreement in most of the cases. 5?- or 3?-phosphate is found to be significant in determining the CD of dinucleotides. Acetyl group increases stacking affinity probably through enhanced solvation. The low stability of hybrid helix formed from deoxyoligoguanylic acids and poly(ribocytidylic acid) is reflected in the thermodynamic parameters. The stability of dG·rC is smaller than that of rG·rC by a factor of two. ?G° for hydrogen bonding is approximately the same for both the base pairs. The difference in stability lies in differences in stacking enthalpy and entropy changes. Since the stability constant obtained from melting data of trimer and tetramer is ten times smaller than that calculated from tetramer and pentamer, it can be concluded that helix nucleation consists of formation of 3–4 base pairs.