Measurement and interpretation of Q0 and Q1 band property changes of two cationic metalloporphyrins upon binding with B-DNA: electronic MCD, CD, and optical absorption.

Room-temperature Q-band electronic MCD, CD, and optical spectra are reported for the first time for two free and nucleic acid-bound cationic metalloporphyrins. Metalloporphyrins are the high-symmetry (C4v or D4h), four-coordinate tetragonal type MP(X) [M = CuII and PtII; P(X) = meso-tetrakis(X-N-methylpyridyl)porphine; X = 2 or 4], and the nucleic acid is native, B-form calf thymus DNA (CT DNA). For intercalation system PtP(4)/CT DNA, large optical (lambda 0, epsilon max) and MCD (lambda peak, lambda trough, A(aj), A(aj)/D(aj), and delta[theta]Mp-t/epsilon max) band parameter shifts, as well as a single negative (-) induced CD peak for each of Q0 and Q1, were observed upon binding of the porphyrin to chiral DNA. The directions and magnitudes of these changes are comparable to those observed for the Soret (B0) band of this system. Decreases of MCD/optical ratio delta[theta]Mp-t/epsilon max (varies; is directly proportional to A(aj)/D(aj)) of 30% (Q0) and 50% (Q1) upon intercalation indicate substantial reductions of the Q0[1Eu(a) (0,0), approximately 1a1u1 4eg1] and Q1[1Eu(a)(0,1), approximately 1a1u1 4eg1] excited state angular momenta, . It is of additional interest that intercalation leads to intensity cancellation of one of the four A-term lobes, the (+)lobe of the Q0 MCD (+)pseudo-A-term, which was also observed previously for intercalation systems PdP(4)/poly(G-C)2 and /CT DNA. Application of the CD sector method to the constituent x- and y-polarized porphyrin edtms, square root of D(aj), of the Q0 (edtms mu0x and mu0y) and Q1 (edtms mu1x and mu1y) CD bands leads to the conclusion that PtP(4) is symmetrically intercalated between adjacent GC base pairs, specifically at 5'GC3' sites, with each of two adjacent 4-N-methylpyridyl groups extending into each of the major and minor grooves. For outside binder CuP(2), small optical and MCD band parameter shifts and smaller, single positive (+) induced CD peaks are observed for Q0 and Q1 upon interaction with CT DNA, similar to what is found for B0. Little or no change in the MCD/optical ratio, delta[theta]Mp-t/epsilon max, informs that external binding has only small effects on the excited state angular momenta, , of these bands. The composite of MCD, CD, and optical spectra are consistent with CuP(2) binding to CT DNA by one or both of two competing external modes, i.e., by an AT-specific, edge-on minor groove mode at 5'TA3' sites and/or a face-on major groove mode with high selectivity for 5'AT3' or 5'CG3' sites. For Q0 and Q1 of each of the CuP(2)/CT DNA and PtP(4)/CT DNA systems, we observe that the genuine MCD (+) A-terms of these free MP(X)s retain their (+) sign on becoming pseudo-A-terms upon binding to the low-symmetry, asymmetric DNA sites, and this indicates that the external and intercalative modes result in the 4eg splitting, or delta LUMO, being less than the magnitude of 1a1u-3a2u energy separation, or delta HOMO.