Department of Chemistry, University of Sheffield, S3 7HF, UK.
Org Biomol Chem. 2014 Mar 7;12(9):1440-7. doi: 10.1039/c3ob42246a.
The association constants for formation of 1 : 1 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 zinc-nitrogen 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 mol(-1) to the free energy change on complexation, and ester-phenol H-bonds contribute up to 3 kJ mol(-1). Porphyrin-ligand 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 porphyrin-pyridine 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.
已通过在两种不同溶剂（甲苯和 1,1,2,2-四氯乙烷（TCE））中的紫外/可见滴定实验测量了 5 种不同咪唑配体和 8 种不同卟啉之间形成 1:1 配合物的缔合常数。配备氢键受体（酯或酰胺）的配体和配备氢键供体（苯酚）的卟啉除了锌-氮配位相互作用之外，还可以形成氢键。使用化学双突变体循环确定这些氢键对配合物整体稳定性的自由能贡献。酰胺-苯酚氢键对配合物自由能变化的贡献高达 5 kJ mol(-1)，而酯-苯酚氢键的贡献高达 3 kJ mol(-1)。几何互补性差的卟啉-配体组合不会形成可检测的氢键相互作用。通过比较形成分子内相互作用的平衡常数与相应的分子间相互作用，估计了配合物中形成氢键的有效摩尔浓度（EM）：值在 3 mM 至 200 mM 之间，与先前获得的卟啉-吡啶配合物的结果相当。灵活和刚性配体系统测量的 EM 值相当。这表明在柔性配体中限制构象灵活性和在刚性配体中限制几何应变之间存在权衡，这导致相似的结合亲和力。
