Ellipticine compounds, derivatives of pyrido-(4-3b) carbazole are used in the human cancer therapy. Most of these drugs interact directly with DNA molecule. CNDO method, alongwith second order perturbation theory and multicentered-multipole approxiation have been used to compute intermolecular interaction energies of ellipticine with DNA ase pairs (GC and AT) in both normal and inverted cases. An attempt has been made to correlate the drug-nucleic acid interactions for ellipticine to locate site of drug action and binding pattern on the basis of intermolecular forces

The biological activity of oxoformycin B has been exafned on the basis of the model developed for the incorporation of nucleoside analogues during transcription. Claverie’s simplified formula has been employed for intermolecular interaction energy calculation. The pairing energy of oxoformycin B base with complementary bases as well as the association energy with nucleic acid base pairs have been calculated. The results are compared with those of similar computation with normal bases. In addition to the in-plane interaction the vertical interaction energy between the analogue and the normal bases has been computed to specify the particular position of the analogue in the chain. On the basis of the model an attempt has been made to explain the mechanism of the biological action of oxoformycin B and to compare the biological activity of pyrazolopyrimidine nucleoside analogues.

The quantum mechanical perturbation method has been utilized to study the biological activity of 8-azapurine (8-azaguanosine, 8-azaadenosine and 8-aza-2,6-diamino-purine) nucleoside antibiotics. The in-plane (hydrogen bonding) and stacking energy of 8-azapurine bases have been evaluated with nucleic acid bases and base pairs in all possible orientations. The energy values and the sites of association of analogous bases, obtained by optimization of energy values as well as the sites of association of nucleic acid bases during the transcription process have been compared. The model developed earlier for the incorporation of nucleoside analogues has been used to find out the inhibitory effects of the drug on nucleic acid and protein synthesis. It has been observed that the activity of 8-azapurines are of the following order

8-azaguanine > 8-aza-2,6-diaminopurine > 8-azaadenine

and these analogues show preference for binding near a guanine or cytosine in the chain. The results are in agreement with the experimental observations