The association constants for formation of 1â:â1 complexes between five different imidazole ligands and eight different porphyrins have been measured by UV/vis titration experiments in two different solvents, toluene and 1,1,2,2-tetrachloroethane (TCE). Ligands equipped with H-bond acceptors (ester or amide) and porphyrins equipped with H-bond donors (phenol) can make H-bonds in addition to the zincânitrogen coordination interaction. The free energy contributions of these H-bonds to the overall stabilities of the complexes were determined using chemical double mutant cycles. Amide-phenol H-bonds contribute up to 5 kJ molâ1 to the free energy change on complexation, and ester-phenol H-bonds contribute up to 3 kJ molâ1. Porphyrinâligand combinations with poor geometric complementarity do not make detectable H-bonding interactions. Effective molarities (EM) for the formation of H-bonds in the complexes were estimated by comparing the equilibrium constants for formation of the intramolecular interaction with the corresponding intermolecular interaction: the values are between 3 mM and 200 mM, which is comparable to previous results obtained for porphyrinâpyridine complexes. The values of EM measured for flexible and rigid ligand systems are comparable. This suggests that there is a trade off between restriction of conformational mobility in the flexible ligands and geometric strain in the rigid ligands, which results in similar binding affinities.
在
甲苯和
1,1,2,2-四氯乙烷(TCE)两种不同溶剂中,通过紫外/可见光滴定实验测量了五种不同
咪唑配体与八种不同
卟啉之间形成1:1复合物的结合常数。配有
氢键受体(
酯或
酰胺)的
配体和配有
氢键供体(
苯酚)的
卟啉除了
锌氮配位相互作用外,还能产生
氢键。利用
化学双突变循环测定了这些
氢键对复合物整体稳定性的自由能贡献。
酰胺-
苯酚 H 键对络合自由能变化的贡献高达 5 kJ molâ1 ,
酯-
苯酚 H 键对络合自由能变化的贡献高达 3 kJ molâ1 。几何互补性较差的
卟啉配体组合无法产生可检测到的
氢键相互作用。通过比较分子内相互作用与相应的分子间相互作用形成的平衡常数,我们估算出了在复合物中形成 H 键的有效摩尔数(
EM):其值介于 3 mM 与 200 mM 之间,这与之前在
卟啉-
吡啶复合物中获得的结果相当。柔性
配体系统和刚性
配体系统测得的
EM 值相当。这表明,在柔性
配体的构象流动性限制和刚性
配体的几何应变之间存在权衡,从而产生了相似的结合亲和力。