Kano Koji, Nishiyabu Ryuhei, Doi Ryoji
Department of Molecular Science and Technology, Doshisha University, Kyotanabe, Kyoto 610-0321, Japan.
J Org Chem. 2005 Apr 29;70(9):3667-73. doi: 10.1021/jo0500535.
[structure: see text] The mechanism for formation of extremely stable 1:2 inclusion complexes of water-soluble meso-tetraarylporphyrins with heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin (TMe-beta-CD) in aqueous solutions has been studied by means of NMR spectroscopy and isothermal titration calorimetry. To simplify the system, 5,10,15-tris(3,5-dicarboxylatophenyl)-20-phenylporphyrin (1) was used as a guest porphyrin, because 1 forms only a 1:1 inclusion complex with cyclodextrin (CD). As host compounds, native beta-CD and the O-methylated-beta-CDs such as heptakis(2,3-di-O-methyl)- (2,3-DMe-beta-CD), heptakis(2,6-di-O-methyl)- (2,6-DMe-beta-CD), and TMe-beta-CDs were used. The thermodynamic parameters for complexation such as binding constants (K) and enthalpy (DeltaH degrees ) and entoropy changes (DeltaS degrees ) were determined by means of isothermal titration calorimetry. The K value for complexation of 1 with CD increases in the order beta-CD (K = (1.2 +/- 0.1) x 10(3) M(-)(1)) < 2,6-DMe-beta-CD ((1.2 +/- 0.1) x 10(4) M(-)(1)) << TMe-beta-CD ((6.9 +/- 0.4) x 10(6) M(-)(1)) < 2,3-DMe-beta-CD ((8.5 +/- 0.5) x 10(6) M(-)(1)), indicating participation of the secondary OCH(3) groups in extremely strong complexation of 1 with CD. Complex formation of 1 with beta-CD and 2,6-DMe-beta-CD is an enthalpically and entropically favorable process, while that with TMe-beta-CD and 2,3-DMe-beta-CD is an enthalpically much more favorable but an entropically less favorable process. The thermodynamic parameters suggest that inclusion of 1 into the cavities of TMe-beta-CD and 2,3-DMe-beta-CD is promoted by van der Waals interactions, which are stronger than those in the cases of beta-CD and 2,6-DMe-beta-CD. (13)C NMR spectra show that the conformations of both TMe-beta-CD and 2,3-DMe-beta-CD are altered upon inclusion of 1, while those of beta-CD and 2,6-DMe-beta-CD are mostly retained. On the basis of these results, it can be concluded that induced-fit type complexation of 1 with TMe-beta-CD and 2,3-DMe-beta-CD causes extremely strong binding of the host to the guest.
[结构：见正文] 通过核磁共振光谱法和等温滴定量热法研究了水溶性中位四芳基卟啉与七（2,3,6-三-O-甲基）-β-环糊精（TMe-β-CD）在水溶液中形成极其稳定的1:2包合物的机制。为简化体系，使用5,10,15-三（3,5-二羧基苯基）-20-苯基卟啉（1）作为客体卟啉，因为1与环糊精（CD）仅形成1:1包合物。作为主体化合物，使用了天然β-CD和O-甲基化的β-CD，如七（2,3-二-O-甲基）-（2,3-DMe-β-CD）、七（2,6-二-O-甲基）-（2,6-DMe-β-CD）和TMe-β-CD。通过等温滴定量热法测定了络合的热力学参数，如结合常数（K）、焓（ΔH°）和熵变（ΔS°）。1与CD络合的K值按以下顺序增加：β-CD（K = (1.2 ± 0.1) × 10³ M⁻¹） < 2,6-DMe-β-CD（(1.2 ± 0.1) × 10⁴ M⁻¹） << TMe-β-CD（(6.9 ± 0.4) × 10⁶ M⁻¹） < 2,3-DMe-β-CD（(8.5 ± 0.5) × 10⁶ M⁻¹），表明仲OCH₃基团参与了1与CD的极强络合。1与β-CD和2,6-DMe-β-CD的络合形成是一个焓和熵都有利的过程，而与TMe-β-CD和2,3-DMe-β-CD的络合形成是一个焓更有利但熵不太有利的过程。热力学参数表明，1被包入TMe-β-CD和2,3-DMe-β-CD的空腔中是由范德华相互作用促进的，这种相互作用比β-CD和2,6-DMe-β-CD的情况更强。¹³C NMR光谱表明，在包入1后，TMe-β-CD和2,3-DMe-β-CD的构象都发生了改变，而β-CD和2,6-DMe-β-CD的构象大多保留。基于这些结果，可以得出结论，1与TMe-β-CD和2,3-DMe-β-CD的诱导契合型络合导致主体与客体的极强结合。
