INSIGHT INTO EXTERNALLY BOUND RAKIS(2-N-METHYLPYRIDYL)PORPHYRINATOPALLADIUM(II), PDP (2), WITH B-FORM DNA DUPLEXES POLY(G-C)(2), POLY(A-T)(2), AND CT DNA BY USING COMBINED MCD, CD, AND OPTICAL-DATA

The Soret (B-0) region of free and externally DNA-bound 5,10,15,20-tet rakis (2-N-methylpyridyl) porphyrinatopalladium( Il), PdP(2), was inve stigated by electronic magnetic circular dichroism (MCD), natural circ ular dichroism (CD), and optical(W-visible) absorption spectroscopies, We conclude that four-coordinate, ''thick'' PdP(2) binds to the exter ior of each of poly(A-T)(2) and calf thymus DNA (CT DNA) by two distin ctly different AT-specific minor and major groove modes, with site 5'T A3' being favored for both modes. The minor groove mode involves an ed ge-on orientation of PdP(2), for which porphyrin electric dipole trans ition moments (edtms) mu(x) (most perturbed direction of the bound por phyrin) and mu(y), (least perturbed direction) have approximate orient ation angles of alpha/beta/beta' = similar to 90 degrees/0 degrees/0 d egrees and similar to 45 degrees/0 degrees/90 degrees, respectively. M ajor groove binding is by a face-on mode, which results in the porphyr in plane being approximately parallel to the helix axis, such that mu( x), (most perturbed direction) and mu(y) (least perturbed direction) h ave approximate orientation angles of alpha/beta/beta' = similar to 45 degrees/180 degrees/90 degrees and similar to 45 degrees/180 degrees/ 270 degrees, respectively. The Soret MCD and optical band alterations upon binding (i.e., sign retention of the tetragonal, genuine MCD (+) A-term on becoming the (+) pseudo-A-term of similar amplitude and smal l DNA-induced optical red (Delta lambda) and hypochromic (H) shifts) a re all consistent with exterior binding perturbations of the porphyrin 's p pi MOs (1a(1u)3a(2u) 4e(g)) by the A and T bases of each polymer being weaker than caused by intercalation. Furthermore, that the (+) A -term of PdP(2) retains the ( +) sign upon binding informs that the 4e (g) splitting, or Delta LUMO, is less than the energy separation \ 1a( 1u)-3a(2u) \, or Delta HOMO. For the third system, PdP(a)/poly( G-C)(2 ), the B-0 CD spectrum has two extremely weak (+) and (-) CD bands at higher and lower energy, respectively, indicating that weak outside bi nding (wob) interactions are taking place between the cationic porphyr in and the electron-rich phosphate backbone of this rigid polymer. The composite of our CD, MCD, and optical data are suggestive of a face-o n mode at the GC major groove. Band parameter extraction is performed on the Soret CD and MCD bands of each of the three bound systems, and it is determined that (1) very little spatial rotation of molecular ch arge is induced during CD excitation and (2) the excited state angular momentum, [L-j], changes very Little upon binding of PdP(2) to each d uplex. These findings are also consistent with each PdP(2)/B-DNA inter action not being very strong. (C) 1998 John Wiley & Sons, Inc.