Instituto de Química, Universidad Nacional Autónoma de México, Apdo. Postal 70213, 04510 Coyoacán, Circuito Exterior, México D.F., México.
J Am Chem Soc. 2009 Dec 23;131(50):18129-38. doi: 10.1021/ja903950t.
Specific interactions between molecules, including those produced by a given solute, and the surrounding solvent are essential to drive molecular recognition processes. A simple molecule such as benzene is capable of recognizing and differentiating among very similar entities, such as methyl 2,3,4,6-tetra-O-methyl-alpha-D-galactopyranoside (alpha-Me(5)Gal), methyl 2,3,4,6-tetra-O-methyl-beta-D-galactopyranoside (beta-Me(5)Gal), 1,2,3,4,6-penta-O-acetyl-beta-D-galactopyranose (beta-Ac(5)Gal), and methyl 2,3,4,6-tetra-O-methyl-alpha-D-mannopyranoside (alpha-Me(5)Man). In order to determine if these complexes are formed, the interaction energy between benzene and the different carbohydrates was determined, using Calvet microcalorimetry, as the enthalpy of solvation. These enthalpy values were -89.0 +/- 2.0, -88.7 +/- 5.5, -132.5 +/- 6.2, and -78.8 +/- 3.9 kJ mol(-1) for the four complexes, respectively. Characterization of the different complexes was completed by establishing the molecular region where the interaction takes place using NMR. It was determined that beta-Me(5)Gal is stabilized by the CH/pi interaction produced by the nonpolar region of the carbohydrate on the alpha face. In contrast, alpha-Me(5)Man is not specifically solvated by benzene and does not present any stacking interaction. Although alpha-Me(5)Gal has a geometry similar to that of its epimer, the obtained NMR data seem to indicate that the axial methoxy group at the anomeric position increases the distance of the benzene molecules from the pyranose ring. Substitution of the methoxy groups by acetate moieties, as in beta-Ac(5)Gal, precludes the approach of benzene to produce the CH/pi interaction. In fact, the elevated stabilization energy of beta-Ac(5)Gal is probably due to the interaction between benzene and the methyl groups of the acetyls. Therefore, methoxy and acetyl substituents have different effects on the protons of the pyranose ring.
特定分子之间的相互作用，包括特定溶质产生的相互作用，以及周围溶剂的相互作用，对于驱动分子识别过程至关重要。像苯这样的简单分子能够识别和区分非常相似的实体，例如 2,3,4,6-四-O-甲基-α-D-吡喃半乳糖（α-Me(5)Gal）、2,3,4,6-四-O-甲基-β-D-吡喃半乳糖（β-Me(5)Gal）、1,2,3,4,6-五-O-乙酰基-β-D-吡喃半乳糖（β-Ac(5)Gal）和 2,3,4,6-四-O-甲基-α-D-吡喃甘露糖（α-Me(5)Man）。为了确定这些配合物是否形成，使用 Calvet 微量量热法确定了苯与不同碳水化合物之间的相互作用能，作为溶剂化焓。这些焓值分别为-89.0±2.0、-88.7±5.5、-132.5±6.2 和-78.8±3.9 kJ mol(-1)。通过使用 NMR 确定发生相互作用的分子区域，完成了对不同配合物的表征。确定 β-Me(5)Gal 是由碳水化合物非极性区域产生的 CH/π 相互作用稳定的。相比之下，α-Me(5)Man 不是由苯特异性溶剂化的，也没有任何堆积相互作用。尽管 α-Me(5)Gal 的几何形状与其差向异构体相似，但获得的 NMR 数据似乎表明，在端基位的甲氧基增加了苯分子与吡喃糖环之间的距离。在 β-Ac(5)Gal 中，用乙酰基取代甲氧基，阻止了苯与 CH/π 相互作用。事实上，β-Ac(5)Gal 的稳定化能升高可能是由于苯与乙酰基的甲基之间的相互作用。因此，甲氧基和乙酰基取代基对吡喃糖环上的质子有不同的影响。
