Honciuc Andrei, Schwartz Daniel K
Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309, USA.
J Am Chem Soc. 2009 Apr 29;131(16):5973-9. doi: 10.1021/ja900607g.
Individual molecules of fluorophore-labeled alkanoic acids with various chain lengths, BODIPY-(CH(2))(n)-COOH (abbreviated as fl-Cn), were observed to adsorb and move at the methylated fused silica-water interface as a function of temperature using total internal reflection fluorescence microscopy. The statistical analysis of squared-displacement distributions indicated that the molecular trajectories were consistent with a diffusive model involving two intertwined modes. The slower mode, typically responsible for <50% of the molecular diffusion time, had a diffusion coefficient of <0.005 mum(2)/s and could not be distinguished from the apparent motions of immobilized molecules because of the limitations of experimental resolution. The faster mode exhibited diffusion coefficients that increased with temperature for all chain lengths, permitting an Arrhenius analysis. Both the effective activation energies and kinetic prefactors associated with the fast-mode diffusion coefficients increased systematically with chain length for fl-C2 through fl-C10; however, fl-C15 did not follow this trend but instead exhibited anomalously small values of both parameters. These observations were considered in the context of hydrophobic interactions between the adsorbate molecules and the methylated surface in the presence of water. Specifically, it was hypothesized that fl-C2, fl-C4, and fl-C10 adopted primarily extended molecular conformations on the hydrophobic surface. The increases in activation energy and entropy with chain length for these molecules are consistent with a picture of the transition state in which the molecule partially detaches from the surface and exhibits greater conformational freedom. In contrast, the small activation energy and entropy for fl-C15 are consistent with a scenario in which the surface-bound molecule adopts a compact/globular conformation with limited surface contact and conformational flexibility.
利用全内反射荧光显微镜，观察到不同链长的荧光团标记链烷酸的单个分子，即BODIPY-(CH(2))(n)-COOH（简称为fl-Cn），在甲基化熔融石英-水界面上随温度变化的吸附和移动情况。平方位移分布的统计分析表明，分子轨迹与涉及两种交织模式的扩散模型一致。较慢的模式通常占分子扩散时间的不到50%，其扩散系数小于0.005μm(2)/s，由于实验分辨率的限制，无法与固定分子的表观运动区分开来。较快的模式在所有链长下的扩散系数均随温度升高，从而可以进行阿伦尼乌斯分析。对于fl-C2到fl-C10，与快速模式扩散系数相关的有效活化能和动力学预因子均随链长系统地增加；然而，fl-C15并不遵循这一趋势，而是这两个参数的值异常小。在水存在的情况下，根据吸附质分子与甲基化表面之间的疏水相互作用来考虑这些观察结果。具体而言，推测fl-C2、fl-C4和fl-C10在疏水表面上主要采取伸展的分子构象。这些分子的活化能和熵随链长的增加与过渡态的情况一致，即分子部分脱离表面并表现出更大的构象自由度。相比之下，fl-C15的活化能和熵较小，这与表面结合分子采取紧密/球状构象、表面接触和构象灵活性有限的情况一致。
