Diffusion and Aggregation of Sodium Fluorescein in Aqueous Solutions
Diffusion and Aggregation of Sodium Fluorescein in Aqueous Solutions
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DOI：
10.1021/jp207459k
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发表时间：
2011-11-10
影响因子：
3.3
通讯作者：
Cavallotti, Carlo
中科院分区：
文献类型：
--
作者：
Casalini, Tommaso;Salvalaglio, Matteo;Cavallotti, Carlo
采用密度泛函理论（DFT）和分子动力学（MD）模拟研究了荧光素钠在水溶液中的扩散和聚集行为。首先，在隐式水的DFT计算被用来确定最低能量结构和溶质的原子电荷，然后被用作显式水分子动力学模拟的输入。使用爱因斯坦方程计算荧光素钠的自扩散系数，从24 ns轨迹计算均方位移。计算的扩散系数为0.42。10（-5）cm（2）s（-1），与文献实验数据吻合较好。模拟证实了荧光素形成二聚体的趋势。为了实现更深入的理解聚集现象，二聚体的几何形状进行了研究，通过DFT计算在真空和隐式水使用不同的泛函和溶剂化理论。结果表明，二聚不发生在真空中，电荷排斥占主导地位，和最低能量二聚体结构是对称的，并通过边到面π-π相互作用稳定。在DFT水平和通过MD模拟使用伞采样计算相互作用能。用WHAM和Umbrella积分方案计算的自由相互作用能为-1.3 kcal/mol，与实验数据吻合良好，而使用DFT计算确定的值明显较小，并且在很大程度上取决于用于确定溶质-溶剂边界表面的所选泛函和计算方法。
The diffusion and aggregation of sodium fluorescein in aqueous solutions was investigated adopting density functional theory (DFT) and molecular dynamics (MD) simulations. First, DFT calculations in implicit water were used to determine minimum energy structure and atomic charges of the solute, which were then used as input for explicit water MD simulations. The self-diffusion coefficient of sodium fluorescein was calculated using the Einstein equation, computing the mean square displacement from 24 ns trajectories. The calculated diffusion coefficient, 0.42 . 10(-5) cm(2) s(-1),is in good agreement with literature experimental data. The simulations confirmed the tendency of fluorescein to form dimers. In order to achieve a deeper understanding of aggregation phenomena, the dimer geometry was investigated through DFT calculations both in vacuo and in implicit water using different functionals and solvation theories. The results showed that dimerization does not occur in vacuo, as charge repulsion dominates, and that the minimum energy dimer structure is symmetric and stabilized by edge-to-face pi-pi interactions. The interaction energy was computed both at the DFT level and through MD simulations using Umbrella Sampling. The free interaction energy calculated with the WHAM and Umbrella Integration protocol, -1.3 kcal/mol, is in good agreement with experimental data, while the value determined using DFT calculations is significantly smaller and depends largely from the chosen functional and the computational methodology used to determine the solute-solvent boundary surface.