5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrinato-palladium(II) as a differentiation probe for sensing binding modes with B-DNA duplexes: ElectronicMCD and CD spectra

We report our detailed electronic MCD, CD, and optical spectroscopic measur ements and analysis of the porphyrin Soret (B-0) region of four-coordinate 5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrinatopalladium(II), PdP(4), an d its bound states with B-DNA duplexes poly(A-T)(2), paly(G-C)(2), and calf thymus DNA (CT DNA). For system PdP(4)/poly(A-T)(2) it was possible to con clude that the porphyrin is bound edge-on in the major groove, specifically at the 5'AT3' site. For this orientation the porphyrin's electric dipole t ransition moments (edtm), mu(x) (most perturbed direction) and mu(y) (least perturbed direction), have tilt angles alpha similar to 90 degrees and sim ilar to 45 degrees, respectively, relative to the helix axis. It was furthe r concluded from the small shifts of B-0 optical and MCD band intensities a nd wavelengths and from the net MCD (+)A-term sign retention upon binding t hat the porphyrin's frontier p pi MOs (1a(1u) 3a(2u) 4e(g)) are only weakly perturbed by the heterocyclic bases of poly(A-T)(2), and therefore that th e LUMO (4e(g)) splitting is less than the \1a(1u)-3a(2u)\ energy separation , Delta HOMO, that is, Delta LUMO < Delta HOMO for the bound state in PdP(4 )/poly(A-T)(2). For intercalation systems PdP(4)/poly(G-C)(2) and /CT DNA, with PdP(4) centered in the intercalation "pocket" and having two of its 4- N-methylpyridyls extending into each of the major and minor grooves, the ed tms mu(x) and mu(y) were determined to be oriented perpendicular (gamma sim ilar to 0 degrees) and parallel (gamma similar to 90 degrees) to the hydrog en bonds of the base pairs, respectively. Intercalation is characterized by a much stronger binding interaction, viz., the B-0 optical band and net MC D extrema wavelength shifts are relatively large, and the net MCD (+) A-ter m of PdP(4) is substantially quenched as it becomes the (-) pseudo-A-term o f intercalated PdP(4)/poly(G-C)(2). This A-term sign reversal informs that the porphyrin MOs are so strongly perturbed by the GC base pairs that Delta LUMO > Delta HOMO, which gives rise to a (-) pseudo-A-term. Also, the find ings demonstrate (1) the potential of PdP(4) as a sensitive, discriminating analytical probe of DNA sequences and (2) the diagnostic capability of the composite of five spectra [net MCD, CD, and optical of free and bound PdP( 4)] in differentiating the site and sequence selectivity and preferred bind ing mode of this porphyrin. (C) 1999 John Wiley & Sons, Inc.