Biology and Soft Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
J Chem Phys. 2012 Aug 14;137(6):064902. doi: 10.1063/1.4742190.
Atomistic molecular dynamics (MD) simulations and contrast variation small angle neutron scattering (SANS) have been combined to investigate the Generation-5 polyelectrolyte polyamidoamine starburst dendrimer. This work reveals the dendrimer conformational dependence on counterion association at different levels of molecular charge. The accuracy of the simulations is verified through satisfactory comparison between modeled results, such as excess intra-dendrimer scattering length density distribution and hydration level, and their experimental counterparts. While the counterion distributions are not directly measureable with SANS, the spatial distribution of the counterions and their dendrimer association are extracted from the validated MD equilibrium trajectories. It is found that the conformation of the charged dendrimer is strongly dependent on the counterion association. Sensitivity of the distribution of counterions around charged amines to the counterion valency is qualitatively explained by adopting Langmuir adsorption theory. Moreover, via extending the concept of electrical double layer for compact charged colloids, we define an effective radius of a charged dendrimer including the spatial distribution of counterions in its vicinity. Within the same framework, the correlation between the strength of intra-dendrimer electrostatic repulsion and the counterion valency and dynamics is also addressed.
原子分子动力学（MD）模拟和对比变化小角中子散射（SANS）已结合使用来研究第五代聚电解质聚酰胺胺星型树枝状大分子。这项工作揭示了树枝状大分子构象对不同分子电荷水平下抗衡离子缔合的依赖性。通过将模拟结果（如过量的树突内散射长度密度分布和水合水平）与实验结果进行令人满意的比较，验证了模拟的准确性。虽然 SANS 不能直接测量抗衡离子分布，但从经过验证的 MD 平衡轨迹中提取了抗衡离子和它们与树枝状大分子的缔合的空间分布。结果发现，带电荷的树枝状大分子的构象强烈依赖于抗衡离子的缔合。通过采用朗缪尔吸附理论定性地解释了带电荷胺周围抗衡离子分布对抗衡离子价的敏感性。此外，通过扩展紧凑带电胶体的双电层概念，我们定义了包括其附近抗衡离子空间分布的带电树枝状大分子的有效半径。在相同的框架内，还讨论了树突内静电排斥的强度与抗衡离子价和动力学之间的关系。
