Radhakrishnan A V, Madhukar S, Chowdhury A, Raghunathan V A
Raman Research Institute, Bangalore 560 080, India.
Phys Rev E. 2022 Jun;105(6-1):064504. doi: 10.1103/PhysRevE.105.064504.
We have studied the structure of complexes of the cationic surfactant dodecyltrimethylammonium bromide (DTAB) with DNA as a function of surfactant to DNA base molar ratio (R) and salt concentration. Small-angle x-ray scattering data show the formation of nematic gels at lower and higher salt concentrations, irrespective of the value of R. Two crystalline phases are observed over intermediate salt concentrations; a square (S) phase for R>3 and a hexagonal (H_{S}) phase for lower R. Electron density maps of these phases show intercalated structures, with DTAB micelles sandwiched between long DNA strands. The composition of these complexes, estimated using elemental analysis, indicates that the micelles are not very long, and they occupy only about half of the interstitial volume between the DNA strands. This phase behavior is strikingly different from that of complexes of DNA with longer chain surfactants cetyltrimethylammonium bromide (CTAB) and tetradecyltrimethylammonium bromide (TTAB), which show only a hexagonal (H) phase over similar ranges of R and salt concentration, the H_{S} structure observed in the present study being a sqrt[3]×sqrt[3] superlattice of the H structure. Madelung energies of the S and H structures, calculated from the electrostatic interaction between their cylindrical constituents, suggest that the former is preferred in DTAB-DNA complexes due to the smaller micellar radius of DTAB. The propensity of DTAB to form short micelles seems also to favor the H_{S} phase at lower R. These results illustrate the important role of micellar size in determining the structure of these two-dimensional macro-ion crystals.
我们研究了阳离子表面活性剂十二烷基三甲基溴化铵（DTAB）与DNA形成的复合物的结构，该结构是表面活性剂与DNA碱基摩尔比（R）和盐浓度的函数。小角X射线散射数据表明，无论R值如何，在较低和较高盐浓度下都会形成向列型凝胶。在中等盐浓度范围内观察到两种晶相；R>3时为正方形（S）相，R较低时为六边形（Hₛ）相。这些相的电子密度图显示出插层结构，DTAB胶束夹在长DNA链之间。使用元素分析估计这些复合物的组成，结果表明胶束不是很长，它们仅占据DNA链之间间隙体积的大约一半。这种相行为与DNA与长链表面活性剂十六烷基三甲基溴化铵（CTAB）和十四烷基三甲基溴化铵（TTAB）形成的复合物显著不同，在相似的R和盐浓度范围内，后者仅显示六边形（H）相，本研究中观察到的Hₛ结构是H结构的√3×√3超晶格。根据S和H结构的圆柱形成分之间的静电相互作用计算得出的马德隆能量表明，由于DTAB的胶束半径较小，前者在DTAB-DNA复合物中更受青睐。DTAB形成短胶束的倾向似乎也有利于在较低R值时形成Hₛ相。这些结果说明了胶束大小在决定这些二维大离子晶体结构中的重要作用。
