Notestein Justin M, Katz Alexander, Iglesia Enrique
Department of Chemical Engineering, University of California at Berkeley, Berkeley, California 94720-1462, USA.
Langmuir. 2006 Apr 25;22(9):4004-14. doi: 10.1021/la053093c.
Calixarenes grafted on silica are energetically uniform hosts that bind aromatic guests with 1:1 stoichiometry, as shown by binding energies that depend upon the calixarene upper rim composition but not on their grafted surface density (0.02-0.23 nm(-2)). These materials are unique in maintaining a hydrophilic silica surface, as probed by H2O physisorption measurements, while possessing a high density of hydrophobic binding sites that are orthogonal to the silica surface below them. The covalently enforced cone-shaped cavities and complete accessibility of these rigidly grafted calixarenes allow the first unambiguous measurements of the thermodynamics of guest interaction with the same calixarene cavities in aqueous solution and vapor phase. Similar to adsorption into nonpolar protein cavities, adsorption into these hydrophobic cavities from aqueous solution is enthalpy-driven, which is in contrast to entropy-driven adsorption into water-soluble hydrophobic hosts such as beta cyclodextrin. The adsorption thermodynamics of several substituted aromatics from vapor and liquid are compared by (i) describing guest chemical potentials relative to pure guest, which removes differences among guests because of aqueous solvation and van der Waals contacts in the pure condensed phase, and (ii) passivating residual guest binding sites on exposed silica, titrated by water during adsorption from aqueous solution, using inorganic salts before vapor adsorption. Adsorption isotherms depend only upon the saturation vapor pressure of each guest, indicating that guest binding from aqueous or vapor media is controlled by van der Waals contacts with hydrophobic calixarene cavities acting as covalently assembled condensation nuclei, without apparent contributions from CH-pi or other directional interactions. These data also provide the first direct quantification of free energies for interactions of water with the calixarene cavity interior. The calixarene-water interface is stabilized by approximately 20 kJ/mol relative to the water-vapor interface, indicating that water significantly competes with the aromatic guests for adsorption at these ostensibly hydrophobic cavities. This result is useful for understanding models of water interactions with other concave hydrophobic surfaces, including those commonly observed within proteins.
接枝在二氧化硅上的杯芳烃是能量均匀的主体，它们以1:1的化学计量比结合芳香族客体，这从结合能可以看出，结合能取决于杯芳烃的上缘组成，而不取决于它们的接枝表面密度（0.02 - 0.23 nm(-2)）。通过H2O物理吸附测量发现，这些材料的独特之处在于保持了亲水性的二氧化硅表面，同时拥有高密度的疏水结合位点，这些位点与它们下方的二氧化硅表面相互垂直。这些刚性接枝的杯芳烃通过共价作用形成的锥形空腔以及完全可及性，使得首次能够明确测量客体在水溶液和气相中与相同杯芳烃空腔相互作用的热力学。与吸附到非极性蛋白质空腔中类似，从水溶液吸附到这些疏水空腔中是由焓驱动的，这与吸附到水溶性疏水主体（如β-环糊精）中由熵驱动形成对比。通过以下方式比较了几种取代芳烃在气相和液相中的吸附热力学：（i）描述相对于纯客体的客体化学势，这消除了由于水溶液溶剂化和纯凝聚相中范德华接触导致的客体之间的差异；（ii）在气相吸附之前，使用无机盐钝化暴露的二氧化硅上残留的客体结合位点，这些位点在从水溶液吸附过程中会被水滴定。吸附等温线仅取决于每个客体的饱和蒸气压，表明从水相或气相介质中的客体结合是由与疏水杯芳烃空腔的范德华接触控制的，疏水杯芳烃空腔起到共价组装的凝聚核的作用，没有明显的CH-π或其他定向相互作用的贡献。这些数据还首次直接定量了水与杯芳烃空腔内部相互作用的自由能。相对于水 - 气界面，杯芳烃 - 水界面稳定了约20 kJ/mol，表明水在这些表面上看似疏水的空腔处与芳香族客体显著竞争吸附。这一结果有助于理解水与其他凹面疏水表面（包括蛋白质中常见的那些）相互作用的模型。
