Pasternack R F, Goldsmith J I, Szép S, Gibbs E J
Department of Chemistry, Swarthmore College, Swarthmore, Pennsylvania 19081, USA.
Biophys J. 1998 Aug;75(2):1024-31. doi: 10.1016/S0006-3495(98)77591-5.
Assemblies of trans-bis(N-methylpyridinium-4-yl)diphenylporphine ions on the surface of calf thymus DNA have been studied using several spectroscopic techniques: absorbance, circular dichroism, and resonance light scattering. The aggregation equilibrium can be treated as a two-state system-monomer and assembly-each bound to the nucleic acid template. The aggregate absorption spectrum in the Soret region is resolved into two bands of Lorentzian line shape, while the DNA-bound monomer spectrum in this region is composed of two Gaussian bands. The Beer-Lambert law is obeyed by both porphyrin forms. The assembly is also characterized by an extremely large, bisignate induced circular dichroism (CD) profile and by enhanced resonance light scattering (RLS). Both the CD and RLS intensities depend linearly on aggregate concentration. The RLS result is consistent with a model for the aggregates as being either of a characteristic size or of a fixed distribution of sizes, independent of total porphyrin concentration or ionic strength. Above threshold values of concentration and ionic strength, the mass action expression for the equilibrium has a particularly simple form: K' = cac-1; where cac is defined as the "critical assembly concentration."offe dependence of the cac upon temperature and ionic strength (NaCl) has been investigated at a fixed DNA concentration. The value of the cac scales as the inverse square of the sodium chloride concentration and, from temperature dependence studies, the aggregation process is shown to be exothermic.
已使用多种光谱技术研究了小牛胸腺DNA表面反式双（N-甲基吡啶鎓-4-基）二苯基卟啉离子的组装体：吸光度、圆二色性和共振光散射。聚集平衡可视为一个双态系统——单体和组装体，它们各自与核酸模板结合。Soret区域的聚集体吸收光谱分解为两个洛伦兹线形的谱带，而该区域与DNA结合的单体光谱由两个高斯谱带组成。两种卟啉形式均符合比尔-朗伯定律。该组装体的特征还在于具有极大的、双符号诱导圆二色性（CD）谱图以及增强的共振光散射（RLS）。CD和RLS强度均与聚集体浓度呈线性关系。RLS结果与聚集体模型一致，即聚集体具有特征尺寸或固定的尺寸分布，与总卟啉浓度或离子强度无关。在浓度和离子强度的阈值以上，平衡的质量作用表达式具有特别简单的形式：K' = cac-1；其中cac被定义为“临界组装浓度”。在固定的DNA浓度下，研究了cac对温度和离子强度（NaCl）的依赖性。cac的值与氯化钠浓度的平方成反比，并且从温度依赖性研究可知，聚集过程是放热的。
