Semiempirical calculations suggest that the intercalation complexes of phenanthridinium cations 1-4 with G-C/C-G and 1 with A-U/U-A are stabilized by frontier orbital interactions between the LUMO of the intercalator and the HOMOs of the adjacent purine bases. The charge on the ring nitrogen of 1-4 appears to be necessary for the orbital interactions, lowering the LUMO, facilitating mixing of this orbital with the HOMOs of the adjacent purine bases to give an extended HOMO stabilizing the complex and resulting in the bathochromic shift in the electron absorption spectrum. Noncationic phenanthridine 5 shows no frontier orbital interactions in the forced intercalation complex with G-C/C-G. The results of the calculations parallel experimental T(m) values.
Bibliographical noteFunding Information:
This work was supported by grant AI-27196 of the National Instituteso f Health, U.S.A. (L.S.), grant of the ResearchC orporation,U .S.A. (L.S.), and the New Zealand Lotteries board and the PGS Fund for Grants (J.M.C.). We thank Farial Tanious and Dr David