Barnes N R, Schreiner A F, Finnegan M G, Johnson M K
Department of Chemistry, North Carolina State University, Raleigh 27695-8204, USA.
Biospectroscopy. 1998;4(5):341-52. doi: 10.1002/(SICI)1520-6343(1998)4:5%3C341::AID-BSPY5%3E3.0.CO;2-E.
The Soret (B0) region of free and externally DNA-bound 5,10,15,20-tetrakis(2-N-methylpyridyl) porphyrinatopalladium(II), PdP(2), was investigated by electronic magnetic circular dichroism (MCD), natural circular dichroism (CD), and optical (UV-visible) absorption spectroscopies. We conclude that four-coordinate, "thick" PdP(2) binds to the exterior of each of poly(A-T)2 and calf thymus DNA (CT DNA) by two distinctly different AT-specific minor and major groove modes, with site 5'TA3' being favored for both modes. The minor groove mode involves an edge-on orientation of PdP(2), for which porphyrin electric dipole transition moments (edtms) mu x (most perturbed direction of the bound porphyrin) and mu y (least perturbed direction) have approximate orientation angles of alpha/beta/beta' = approximately 90 degrees/0 degree/0 degree and approximately 45 degrees/0 degree/90 degrees, respectively. Major groove binding is by a face-on mode, which results in the porphyrin plane being approximately parallel to the helix axis, such that mu x (most perturbed direction) and mu y (least perturbed direction) have approximate orientation angles of alpha/beta/beta' = approximately 45 degrees/180 degrees/90 degrees and approximately 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 small DNA-induced optical red (delta lambda) and hypochromic (H) shifts) are all consistent with exterior binding perturbations of the porphyrin's p pi MOs (1a1u 3a2u 4eg) 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 4eg splitting, or delta LUMO, is less than the energy separation magnitude of 1a1u-3a2u, or delta HOMO. For the third system, PdP(2)/poly (G-C)2, the B0 CD spectrum has two extremely weak (+) and (-) CD bands at higher and lower energy, respectively, indicating that weak outside binding (wob) interactions are taking place between the cationic porphyrin and the electron-rich phosphate backbone of this rigid polymer. The composite of our CD, MCD, and optical data are suggestive of a face-on 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 charge is induced during CD excitation and (2) the excited state angular momentum, 'Lj', changes very little upon binding of PdP(2) to each duplex. These findings are also consistent with each PdP(2)/B-DNA interaction not being very strong.
通过电子磁圆二色性（MCD）、自然圆二色性（CD）和光学（紫外-可见）吸收光谱，研究了游离的和外部结合DNA的5,10,15,20-四（2-N-甲基吡啶基）卟啉钯（II）（PdP(2)）的索雷特（B0）区域。我们得出结论，四配位的“厚”PdP(2)通过两种明显不同的AT特异性小沟和大沟模式与聚（A-T）2和小牛胸腺DNA（CT DNA）的外部结合，两种模式都倾向于5'TA3'位点。小沟模式涉及PdP(2)的边缘取向，对于该取向，卟啉电偶极跃迁矩（edtms）μx（结合卟啉的最受扰动方向）和μy（最不受扰动方向）的近似取向角分别为α/β/β'≈90度/0度/0度和≈45度/0度/90度。大沟结合是通过面对面模式，这导致卟啉平面大致平行于螺旋轴，使得μx（最受扰动方向）和μy（最不受扰动方向）的近似取向角分别为α/β/β'≈45度/180度/90度和≈45度/180度/270度。结合时索雷特MCD和光谱带的变化（即，四方的、真正的MCD（+）A项在变为幅度相似的（+）伪A项以及小的DNA诱导的光学红移（δλ）和减色（H）位移时符号保留）都与每个聚合物的A和T碱基对卟啉的pπ分子轨道（1a1u 3a2u 4eg）的外部结合扰动弱于嵌入引起的扰动一致。此外，PdP(2)的（+）A项在结合时保留（+）符号表明4eg分裂或δLUMO小于1a1u - 3a2u的能量分离幅度或δHOMO。对于第三个体系PdP(2)/聚（G-C）2，B0 CD光谱在较高和较低能量处分别有两个极弱的（+）和（-）CD带，表明阳离子卟啉与这种刚性聚合物的富电子磷酸骨架之间发生了弱的外部结合（wob）相互作用。我们的CD、MCD和光学数据的综合表明在GC大沟处存在面对面模式。对三个结合体系中每个体系的索雷特CD和MCD带进行了带参数提取，并且确定（1）在CD激发期间分子电荷的空间旋转非常小，以及（2）PdP(2)与每个双链体结合时激发态角动量“Lj”变化非常小。这些发现也与每个PdP(2)/B-DNA相互作用不是很强一致。
