Department of Chemistry, Yeungnam University Gyeongsan City, Gyeong-buk, 712-749 Republic of Korea.
J Phys Chem B. 2012 Oct 18;116(41):12510-21. doi: 10.1021/jp3081063. Epub 2012 Oct 8.
The binding modes of cationic porphyrins, namely M-meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (MTMPyP, where M = free base, Cu(II), Ni(II), Co(III), Mn(III), V(IV)═O, and Ti(IV)═O), to native DNA were systematically investigated by polarized light spectroscopies, viz. circular and linear dichroism spectroscopy (CD and LD, respectively). At low [porphyrin]/[DNA] ratio, planar porphyrins including TMPyP, CuTMPyP, and NiTMPyP exhibited a negative CD signal in the Soret region and large negative wavelength-dependent LD(r) signal which is characteristics of their intercalative binding mode. In the intercalation pocket, the molecular plane of porphyrin is skewed to a large extent as it was judged from a large wavelength-dependency of the LD(r) magnitude in the Soret region. As the mixing ratio was increased, a bisignate CD spectrum became apparent for all of the planar porphyrins, while the shape of LD(r) remained the same, indicating the coupling of the electric transition moments of the intercalated porphyrins. Thus, coupling can occur between the porphyrins when they are separated at least two DNA base-pairs according to the nearest neighboring site exclusion model. This coupling interaction was the weakest for NiTMPyP. The porphyrins with axial ligands, namely VOTMPyP, TiOTMPyP, MnTMPyP, and CoTMPyP, bind to the exterior of the DNA at a low [porphyrin]/[DNA] ratio, which is indicated by the positive CD signal in the Soret region. The absorption and LD spectra in the Soret region were treated as the sum of two transitions to calculate the angle between each of the electric transition moments of the porphyrin, i.e., the B(x) and B(y) transitions relative to the local DNA helix axis. The angle of one of the two electric transitions of the porphyrin is in the range of 56°∼59°, whereas the other is in the range of 59°∼65°. Increasing the porphyrin density resulted in the appearance of an interaction between the bound porphyrins, which was indicated by the distorted CD spectra. In contrast with the planar porphyrins, which are parallel to each other in the intercalation pocket when they interact by themselves, the molecular plane of the porphyrin with the axial ligand tilts more toward the local DNA helix axis as the population of porphyrins was increased. CoTMPyP exhibited complicated CD and LD spectra at high [porphyrin]/[DNA] ratios that were impossible to explain by the presence of two porphyrin species: those with a monomeric external binding and electric coupling. Since the CoTMPyP-DNA complex started to aggregate at relatively low [porphyrin]/[DNA] ratios, the complication may due to the involvement of extensive porphyrin aggregation.
阳离子卟啉，即 M-中位-四（N-甲基吡啶-4-基）卟啉（MTMPyP，其中 M = 游离基、Cu(II)、Ni(II)、Co(III)、Mn(III)、V(IV)═O 和 Ti(IV)═O）与天然 DNA 的结合模式通过偏振光光谱学，即圆二色性光谱（CD）和线二色性光谱（LD），得到了系统的研究。在低[卟啉]/[DNA]比下，平面卟啉，包括 TMPyP、CuTMPyP 和 NiTMPyP，在 Soret 区表现出负的 CD 信号和大的负波长依赖 LD(r)信号，这是它们嵌入结合模式的特征。在嵌入口袋中，卟啉的分子平面在很大程度上发生倾斜，这可以从 Soret 区 LD(r)幅度的大波长依赖性来判断。随着混合比的增加，所有平面卟啉都出现了双信号 CD 光谱，而 LD(r)的形状保持不变，表明嵌入卟啉的电跃迁矩发生了耦合。因此，根据最近邻位排斥模型，当卟啉彼此分离至少两个 DNA 碱基对时，它们之间可以发生偶联。这种偶联相互作用对于 NiTMPyP 来说是最弱的。具有轴向配体的卟啉，即 VOTMPyP、TiOTMPyP、MnTMPyP 和 CoTMPyP，在低[卟啉]/[DNA]比下结合到 DNA 的外部，这表明在 Soret 区有正的 CD 信号。Soret 区的吸收和 LD 光谱被处理为两个跃迁的和，以计算每个卟啉的电跃迁矩之间的角度，即相对于局部 DNA 螺旋轴的 B(x)和 B(y)跃迁。卟啉的两个电跃迁之一的角度在 56°∼59°范围内，而另一个在 59°∼65°范围内。随着卟啉密度的增加，出现了结合卟啉之间的相互作用，这表明 CD 光谱发生了扭曲。与在嵌入口袋中彼此平行的平面卟啉不同，当它们通过自身相互作用时，具有轴向配体的卟啉的分子平面更倾向于局部 DNA 螺旋轴，随着卟啉的增加。CoTMPyP 在高[卟啉]/[DNA]比下表现出复杂的 CD 和 LD 光谱，这不可能用两种卟啉物种的存在来解释：一种是单体外部结合和电偶联。由于 CoTMPyP-DNA 复合物在相对较低的[卟啉]/[DNA]比下开始聚集，这种复杂性可能是由于卟啉聚集的广泛参与。
